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Invasive alien plant species, fragmentation and scale effects on urban forest community composition in Durban, South Africa

Invasive alien plant species, fragmentation and scale effects on urban forest community... Background: Urban forests are under increased pressure from invasion by exotic (alien) species. The vegetation present in the matrix of urban sites is a rich source of alien invasive propagules, which increases the risk of alien invasion in forests within an urban space, leading to a decline in indigenous species. Therefore, determining the distribution patterns of native and exotic species as influenced by environmental factors can assist in quantifying the impact of exotic species at broad scales based on responses on a finer scale. Quantifying the effects of multiple environmental factors on the distribution patterns of both indigenous and alien species in the ecosystem may help in prescribing suitable management efforts. Methods: Fifteen forest patches were sampled in the eThekwini (Durban) Municipality and data collected from 74 100-m plots with different degrees of invasion. Indigenous and alien species of trees, shrubs and climbers occurring in ten and more plots were considered for analysis and the CANOCO 5.1 package was used to run various constrained ordination analyses. Variation partitioning analysis was used to assess the impact of environmental variables at different spatial scales, namely the plot and patch scales. Results: Canopy gaps are the major controlling factor for invasive alien plants (IAPs) occurrence at fine scale. At patch level, residential and industry areas outside the boundaries (buffer area) of forest patches have a high influence on the distribution of IAPs. Communities dominated by the invasive Chromolaena odorata (L.) R.M. King are most common on the lowland coastal forests while communities dominated by either Litsea glutinosa (Lour.) C.B.Rob. or Cardiospermum grandiflorum Sw. can prevail in both lowland coastal and scarp forests. Conclusions: Canopy gaps in lowland forests can facilitate the transition of native forests to novel communities containing a variety of alien plant species. Communities of shrub, climber and tree IAPs occur in lowland coastal forests while climber and tree IAPs dominate the high-elevation scarp forests. The resilience shown by some native species to the tree, shrub and climber IAPs by remaining when IAPs establish makes these species very suitable for restoration projects. Forest patches surrounded by a high incidence of residential and industrial areas in the buffer matrix are likely to have a high diversity of IAPs. Larger patch size and high connectivity to nearby native forests are key in reducing invasion by IAPs. Keywords: Species community, Patch connectivity index, Invasive species * Correspondence: vimbi28@gmail.com School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, University Road Westville, Private Bag X 54001, Durban 4000, South Africa © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 2 of 14 Background along drainage channels or paths (Grunewald et al. 2018). Urbanisation generally has negative impacts on ecosys- More often, plant communities with high resource avail- tems, mainly because of habitat loss (Boon et al. 2016; ability allow for homogenous growth of early successional Lehosmaa et al. 2017; Nor et al. 2017; Galic et al. 2018). alien plants or a high diversity of invasive alien plant spe- Urban expansion is primarily driven by population cies (IAPs) (Muster et al. 2014). In this study, the acronym growth (Ersoy 2016) and the consequent increase in de- ‘IAPs’ refers to naturalised plants that regenerate in large mand for services (EPCPD 2016; Lehosmaa et al. 2017). numbers in distant locations from the location where par- The reduction of vegetation in urban areas and soil ma- ent plants were introduced (Richardson et al. 2000). These nipulation due to anthropogenic development/expansion communities normally have high densities of IAP propa- lead to the overall reduction of urban green space gules (Heinrichs and Pauchard 2015). Plant communities (Threlfall et al. 2016). Green spaces in urban areas sup- with a high density of propagules normally exert propa- port many plant species, both native and non-native. gule pressure to nearby forest reserves (Heinrichs and Some of the native species can be endemic or endan- Pauchard 2015;Padayacheeet al. 2017). Landscape con- gered and should be protected due to the vulnerability nectivity is used to describe the structural and functional to extinction in their communities (Lepczyk et al. 2017). continuity of ecosystems (Ziółkowska et al. 2014;Nor et The overall diversity of native species is determined by al. 2017) from landscape to regional scale (LaPoint et al. the size, quantity and quality of urban green spaces. 2015). Structural connectivity is measured by landscape Green space networks in cities are recognised around metrics such as patch size, isolation and the identification the globe to be fundamental for urban ecosystem func- of linear features that may act as conduits or barriers to tioning (Boon et al. 2016) and for the health and well- IAP movement (LaPoint et al. 2015). This means that con- being of residents through food provision and pollution nectivity is a vital element to maintain gene flow and to fa- removal (Threlfall et al. 2016; Nor et al. 2017). From a cilitate species movement, distribution, dispersal and spatial-ecology perspective, the reduction of green recolonisation (LaPoint et al. 2015; Naicker et al. 2016; spaces does not only lead to reduced sizes of forest Nor et al. 2017). Consequently, it may also influence the patches. The reduction in green space causes an increase success of establishment and spread of IAPs (Vidra 2004; in the ratio of the area of edge habitats to total patch Bierwagen 2005;Procheş et al. 2005). size (Vidra 2004; Bergsten et al. 2013). Forest centres In forest communities, the invasion levels by exotic having closed canopies act as physical barriers to disper- species depend on the integrity of the canopy cover sal pathways, and the prevailing light and moisture con- whereby forests with many and relatively large canopy ditions act as environmental barriers to the gaps are readily invaded. This is because invasive species establishment of alien plants. Forest edges have lower establish and grow rapidly in large canopy gaps and canopy cover and are closer to transportation corridors along forest margins (Geldenhuys 2004). Some forest and thus they may have weaker physical and environ- sites experiencing branch or tree falls that are small in mental barriers to invasion (Hansen and Clevenger nature have shown to be sufficient to allow low light de- 2005). Forest edges create a favourable micro-climate for manding tree invasive species to grow to the canopy establishment of invasive alien plants (IAPs) (Vidra (van Wyk et al. 1996). Canopy gaps facilitate the coexist- 2004; Bierwagen 2005; Bergsten et al. 2013). This implies ence of large numbers of species because they remove that forest patches that are highly fragmented can have barriers of dispersal and recruitment while advancing altered successional processes, whereby early succes- the growth of early-successional species which are sup- sional exotic species alter and increase the risk of decline pressed under the forest canopy (Lawes et al. 2004). For- in native species (Singer et al. 2016). The Durban Metro- est edges that are dense and forest canopies that are politan Open Space System (D’MOSS) occurs along a closed often create physical barriers for light demanding variety of habitat types and includes areas of high bio- IAPs and limit their dispersal pathways (Hansen and diversity value linked to form an ecologically viable net- Clevenger 2005). Late-successional species are lost work. The network of areas (ca. 75,000 ha; one third of through disturbance regimes that cause canopy gaps, the municipality) has a high biodiversity value and has leading to the establishment of light-demanding pioneer been under municipal management for over three de- species which are adapted to colonising new disturbed cades (Boon et al. 2016; EPCPD 2016). sites (Lawes et al. 2004). Furthermore, IAPs are likely to Physical features of urban areas such as buildings, large become dominant near forest paths because of the phys- impermeable concrete surfaces, pollution, high human ical disturbance of the soil and vegetation which in- population and traffic density are known for promoting creases light conditions and this favours the growth of the establishment of alien plant species. These physical IAPs (Hansen and Clevenger 2005). features may alter resource availability, such as increased The functional types of IAPs namely trees, shrubs and light where trees are cut and high soil moisture occurring climbers are considered to have a negative impact on Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 3 of 14 forest integrity, as illustrated by several species estab- mind, it is important to identify plant communities that lished in coastal KwaZulu-Natal (Geldenhuys 2013). The are vulnerable to invasion due to prevailing environmental tree IAP Litsea glutinosa (Lour.) C.B.Rob. establishes factors (Hero et al. 2014). This study aims to describe vigorously in humid areas of sub-tropical forests. This existing distribution patterns of the dominant IAPs across light-demanding plant takes advantage of small canopy two different spatial scales namely patch (forest reserve) gaps in the forest, and its vegetative reproduction by and plot (on site within a forest reserve). The specific ob- root-suckering makes it an aggressive invader that poses jectives of the study are (i) to identify controlling environ- a threat to biodiversity loss under canopy (Jacq et al. mental factors at fine and broad scale, defining the IAPs 2005). Similarly, Melia azedarach L. invades urban open distribution, and (ii) to determine community associations space, wasteland and riparian zones (Henderson 2001). between native species and IAPs as an indicator of native When the species undergoes physiological damage, it plant species resilience. can reproduce by vegetative means through stump and root sprouts for its persistence in the environment Methods (Tourn et al. 1999). The shrub Chromolaena odorata Physical location (L.) R.M. King is restricted to frost-free areas and dis- The eThekwini Municipal Area in which the Durban plays great plasticity in terms of growth form and in City falls under (Bengtsson et al. 2003) is situated in the terms of the vegetation types it invades (Zachariades et KwaZulu-Natal (KZN) province of South Africa (29° 52′ al. 2002). The shrub Lantana camara L. grows aggres- S, 31° 01′ E). It is situated in the Maputaland-Pondolan- sively and causes ecological destruction through loss of d-Albany regional centre, which is one of the 35 global species diversity and more so due to its ability to domin- biodiversity hotspots (Boon et al. 2016; EPCPD 2016). ate the understorey (Totland et al. 2005). Another ever- The climate is subtropical with wet and warm summers, green perennial branched shrub (Olckers 1999), which is and mild dry winters. Temperature in eThekwini Munici- aggressive, extremely resilient, invades riparian, forest, pality varies along the steep elevational gradient, from grass and agricultural land is Solanum mauritianum warm to cool as one moves from the coast to inland which Scop. (Witkowski and Garner 2008; Peerbhay et al. has a maximum elevation of 904 m (EPCPD 2016). The 2016). The climber Cardiospermum grandiflorum mean annual temperature range is 16–20 °C, while the Sw. grows vigorously in damp conditions preferably mean annual rainfall range is 550–1200 mm per annum along forest margins and watercourses of the subtropics (INR 2004). A total of 15 forest patches (mostly nature re- (Simelane et al. 2011), climbing to heights of 6 m up to serves, as well as one golf estate and one conservancy) 10 m (Henderson 2001; Krings and Braham 2005). Once which are included in the D’MOSS and are managed by the invader is well established, it forms a smothering different authorities, were selected for sampling (Table 1). curtain which outcompete indigenous species for sun- light (Simelane et al. 2011). Another climber, Ipomoea Species sampling purpurea (L.) Roth, grows vigorously twining above the After an initial exploratory stage, the most abundant vegetation around them for light (Defelice 2001). Fur- and widespread IAP species in each of the growth thermore, the vine is adapted to low light intensities forms considered were included as target species. These found in the forest understorey (Defelice 2001). The were two tree IAPs namely Litsea glutinosa and Melia plastic physiological and morphological traits of some azedarach,three shrubs namely Chromolaena odorata, IAPs allow for the efficient use of available resources for Lantana camara and Solanum mauritianum and two fast growth such as soil moisture, nutrients and light; climbers namely Cardiospermum grandiflorum and Ipo- hence, they can outperform less plastic native species moea purpurea. Seventy-four plots measuring 100 m (Leishman et al. 2007; Funk 2008; Kraft et al. 2015). (10 m × 10 m) were surveyed, and a plot was established Some native pioneer species exhibit traits like those of at every site where the target species were encountered exotic species such as light and water efficiency. These in the forest patch. In the plots, the community struc- traits make them to adapt, grow and persist in altered ture (strata/vertical layers) was determined and the per- forest conditions that result in either increased or de- centage canopy cover in each stratum was estimated. creased resources essential for growth due to natural All native and invasive species in the shrub and tree and anthropogenic environmental disturbances (Everard layers were recorded for each stratum. The strata were et al. 1995; Schulten et al. 2014). categorised according to height as follows: lower trees The assembly of natural plant communities is deter- (> 6 m), tall shrubs (> 2–6 m), low shrubs (0.5–2m) and mined by several biotic and abiotic factors and their all climbers (regarded as a separate stratum because interaction effects along environmental gradients they grow across all layers). The total cover from all (Olthoff et al. 2015), which occur at multiple spatial species within a plot was assessed according to Tans- scales (Dray et al. 2012). In this context, having IAPs in ley’s DAFOR scale (Emson et al. 2018): D = Dominant Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 4 of 14 Table 1 Fifteen forest patches of the sampling sites within D’MOSS and their details on geographic and management authority of each. EKZNW stands for Ezemvelo KwaZulu-Natal Wildlife, PO for privately owned and EM for eThekwini Municipality Forest patch Patch Forest Management Patch size Connectivity Landscape index Average elevation codes type agency (ha) index (793757) (m asl) Springside Spri Scarp EM 21.1 38,046 Very low 584.0 Krantzkloof Kra Scarp EKZNW 658.1 2,928,636 Very high 455.7 Iphithi Ipit Scarp PO 9.5 11,433 Very low 545.3 Bluff Golf Estate Bluf Coastal PO 68.5 7240 Very low 21.5 Buffelsdraai Buf Coastal EM 787 3,573,938 Very high 252.2 Burman Bush Bur Coastal EM 51.2 248,445 Low 47.3 New Germany New Coastal EM 104.5 608,476 Average 358.5 Palmiet Palm Coastal EM 63.5 206,754 Low 153.4 Paradise Valley Para Coastal EM 153.5 533,837 Average 228.0 Pigeon Valley Pig Coastal EM 13.9 79,324 Very low 84.3 Roosfontein Ros Coastal EM 215.6 1,682,652 Very high 158.0 Virginia Bush Vig Coastal EM 49.8 208,275 Low 83.8 Umhlanga Lagoon Lag Coastal EKZNW 30.9 137,533 Low 16.0 North Park Nor Coastal EKZNW 48.9 330,616 Low 158.3 Stainbank StaB Coastal EKZNW 224.2 524,635 Average 80.5 Low range 200000-300000 High range 600000-800000 Landscape index¹ (793757) is used to determine if forest patch has a low connectivity or high connectivity (5); A = Abundant (4), F = Frequent (3), O = Occasional calculate the total area (hectares) of land represented by (2) and R = Rare (1). The overall species cover values the polygons for each land use and to compute the num- were obtained by determining the average cover from ber of forest patches found in the matrix (NP). Class all plots and used as a dependent variable with the en- area represents how much of the landscape is comprised vironmental variables treated as independent. Species by a patch type, which determines the size of a patch were identified using the most recent regional tree within a landscape. In forest fragmentation, it is import- guide (Boon 2010). The Plants of Southern Africa web- ant to know the total size of a target forest patch that site from the South African National Biodiversity exists within the landscape. The landscape similarity Institute was accessed to confirm plants scientific index or buffer index is the percentage of the landscape names and floristic details (SANBI 2016). Furthermore, represented by one land-use type (McGarigal and Bar- the Angiosperm Phylogeny website was accessed to bara 1995; Rutledge 2003). For this study, there are four confirm the most recent family assignment of the landscape similarity indices, namely forest, grassland, species (Stevens 2017). residential and industrial areas. Environmental factors of plot scale and patch scale Connectivity calculations of forest patches The plot-level environmental factors of geographic scale For each forest patch sampled, a proximity buffer of 500 recorded included forest type (coastal or scarp), eleva- m was delineated for the identification of forest patches tion (m), latitude and longitude (measured with a GPS). to be included in the calculation. The formula of patch Environmental variables were recorded as a percentage connectivity incorporates the isolation of individual for- cover in each plot. These were exposed rock, gravel or est patches in relation to other forest patches found surface stones, erosion and canopy gaps (inverse of can- within the buffer. The patch connectivity index (PCI) opy cover). At the patch level, the spatial variables for was calculated as an area-based index weighted by dis- each nature reserve were obtained from digital cartog- tance and calculated as: raphy available in Google Earth Pro 7.2. The software Arc GIS 10.3.1 was used to convert layers to KML files j Connectivity Index ¼ and to create buffers of 500 m for each forest patch Dist shape file. These buffers were embedded on Google Earth images of 2017 to create land-use polygons found where A denotes the area (m ) of a focal patch and Dist j j within the buffer area. ArcGIS software was used to is the squared distance (m ) from the focal patch edge to Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 5 of 14 another edge of a nearby patch in the landscape (Mun- analysis (CCA) (Schulten et al. 2014). The CCA allows for guía-Rosas and Montiel 2014). In this calculation, the the measurement of the amount of variation in the species closest three patches to each focal patch were sampled distribution data that can be explained by a set of environ- to obtain their distances, which were then squared and mental variables (Borcard et al. 1992). Environmental vari- calculated to obtain the focal forest patch index. Calcula- ables used for the data analysis are shown in Table 2,and tions relevant here are the mean connectivity index for they are differentiated according to their spatial scale of each class (forest patch) and for all patches in the land- occurrence, either at fine (plot) or broad (patch) scale. scape. The patch connectivity index is used to calculate The plot scale analysis determines which niche factors the degree of isolation. The landscape index is calculated influence the establishment of invasive species while as the average of all patch connectivity indices found the patch scale analysis determines which landscape within the landscape (Table 1). When a forest patch has attributes facilitate or impede the establishment of a very low landscape index in comparison to that of the those species. This shows the scale of influence in the surrounding landscape, it means that the forest patch is establishment and distribution of species (Ersoy 2016; highly isolated and when the landscape index of the Krishnadas et al. 2016). patch is higher than that of the landscape it means it is Variation partitioning consists of a series of canonical less fragmented (McGarigal and Barbara 1995). ordinations that are constrained by a single set of ex- planatory variables with the complementary set used as Numerical data analyses co-variables. The variable sets that were used in this Floristic similarity analysis are the environmental conditions at a plot scale The results of hierarchical clustering can be visualised and the variables explaining the spatial structure across using a dendrogram that indicates which plots are simi- the patch scale (Borcard et al. 1992; Legendre and Le- lar on the basis of their species composition. Species that gendre 1998). The variation in species distribution is ex- occurred in ten or more plots were selected and used to plained by the spatial structuring (Borcard et al. 1992). classify plots into clusters. The dominant plant species These are meant to test at which spatial level the occurring in each group were then used to name the species patterns of response are retained as influenced plant communities that were in those clusters. The by the environmental factors operating at the two Paleontological Statistics Software Package (PAST v3.18) spatial scales (Dray et al. 2012). The spatial structur- was used to analyse the dataset of plots with their spe- ing of environmental factors addresses the problem of cies composition into clusters (Legendre and Legendre the relative contribution of different overlaid effects 1998; Hammer et al. 2001). The clustering of plant com- in space (Borcard et al. 1992). munities was performed using Ward’s algorithm as a linkage method and with Euclidean distance as a dis- tance measure. Ward’s method uses an information cri- Table 2 Environmental factors (and their abbreviations) used in terion to join clusters in such a way that within-group the current study along with a description of their geographic variance is minimised (Hammer et al. 2001). The 15% 2 scale. The fine scale refers to plot area (100 m ), and the broad dissimilarity value was used as a cut-off in defining clus- scale refers to patch size (ranges from 9.5 to 787 ha) ters as it best separated clusters of manageable and com- Environmental factor Abbreviation Spatial scale parable size in terms of number of species. Tree IAPs T-IAPs Fine Shrub IAPs S-IAPs Fine Ordination and variation partitioning of vegetation plot Climbers IAPs C-IAPs Fine data Canopy cover Canopy C Fine Ordination is the arrangement of data units along one or more axes that express the range of variation existing Canopy gap Canopy G Fine within the dataset. The final result of an ordination is a Elevation Altitude Fine graph that shows the variation of data units along two or Gravel Gravel Fine more axes in a scatter-diagram. Constrained ordination Surface rocks Rock Fine characterises main trends of variation of data sets with re- Patch size PS Broad spect to environmental factors and limits the illustration Patch connectivity index PCI Broad of unexplained variation (Legendre and Legendre 1998). The CANOCO 5.1 program package was used to run the Forest land similarity index For-LSI Broad ordination analysis (Ter Braak and Šmilauer 2012). Con- Grass land similarity index Gra-LSI Broad strained ordinations were carried out on 32 species that Residence land similarity index Res-LSI Broad occurred in ten or more plots of the whole data set, using Industry land similarity index Ind-LSI Broad the ordination method of canonical correspondence Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 6 of 14 Results correlated land-similarity indices of residential area Species composition and diversity and industrial area, canopy cover and elevation. Na- A total of 357 species from 75 families were identified in tive species communities with an exclusive association the forest plots (Fig. 1). Of these, there were 37 species with community E were A (Albizia adianthifolia), B that were considered abundant based on the number of (Protorhus longifolia)and C(Dalbergia obovate) plots in which they occurred and these species were il- (Fig. 3). The M (Strelitzia nicolai) communities oc- lustrated in further analysis and discussion. The import- curred close to the H communities. ant species are listed in descending order of their overall The results of constrained ordination CCA for all en- average cover in Table 3. vironmental variables are shown in Fig. 4. This analysis indicated tree IAPs, patch size and patch connectivity Floristic clustering index were all key drivers of species distribution pat- A dendrogram with the similarity of plots from the dif- terns. The tree IAPs Melia azedarach and Litsea gluti- ferent sites is shown in Fig. 2. It was obtained by fixing nosa and the climber IAP Cardiospermum grandiflorum and computing a Euclidean distance at 15%, which re- were correlated to canopy cover, while the other IAPs sults in the formation of 13 vegetation clusters. Commu- were correlated to canopy gaps (Fig. 4). The canonical nities are named after the most dominant species axes test of significance is P < 0.01. occurring in those clusters (Table 4). A few of these The variation partitioning was significant (P < 0.01) for communities are dominated by invasive alien plant spe- the two spatial levels tested for the plot (group I) and cies (IAPs), which makes them communities of interest. patch (group II) which has an intersection (III). More These are E (Chromolaena odorata), H (Cardiospermum variables were explained at plot level (42.7%) followed by grandiflorum) and K (Litsea glutinosa) (Table 4). patch level (33.8%) and least by intersection (23.4%). The results of variation partitioning at plot level that Constrained ordination and variation partitioning are significant at P < 0.01 are shown in Fig. 5. The envir- The results of constrained ordination CCA of com- onmental factor of shrub IAPs was positively correlated munity E are showninFig. 3.The environmental to canopy gaps while the environmental factors eleva- variables with the strongest influence in the establish- tion, climber IAPs and tree IAPs were positively corre- ment of patterns in this community were patch size lated to canopy cover. (PS) and patch connectivity index (PCI). The H and The results of variation partitioning at the patch scale K communities were influenced by the positively are shown in Fig. 6. This figure illustrates that the Fig. 1 Map of the study area (eThekwini Municipality) in KwaZulu-Natal, South Africa, indicating the 15 study sites Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 7 of 14 Table 3 The important native and invasive species retained for analysis determined by average rank of cover of DAFOR scale for all plots Species Species Family Functional Plot Overall average abbreviation type occurrence cover a) Native Dalbergia obovata E.Mey. DalbObov Fabaceae Scrambler 42 1.57 Albizia adianthifolia (Schumach.) W.F. Wight AlbiAdia Fabaceae Tree 39 1.05 Monanthotaxis caffra Baill. MonaCaff Annonaceae Scrambler 32 1.00 Psychotria capensis (Eckl.) Vatke PsycCape Rubiaceae Tree 29 0.97 Protorhus longifolia (Bernh.ex C.Krauss) Engl. ProtLong Anacardiaceae Tree 31 0.91 Berkheya bergiana Ehrh. BerkBerg Asteraceae Shrub 21 0.82 Strelitzia nicolai Regel & Körn. StreNico Strelitziaceae Tree 24 0.81 Isoglossa woodii Oerst. IsogWood Acanthaceae Shrub 16 0.78 Dalbergia armata DalbArma Fabaceae Scrambler 26 0.77 Smilax anceps Willd. SmilAnce Smilacaceae Climber 23 0.65 Englerophytum natalense (Sond.) T.D.Penn. EnglNata Sapotaceae Tree 20 0.55 Trimeria grandifolia (Hochst.) Warb. subsp. grandifolia TrimGran Salicaceae Tree 15 0.53 Dioscorea villosa L. DiosVill Dioscoreaceae Climber 23 0.51 Croton sylvaticus Muell. Arg. CrotSylv Euphorbiaceae Tree 26 0.50 Rothmannia globosa (Hochst.) Keay RothGlob Rubiaceae Tree 17 0.49 Senecio tamoides DC. SeneTamo Asteraceae Climber 18 0.49 Canthium ciliatum (D.Dietr.) Kuntze CantCili Rubiaceae Tree 19 0.47 Kiggelaria africana L. KiggAfri Achariaceae Tree 21 0.46 Heteropyxis natalensis Harv. HeteNata Myrtaceae Tree 14 0.45 Zehneria scabra (L.f.) Sond. ZehnScab Curcubitaceae Climber 21 0.45 Combretum molle R.Br. ex G.Don CombMoll Combretaceae Tree 20 0.43 Canthium inerme (L.f.) Kuntze CantIner Rubiaceae Tree 19 0.38 Tabernaemontana ventricosa Hochst. ex A.DC. TabeVetr Apocynaceae Tree 14 0.36 Baphia racemose (Hochst.) Baker BaphRace Fabaceae Tree 11 0.30 Bridelia micrantha (Hochst.) Baill. BridMicr Phyllanthaceae Tree 17 0.30 Ekebergia capensis Sparrm. EkebCape Meliaceae Tree 13 0.19 Dregea floribunda E.Mey. DregFlor Apocynaceae Climber 10 0.19 Trema orientalis (L.) Blume TremOrie Cannabaceae Tree 10 0.16 b) Invasive Chromolaena odorata (L.) R.M.King ChrmOdor Asteraceae Shrub 38 1.09 Lantana camara L. LantCama Verbenaceae Shrub 35 0.93 Cardiospermum grandiflorum Sw. CardGran Sapindaceae Climber 13 0.65 Litsea glutinosa (Lour.) C.B. Rob. LitsSebi Lauraceae Tree 15 0.57 Melia azedarach L. MeliAzed Meliaceae Tree 16 0.42 Ipomoea purpurea (L) Roth IpomPurp Convolvulaceae Climber 10 0.31 Parthenocissus quinquefolia (L.) Planch. PartQuin Vitaceae Climber 18 0.30 Passiflora suberosa L. PassSube Passifloraceae Climber 14 0.30 Solanum mauritianum Scop. SolaMaur Solanaceae Shrub 12 0.27 environmental variables of patch size and patch connect- variables are positively correlated to similarity indices of ivity index are strong drivers of species composition at forest and grass lands and all show a positive influence the patch scale. Furthermore, these environmental on the establishment of the tree IAP Melia azedarach. Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 8 of 14 Fig. 2 Dendrogram showing the results of cluster analysis based on Ward-Euclidean for the species occurring in the plots. Community clusters delimited at 15% dissimilarity are labelled with letters from A to M, and the red line cuts clusters dominated by similar species. Codes on x-axis refer to all plot numbers of patches where the purple are from scarp forest and black from lowland forests The environmental variables land similarity indices of lowland coastal forests, whereby the IAP is correlated to residential and industrial area show a positive influence elevation and canopy cover. The H community also on the establishment of the IAPs Solanum mauritianum, occur in both scarp and lowland coastal forests and the Cardiospermum grandiflorum and Litsea glutinosa. Both vine grows in association with both tree and shrub IAPs sets of environmental factors have weaker correlation communities. The influence of the factors of patch size, with the remaining five IAPs as shown in Fig. 6. patch connectivity index, similarity indices of forest and The Buffelsdraai Conservancy and Stainbank Nature grass lands are prominent in lowland coastal forests. Reserve have a high occurrence of community E plots as shown in Fig. 7. These are situated in lowland coastal Discussion forest, with large canopy gaps. Some of the properties of The environmental variables of the three different func- Buffelsdraai Conservancy are large patch size and high tional types (tree IAPs, shrub IAPs and climber IAPs) patch connectivity in relation to the landscape index. are not correlated, which means that they have different Stainbank Nature Reserve has an average patch size and environmental niches, as illustrated in Figs. 3 and 4.The average connectivity in relation to the landscape index tree IAPs are distributed around the environmental vari- (Table 1). The K community occurs in both scarp and able of canopy cover while the climber and shrub IAPs Table 4 Details of plant communities observed across the coastal lowland and scarp forests, and the number of their representative plots and forest patches each community occurred in Community label Dominant species Abbreviation Plots Patches A Albizia adianthifolia Aa Com 5 1 B Protorhus longifolia Pl Com 3 1 C Dalbergia obovata - Englerophytum natalense Do-En Com 4 3 D Dalbergia armata Da Com 8 4 E Chromolaena odorata Co Com 6 3 F Dalbergia obovata Do Com 7 2 G Isoglossa woodii Iw Com 7 4 H Cardiospermum grandiflorum Cg Com 5 5 I Englerophytum natalense En Com 3 1 J Psychotria capensis Pc Com 7 6 K Litsea glutinosa Ls Com 5 4 L Trimeria grandifolia Tg Com 4 3 M Strelitzia nicolai Sn Com 10 6 Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 9 of 14 Fig. 4 Constrained ordination CCA of species distribution across the environmental factors studied (see Table 2 for details). Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the environmental factors, while red labels are IAPs and black labels are native species Fig. 3 Constrained ordination CCA for vegetation communities across the fifteen forest patches along the Durban Metropolitan Open Space System. Blue arrows and corresponding labels represent the direction and degree of various environmental variables (See Table 2 for details) are distributed around the environmental variable of canopy gaps. The highly invaded communities E, H and K are important because each represents one of the troublesome functional types occurring in forests which are tree, shrub and climber IAPs. Plot-scale environmental effects The environmental factors of high elevation and medium canopy cover are positively correlated and are strong determinants for tree IAPs (Fig. 5). These factors have a negative influence on diversity of IAPs and native species. Small and scattered canopy gaps found in higher elevation scarp forests explain the limited establishment of pioneer native species there (Everard et al. 1995). The IAPs that are most likely to become problem plants in higher elevation scarp forests are shade-tolerant and bird-dispersed species (Geldenhuys 2004, 2013). These IAPs grow in closed forest ecosystems and are ranked top in the hierarchy of species competitiveness, making Fig. 5 The plot scale (fine-scale) spatial variables from the Durban them important species for control (Everard et al. 1995). Metropolitan Open Space System. Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the In canopy gaps, species diversity is high for both alien environmental factors, while red labels are IAPs and black labels and native species. This indicates that forest habitat are native species openness increases the establishment of alien species Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 10 of 14 Fig. 6 The patch scale (broader-scale) spatial variables from the Durban Metropolitan Open Space System. Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the environmental factors, while red labels are IAPs and black labels are native species Fig. 7 Envelop mapping of the three important invasive species communities found in association with native species communities (Charbonneau and Fahrig 2004), which are generally that occur in 15 forest patches found in the Durban Metropolitan shade intolerant (Geldenhuys 2013). In this study, shrub Open Space System. Blue arrows and corresponding labels represent and climber IAPs were found in canopy gaps (Fig. 5), the environmental factors. Community codes are detailed in Table 4 meaning that the success of invasion by these IAPs is at- tributed to canopy gaps, species attributes and ecosys- tem properties (Deckers et al. 2005). Coastal and scarp coastal forests. This finding implies that strong wind ex- forests in Durban and nearby areas are occasionally ex- posure is more severe in the lowland coastal forest than posed to strong winds that can blow down adult trees the scarp forest (Everard et al. 1995). growing on generally shallow soils (Everard et al. 1995; Botzat et al. 2015; Vecchio et al. 2015). The lowland Patch-scale environmental effects coastal forests are characterised by communities of early The size and types of matrices surrounding forest successional species and are thus dominated by patches have an influence on the establishment of inva- shade-intolerant species (Everard et al. 1995). The phys- sive species (Hansen and Clevenger 2005; Bartuszevige ical disruption of the land surface facilitates invasion due et al. 2006; Botzat 2012; Nitoslawski and Duinker 2016). to reduced cover and vigour of native species (Davis et The niches that have high numbers of IAPs correspond al. 2000), and the exposure to light and greater with the areas that have high values for the correlated temperature fluctuations which increases nitrogen min- similarity indices of residence and industry (Fig. 6). eralisation (D'Antonio and Meyerson 2002). Forest gaps Spatial factors such as the close proximity of forest that are created in this manner assist invaders to suc- patches to propagule sources could be the reason why cessfully fill unoccupied niches because of the reduction urban forests that have high residential and industrial of native species due to degradation (Mack and D'Anto- matrix values are subjected to high levels of invasion by nio 1998; Goodall 2000; Wilson et al. 2007; Gaertner et alien species (Vidra 2004). Residential areas also influ- al. 2009). The light in canopy gaps limits late succes- ence invasion of forest patches due to high population, sional species (mostly natives), and the resultant increase pollution (Grunewald et al. 2018) and by the growing of in mineral resource levels will favour fast-growing early exotic plants for aesthetic values by property occupiers successional species and this may include invader spe- (Nitoslawski and Duinker 2016). This explains why cies (D'Antonio and Meyerson 2002). Canopy gaps are human-derived land uses increase propagule pressure more prevalent in the forest patches found in lowland from transformed land matrices into natural forest Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 11 of 14 habitats (González-Moreno et al. 2013). In the current potential propagule sources of IAPs regeneration in study, those forest patches characterised by high patch nearby forest reserves (Botzat 2012). size and patch connectivity index (and, simultaneously, The E community is more pronounced in patches medium land similarity indices of forest and grassland) of lowland coastal forest, and the K community is have poor diversity of invasive species (Fig. 6). More iso- more pronounced in patches of high-elevation scarp lated patches with a high level of land-use change affect forest. Additionally, the H community is found in ecosystem integrity, which provides an opportunity for patches of both types of coastal forests (Fig. 7). A IAPs to establish (Borgmann and Rodewald 2005). study on IAPs ranked the effects of species functional Therefore, the proportion of pioneer invasive species on types and found that the shrub and climber IAPs site are an indicator that some form of disturbance oc- have high ecological impact on species diversity and curs in that forest community, but resilience to invasion distribution extent when compared with the tree is shown by a greater patch size (Hernández-Ruedas et IAPs. Furthermore, the two functional types are diffi- al. 2014). There is a weak influence of patch size, patch cult to control (Macdonald and Jarman, 1985). The connectivity index, and similarity indices of forest and patterns of forest community assembly relate directly grass lands on shrub IAPs distribution at the patch scale to the forest dynamics and to the species adapting to in variation partitioning (Fig. 6). This indicates that the specific environmental factors or disturbance regimes interaction of canopy gap with these variables is crucial (Geldenhuys 2013). Factors that determine the levels for species displacement. The level of connectivity and of invasion are propagule pressure and suitable traits the spatial arrangement of forest corridors do affect the for establishment (Matthews et al. 2009). Further- rate of invasion (Bartuszevige et al. 2006). In the current more, species establishment is determined by suitable study, the poor invasion of forest patches characterised traits for establishment traits and the site condition. by these factors indicates that the more connected forest A few lowland coastal forest patches had occurrence patches are, the more resilient they are to the risks of in- of all three IAP functional type clusters. Invasive spe- vasion due to a lower ratio of edge area to patch size cies that co-exist are likely to vary in their impact which limits the establishment of IAPs. The more edge and their niche preference (Kuebbing et al. 2014). habitat a forest patch has, the more vulnerable it is to in- Multiple invasions lead to high risk of transformation vasion by alien invasive plants (Hansen and Clevenger to novel communities. The K community is associated 2005). Solar radiation changes along forest edges and with canopy cover in both forest types in an eleva- often influences the state of soil moisture and tional range of 80–580 m asl. The species of the K temperature, which creates a micro-climate which pro- vegetation community are shade-tolerant which makes motes the germination of IAP seeds in the seedbank them a great threat to the integrity of natural forest (Bonanomi et al. 2018). (Geldenhuys 2004). This is because shade-tolerant IAPs exhibit advanced regeneration and establishment properties under closed canopies, by taking advantage The distribution of communities of the small canopy gaps to grow and reach the can- The E (Chromolaena odorata) community is influenced opy (West et al. 2000). These species usually form by many environmental factors that are positively cor- dense populations of saplings in the forest understory related with canopy gaps (Fig. 7). Canopy gaps are (Geldenhuys 2013). found mostly in lowland coastal forest patches (Table 1), The ordination association of the IAP community E and the three IAP functional types do occur in them with A, I, D and F communities is an indication that (Fig. 7). The high proximity to non-managed forest these species are resilient to the prevailing forest dynam- patches found outside boundaries of nature reserves act ics of their niche. The ordination association of the IAP as corridors that could be occupied by plants with community K with J, L and M communities is an indica- wind-dispersed invasive seeds. The configuration of a tor that these native species can deal with the same landscape and the proximity to propagule sources in- environmental factors that are promoting the establish- creases dispersal towards nature reserves which in- ment of IAPs. Species that co-occur in these environ- creases invasion risk (Borgmann and Rodewald 2005; mental conditions are those that have developed Botzat 2012). Management of IAPs outside the bound- strategies for coping and survival (Gallien et al. 2014). aries of forest reserves might be poor; therefore, adja- Therefore, these species can be used to pre-empt invasion cent forest patches that are invaded may be propagule in forest restoration projects. The climber IAPs occur in sources of wind-dispersed IAPs as suggested by Mac- canopy gaps and grow to the level of the tree canopy. donald and Jarman (1985). So, such forest patches oc- Climber IAPs grow well in damp environments with a curring outside of nature reserves, even when they have preference for forest margins and canopy gaps, where they low transformation and resembling natural habitats, are get adequate light levels for growth. Furthermore, the Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 12 of 14 tendrils of the vines and lianas help them to twirl around Authors’ contributions LZM designed the study, collected, organised the data for analysis and wrote other plants allowing them to climb over 10 m in height, the manuscript. EJJS is the main supervisor on the study design, data where they then form large canopy cover to outcompete collection, data analysis and manuscript write-up. ŞP co-supervised on other plants for light (Simelane et al. 2011). Competitive the study design, data collection, data analysis and manuscript write-up. All authors read and approved the final manuscript. species replace non-competitive ones in niches that have resources manipulated by disturbance. Native tree species Authors’ information of the coastal forest of KwaZulu-Natal that have shown re- LZM, currently PhD Candidate at the University of KwaZulu-Natal, School of Agricultural, Earth and Environmental Sciences. silience in canopy gaps include Croton syivaticus (Everard EJJS, Senior Lecturer at the University of KwaZulu-Natal, School of Agricultural, et al. 1995), Canthium inerme, Trema orientalis, Albizia Earth and Environmental Sciences. adianthifolia, Bridelia micrantha and Ekebergia capensis ŞP, Professor at the University of KwaZulu-Natal, School of Agricultural, Earth and Environmental Sciences. (Grainger 2012). Forest site restoration is most likely to be successful when these native species are planted to occupy Ethics approval and consent to participate any empty niches. This suggests that those native species Not applicable that remain in invaded environments are likely to persist Consent for publication (Schulten et al. 2014). Not applicable Conclusion Competing interests The authors declare that they have no competing interests. The higher elevations in scarp coastal forests and small canopy gaps favour the occurrence of tree IAPs while Publisher’sNote limiting other alien invasions. In scarp forests, early con- Springer Nature remains neutral with regard to jurisdictional claims in published trol of tree IAPs under canopy can avoid the loss of bio- maps and institutional affiliations. diversity and economic costs before the species can Received: 8 August 2018 Accepted: 30 November 2018 establish and reach canopy level. 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Seed production, seed bank dynamics, resprouting and long-term response to clearing of the alien invasive Solanum mauritianum in a temperate to subtropical riparian ecosystem. South African Journal of Botany, 74(3), 476–484. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png New Zealand Journal of Forestry Science Springer Journals

Invasive alien plant species, fragmentation and scale effects on urban forest community composition in Durban, South Africa

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Springer Journals
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Copyright © 2018 by The Author(s).
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Life Sciences; Forestry
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1179-5395
DOI
10.1186/s40490-018-0124-8
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Abstract

Background: Urban forests are under increased pressure from invasion by exotic (alien) species. The vegetation present in the matrix of urban sites is a rich source of alien invasive propagules, which increases the risk of alien invasion in forests within an urban space, leading to a decline in indigenous species. Therefore, determining the distribution patterns of native and exotic species as influenced by environmental factors can assist in quantifying the impact of exotic species at broad scales based on responses on a finer scale. Quantifying the effects of multiple environmental factors on the distribution patterns of both indigenous and alien species in the ecosystem may help in prescribing suitable management efforts. Methods: Fifteen forest patches were sampled in the eThekwini (Durban) Municipality and data collected from 74 100-m plots with different degrees of invasion. Indigenous and alien species of trees, shrubs and climbers occurring in ten and more plots were considered for analysis and the CANOCO 5.1 package was used to run various constrained ordination analyses. Variation partitioning analysis was used to assess the impact of environmental variables at different spatial scales, namely the plot and patch scales. Results: Canopy gaps are the major controlling factor for invasive alien plants (IAPs) occurrence at fine scale. At patch level, residential and industry areas outside the boundaries (buffer area) of forest patches have a high influence on the distribution of IAPs. Communities dominated by the invasive Chromolaena odorata (L.) R.M. King are most common on the lowland coastal forests while communities dominated by either Litsea glutinosa (Lour.) C.B.Rob. or Cardiospermum grandiflorum Sw. can prevail in both lowland coastal and scarp forests. Conclusions: Canopy gaps in lowland forests can facilitate the transition of native forests to novel communities containing a variety of alien plant species. Communities of shrub, climber and tree IAPs occur in lowland coastal forests while climber and tree IAPs dominate the high-elevation scarp forests. The resilience shown by some native species to the tree, shrub and climber IAPs by remaining when IAPs establish makes these species very suitable for restoration projects. Forest patches surrounded by a high incidence of residential and industrial areas in the buffer matrix are likely to have a high diversity of IAPs. Larger patch size and high connectivity to nearby native forests are key in reducing invasion by IAPs. Keywords: Species community, Patch connectivity index, Invasive species * Correspondence: vimbi28@gmail.com School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, University Road Westville, Private Bag X 54001, Durban 4000, South Africa © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 2 of 14 Background along drainage channels or paths (Grunewald et al. 2018). Urbanisation generally has negative impacts on ecosys- More often, plant communities with high resource avail- tems, mainly because of habitat loss (Boon et al. 2016; ability allow for homogenous growth of early successional Lehosmaa et al. 2017; Nor et al. 2017; Galic et al. 2018). alien plants or a high diversity of invasive alien plant spe- Urban expansion is primarily driven by population cies (IAPs) (Muster et al. 2014). In this study, the acronym growth (Ersoy 2016) and the consequent increase in de- ‘IAPs’ refers to naturalised plants that regenerate in large mand for services (EPCPD 2016; Lehosmaa et al. 2017). numbers in distant locations from the location where par- The reduction of vegetation in urban areas and soil ma- ent plants were introduced (Richardson et al. 2000). These nipulation due to anthropogenic development/expansion communities normally have high densities of IAP propa- lead to the overall reduction of urban green space gules (Heinrichs and Pauchard 2015). Plant communities (Threlfall et al. 2016). Green spaces in urban areas sup- with a high density of propagules normally exert propa- port many plant species, both native and non-native. gule pressure to nearby forest reserves (Heinrichs and Some of the native species can be endemic or endan- Pauchard 2015;Padayacheeet al. 2017). Landscape con- gered and should be protected due to the vulnerability nectivity is used to describe the structural and functional to extinction in their communities (Lepczyk et al. 2017). continuity of ecosystems (Ziółkowska et al. 2014;Nor et The overall diversity of native species is determined by al. 2017) from landscape to regional scale (LaPoint et al. the size, quantity and quality of urban green spaces. 2015). Structural connectivity is measured by landscape Green space networks in cities are recognised around metrics such as patch size, isolation and the identification the globe to be fundamental for urban ecosystem func- of linear features that may act as conduits or barriers to tioning (Boon et al. 2016) and for the health and well- IAP movement (LaPoint et al. 2015). This means that con- being of residents through food provision and pollution nectivity is a vital element to maintain gene flow and to fa- removal (Threlfall et al. 2016; Nor et al. 2017). From a cilitate species movement, distribution, dispersal and spatial-ecology perspective, the reduction of green recolonisation (LaPoint et al. 2015; Naicker et al. 2016; spaces does not only lead to reduced sizes of forest Nor et al. 2017). Consequently, it may also influence the patches. The reduction in green space causes an increase success of establishment and spread of IAPs (Vidra 2004; in the ratio of the area of edge habitats to total patch Bierwagen 2005;Procheş et al. 2005). size (Vidra 2004; Bergsten et al. 2013). Forest centres In forest communities, the invasion levels by exotic having closed canopies act as physical barriers to disper- species depend on the integrity of the canopy cover sal pathways, and the prevailing light and moisture con- whereby forests with many and relatively large canopy ditions act as environmental barriers to the gaps are readily invaded. This is because invasive species establishment of alien plants. Forest edges have lower establish and grow rapidly in large canopy gaps and canopy cover and are closer to transportation corridors along forest margins (Geldenhuys 2004). Some forest and thus they may have weaker physical and environ- sites experiencing branch or tree falls that are small in mental barriers to invasion (Hansen and Clevenger nature have shown to be sufficient to allow low light de- 2005). Forest edges create a favourable micro-climate for manding tree invasive species to grow to the canopy establishment of invasive alien plants (IAPs) (Vidra (van Wyk et al. 1996). Canopy gaps facilitate the coexist- 2004; Bierwagen 2005; Bergsten et al. 2013). This implies ence of large numbers of species because they remove that forest patches that are highly fragmented can have barriers of dispersal and recruitment while advancing altered successional processes, whereby early succes- the growth of early-successional species which are sup- sional exotic species alter and increase the risk of decline pressed under the forest canopy (Lawes et al. 2004). For- in native species (Singer et al. 2016). The Durban Metro- est edges that are dense and forest canopies that are politan Open Space System (D’MOSS) occurs along a closed often create physical barriers for light demanding variety of habitat types and includes areas of high bio- IAPs and limit their dispersal pathways (Hansen and diversity value linked to form an ecologically viable net- Clevenger 2005). Late-successional species are lost work. The network of areas (ca. 75,000 ha; one third of through disturbance regimes that cause canopy gaps, the municipality) has a high biodiversity value and has leading to the establishment of light-demanding pioneer been under municipal management for over three de- species which are adapted to colonising new disturbed cades (Boon et al. 2016; EPCPD 2016). sites (Lawes et al. 2004). Furthermore, IAPs are likely to Physical features of urban areas such as buildings, large become dominant near forest paths because of the phys- impermeable concrete surfaces, pollution, high human ical disturbance of the soil and vegetation which in- population and traffic density are known for promoting creases light conditions and this favours the growth of the establishment of alien plant species. These physical IAPs (Hansen and Clevenger 2005). features may alter resource availability, such as increased The functional types of IAPs namely trees, shrubs and light where trees are cut and high soil moisture occurring climbers are considered to have a negative impact on Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 3 of 14 forest integrity, as illustrated by several species estab- mind, it is important to identify plant communities that lished in coastal KwaZulu-Natal (Geldenhuys 2013). The are vulnerable to invasion due to prevailing environmental tree IAP Litsea glutinosa (Lour.) C.B.Rob. establishes factors (Hero et al. 2014). This study aims to describe vigorously in humid areas of sub-tropical forests. This existing distribution patterns of the dominant IAPs across light-demanding plant takes advantage of small canopy two different spatial scales namely patch (forest reserve) gaps in the forest, and its vegetative reproduction by and plot (on site within a forest reserve). The specific ob- root-suckering makes it an aggressive invader that poses jectives of the study are (i) to identify controlling environ- a threat to biodiversity loss under canopy (Jacq et al. mental factors at fine and broad scale, defining the IAPs 2005). Similarly, Melia azedarach L. invades urban open distribution, and (ii) to determine community associations space, wasteland and riparian zones (Henderson 2001). between native species and IAPs as an indicator of native When the species undergoes physiological damage, it plant species resilience. can reproduce by vegetative means through stump and root sprouts for its persistence in the environment Methods (Tourn et al. 1999). The shrub Chromolaena odorata Physical location (L.) R.M. King is restricted to frost-free areas and dis- The eThekwini Municipal Area in which the Durban plays great plasticity in terms of growth form and in City falls under (Bengtsson et al. 2003) is situated in the terms of the vegetation types it invades (Zachariades et KwaZulu-Natal (KZN) province of South Africa (29° 52′ al. 2002). The shrub Lantana camara L. grows aggres- S, 31° 01′ E). It is situated in the Maputaland-Pondolan- sively and causes ecological destruction through loss of d-Albany regional centre, which is one of the 35 global species diversity and more so due to its ability to domin- biodiversity hotspots (Boon et al. 2016; EPCPD 2016). ate the understorey (Totland et al. 2005). Another ever- The climate is subtropical with wet and warm summers, green perennial branched shrub (Olckers 1999), which is and mild dry winters. Temperature in eThekwini Munici- aggressive, extremely resilient, invades riparian, forest, pality varies along the steep elevational gradient, from grass and agricultural land is Solanum mauritianum warm to cool as one moves from the coast to inland which Scop. (Witkowski and Garner 2008; Peerbhay et al. has a maximum elevation of 904 m (EPCPD 2016). The 2016). The climber Cardiospermum grandiflorum mean annual temperature range is 16–20 °C, while the Sw. grows vigorously in damp conditions preferably mean annual rainfall range is 550–1200 mm per annum along forest margins and watercourses of the subtropics (INR 2004). A total of 15 forest patches (mostly nature re- (Simelane et al. 2011), climbing to heights of 6 m up to serves, as well as one golf estate and one conservancy) 10 m (Henderson 2001; Krings and Braham 2005). Once which are included in the D’MOSS and are managed by the invader is well established, it forms a smothering different authorities, were selected for sampling (Table 1). curtain which outcompete indigenous species for sun- light (Simelane et al. 2011). Another climber, Ipomoea Species sampling purpurea (L.) Roth, grows vigorously twining above the After an initial exploratory stage, the most abundant vegetation around them for light (Defelice 2001). Fur- and widespread IAP species in each of the growth thermore, the vine is adapted to low light intensities forms considered were included as target species. These found in the forest understorey (Defelice 2001). The were two tree IAPs namely Litsea glutinosa and Melia plastic physiological and morphological traits of some azedarach,three shrubs namely Chromolaena odorata, IAPs allow for the efficient use of available resources for Lantana camara and Solanum mauritianum and two fast growth such as soil moisture, nutrients and light; climbers namely Cardiospermum grandiflorum and Ipo- hence, they can outperform less plastic native species moea purpurea. Seventy-four plots measuring 100 m (Leishman et al. 2007; Funk 2008; Kraft et al. 2015). (10 m × 10 m) were surveyed, and a plot was established Some native pioneer species exhibit traits like those of at every site where the target species were encountered exotic species such as light and water efficiency. These in the forest patch. In the plots, the community struc- traits make them to adapt, grow and persist in altered ture (strata/vertical layers) was determined and the per- forest conditions that result in either increased or de- centage canopy cover in each stratum was estimated. creased resources essential for growth due to natural All native and invasive species in the shrub and tree and anthropogenic environmental disturbances (Everard layers were recorded for each stratum. The strata were et al. 1995; Schulten et al. 2014). categorised according to height as follows: lower trees The assembly of natural plant communities is deter- (> 6 m), tall shrubs (> 2–6 m), low shrubs (0.5–2m) and mined by several biotic and abiotic factors and their all climbers (regarded as a separate stratum because interaction effects along environmental gradients they grow across all layers). The total cover from all (Olthoff et al. 2015), which occur at multiple spatial species within a plot was assessed according to Tans- scales (Dray et al. 2012). In this context, having IAPs in ley’s DAFOR scale (Emson et al. 2018): D = Dominant Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 4 of 14 Table 1 Fifteen forest patches of the sampling sites within D’MOSS and their details on geographic and management authority of each. EKZNW stands for Ezemvelo KwaZulu-Natal Wildlife, PO for privately owned and EM for eThekwini Municipality Forest patch Patch Forest Management Patch size Connectivity Landscape index Average elevation codes type agency (ha) index (793757) (m asl) Springside Spri Scarp EM 21.1 38,046 Very low 584.0 Krantzkloof Kra Scarp EKZNW 658.1 2,928,636 Very high 455.7 Iphithi Ipit Scarp PO 9.5 11,433 Very low 545.3 Bluff Golf Estate Bluf Coastal PO 68.5 7240 Very low 21.5 Buffelsdraai Buf Coastal EM 787 3,573,938 Very high 252.2 Burman Bush Bur Coastal EM 51.2 248,445 Low 47.3 New Germany New Coastal EM 104.5 608,476 Average 358.5 Palmiet Palm Coastal EM 63.5 206,754 Low 153.4 Paradise Valley Para Coastal EM 153.5 533,837 Average 228.0 Pigeon Valley Pig Coastal EM 13.9 79,324 Very low 84.3 Roosfontein Ros Coastal EM 215.6 1,682,652 Very high 158.0 Virginia Bush Vig Coastal EM 49.8 208,275 Low 83.8 Umhlanga Lagoon Lag Coastal EKZNW 30.9 137,533 Low 16.0 North Park Nor Coastal EKZNW 48.9 330,616 Low 158.3 Stainbank StaB Coastal EKZNW 224.2 524,635 Average 80.5 Low range 200000-300000 High range 600000-800000 Landscape index¹ (793757) is used to determine if forest patch has a low connectivity or high connectivity (5); A = Abundant (4), F = Frequent (3), O = Occasional calculate the total area (hectares) of land represented by (2) and R = Rare (1). The overall species cover values the polygons for each land use and to compute the num- were obtained by determining the average cover from ber of forest patches found in the matrix (NP). Class all plots and used as a dependent variable with the en- area represents how much of the landscape is comprised vironmental variables treated as independent. Species by a patch type, which determines the size of a patch were identified using the most recent regional tree within a landscape. In forest fragmentation, it is import- guide (Boon 2010). The Plants of Southern Africa web- ant to know the total size of a target forest patch that site from the South African National Biodiversity exists within the landscape. The landscape similarity Institute was accessed to confirm plants scientific index or buffer index is the percentage of the landscape names and floristic details (SANBI 2016). Furthermore, represented by one land-use type (McGarigal and Bar- the Angiosperm Phylogeny website was accessed to bara 1995; Rutledge 2003). For this study, there are four confirm the most recent family assignment of the landscape similarity indices, namely forest, grassland, species (Stevens 2017). residential and industrial areas. Environmental factors of plot scale and patch scale Connectivity calculations of forest patches The plot-level environmental factors of geographic scale For each forest patch sampled, a proximity buffer of 500 recorded included forest type (coastal or scarp), eleva- m was delineated for the identification of forest patches tion (m), latitude and longitude (measured with a GPS). to be included in the calculation. The formula of patch Environmental variables were recorded as a percentage connectivity incorporates the isolation of individual for- cover in each plot. These were exposed rock, gravel or est patches in relation to other forest patches found surface stones, erosion and canopy gaps (inverse of can- within the buffer. The patch connectivity index (PCI) opy cover). At the patch level, the spatial variables for was calculated as an area-based index weighted by dis- each nature reserve were obtained from digital cartog- tance and calculated as: raphy available in Google Earth Pro 7.2. The software Arc GIS 10.3.1 was used to convert layers to KML files j Connectivity Index ¼ and to create buffers of 500 m for each forest patch Dist shape file. These buffers were embedded on Google Earth images of 2017 to create land-use polygons found where A denotes the area (m ) of a focal patch and Dist j j within the buffer area. ArcGIS software was used to is the squared distance (m ) from the focal patch edge to Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 5 of 14 another edge of a nearby patch in the landscape (Mun- analysis (CCA) (Schulten et al. 2014). The CCA allows for guía-Rosas and Montiel 2014). In this calculation, the the measurement of the amount of variation in the species closest three patches to each focal patch were sampled distribution data that can be explained by a set of environ- to obtain their distances, which were then squared and mental variables (Borcard et al. 1992). Environmental vari- calculated to obtain the focal forest patch index. Calcula- ables used for the data analysis are shown in Table 2,and tions relevant here are the mean connectivity index for they are differentiated according to their spatial scale of each class (forest patch) and for all patches in the land- occurrence, either at fine (plot) or broad (patch) scale. scape. The patch connectivity index is used to calculate The plot scale analysis determines which niche factors the degree of isolation. The landscape index is calculated influence the establishment of invasive species while as the average of all patch connectivity indices found the patch scale analysis determines which landscape within the landscape (Table 1). When a forest patch has attributes facilitate or impede the establishment of a very low landscape index in comparison to that of the those species. This shows the scale of influence in the surrounding landscape, it means that the forest patch is establishment and distribution of species (Ersoy 2016; highly isolated and when the landscape index of the Krishnadas et al. 2016). patch is higher than that of the landscape it means it is Variation partitioning consists of a series of canonical less fragmented (McGarigal and Barbara 1995). ordinations that are constrained by a single set of ex- planatory variables with the complementary set used as Numerical data analyses co-variables. The variable sets that were used in this Floristic similarity analysis are the environmental conditions at a plot scale The results of hierarchical clustering can be visualised and the variables explaining the spatial structure across using a dendrogram that indicates which plots are simi- the patch scale (Borcard et al. 1992; Legendre and Le- lar on the basis of their species composition. Species that gendre 1998). The variation in species distribution is ex- occurred in ten or more plots were selected and used to plained by the spatial structuring (Borcard et al. 1992). classify plots into clusters. The dominant plant species These are meant to test at which spatial level the occurring in each group were then used to name the species patterns of response are retained as influenced plant communities that were in those clusters. The by the environmental factors operating at the two Paleontological Statistics Software Package (PAST v3.18) spatial scales (Dray et al. 2012). The spatial structur- was used to analyse the dataset of plots with their spe- ing of environmental factors addresses the problem of cies composition into clusters (Legendre and Legendre the relative contribution of different overlaid effects 1998; Hammer et al. 2001). The clustering of plant com- in space (Borcard et al. 1992). munities was performed using Ward’s algorithm as a linkage method and with Euclidean distance as a dis- tance measure. Ward’s method uses an information cri- Table 2 Environmental factors (and their abbreviations) used in terion to join clusters in such a way that within-group the current study along with a description of their geographic variance is minimised (Hammer et al. 2001). The 15% 2 scale. The fine scale refers to plot area (100 m ), and the broad dissimilarity value was used as a cut-off in defining clus- scale refers to patch size (ranges from 9.5 to 787 ha) ters as it best separated clusters of manageable and com- Environmental factor Abbreviation Spatial scale parable size in terms of number of species. Tree IAPs T-IAPs Fine Shrub IAPs S-IAPs Fine Ordination and variation partitioning of vegetation plot Climbers IAPs C-IAPs Fine data Canopy cover Canopy C Fine Ordination is the arrangement of data units along one or more axes that express the range of variation existing Canopy gap Canopy G Fine within the dataset. The final result of an ordination is a Elevation Altitude Fine graph that shows the variation of data units along two or Gravel Gravel Fine more axes in a scatter-diagram. Constrained ordination Surface rocks Rock Fine characterises main trends of variation of data sets with re- Patch size PS Broad spect to environmental factors and limits the illustration Patch connectivity index PCI Broad of unexplained variation (Legendre and Legendre 1998). The CANOCO 5.1 program package was used to run the Forest land similarity index For-LSI Broad ordination analysis (Ter Braak and Šmilauer 2012). Con- Grass land similarity index Gra-LSI Broad strained ordinations were carried out on 32 species that Residence land similarity index Res-LSI Broad occurred in ten or more plots of the whole data set, using Industry land similarity index Ind-LSI Broad the ordination method of canonical correspondence Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 6 of 14 Results correlated land-similarity indices of residential area Species composition and diversity and industrial area, canopy cover and elevation. Na- A total of 357 species from 75 families were identified in tive species communities with an exclusive association the forest plots (Fig. 1). Of these, there were 37 species with community E were A (Albizia adianthifolia), B that were considered abundant based on the number of (Protorhus longifolia)and C(Dalbergia obovate) plots in which they occurred and these species were il- (Fig. 3). The M (Strelitzia nicolai) communities oc- lustrated in further analysis and discussion. The import- curred close to the H communities. ant species are listed in descending order of their overall The results of constrained ordination CCA for all en- average cover in Table 3. vironmental variables are shown in Fig. 4. This analysis indicated tree IAPs, patch size and patch connectivity Floristic clustering index were all key drivers of species distribution pat- A dendrogram with the similarity of plots from the dif- terns. The tree IAPs Melia azedarach and Litsea gluti- ferent sites is shown in Fig. 2. It was obtained by fixing nosa and the climber IAP Cardiospermum grandiflorum and computing a Euclidean distance at 15%, which re- were correlated to canopy cover, while the other IAPs sults in the formation of 13 vegetation clusters. Commu- were correlated to canopy gaps (Fig. 4). The canonical nities are named after the most dominant species axes test of significance is P < 0.01. occurring in those clusters (Table 4). A few of these The variation partitioning was significant (P < 0.01) for communities are dominated by invasive alien plant spe- the two spatial levels tested for the plot (group I) and cies (IAPs), which makes them communities of interest. patch (group II) which has an intersection (III). More These are E (Chromolaena odorata), H (Cardiospermum variables were explained at plot level (42.7%) followed by grandiflorum) and K (Litsea glutinosa) (Table 4). patch level (33.8%) and least by intersection (23.4%). The results of variation partitioning at plot level that Constrained ordination and variation partitioning are significant at P < 0.01 are shown in Fig. 5. The envir- The results of constrained ordination CCA of com- onmental factor of shrub IAPs was positively correlated munity E are showninFig. 3.The environmental to canopy gaps while the environmental factors eleva- variables with the strongest influence in the establish- tion, climber IAPs and tree IAPs were positively corre- ment of patterns in this community were patch size lated to canopy cover. (PS) and patch connectivity index (PCI). The H and The results of variation partitioning at the patch scale K communities were influenced by the positively are shown in Fig. 6. This figure illustrates that the Fig. 1 Map of the study area (eThekwini Municipality) in KwaZulu-Natal, South Africa, indicating the 15 study sites Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 7 of 14 Table 3 The important native and invasive species retained for analysis determined by average rank of cover of DAFOR scale for all plots Species Species Family Functional Plot Overall average abbreviation type occurrence cover a) Native Dalbergia obovata E.Mey. DalbObov Fabaceae Scrambler 42 1.57 Albizia adianthifolia (Schumach.) W.F. Wight AlbiAdia Fabaceae Tree 39 1.05 Monanthotaxis caffra Baill. MonaCaff Annonaceae Scrambler 32 1.00 Psychotria capensis (Eckl.) Vatke PsycCape Rubiaceae Tree 29 0.97 Protorhus longifolia (Bernh.ex C.Krauss) Engl. ProtLong Anacardiaceae Tree 31 0.91 Berkheya bergiana Ehrh. BerkBerg Asteraceae Shrub 21 0.82 Strelitzia nicolai Regel & Körn. StreNico Strelitziaceae Tree 24 0.81 Isoglossa woodii Oerst. IsogWood Acanthaceae Shrub 16 0.78 Dalbergia armata DalbArma Fabaceae Scrambler 26 0.77 Smilax anceps Willd. SmilAnce Smilacaceae Climber 23 0.65 Englerophytum natalense (Sond.) T.D.Penn. EnglNata Sapotaceae Tree 20 0.55 Trimeria grandifolia (Hochst.) Warb. subsp. grandifolia TrimGran Salicaceae Tree 15 0.53 Dioscorea villosa L. DiosVill Dioscoreaceae Climber 23 0.51 Croton sylvaticus Muell. Arg. CrotSylv Euphorbiaceae Tree 26 0.50 Rothmannia globosa (Hochst.) Keay RothGlob Rubiaceae Tree 17 0.49 Senecio tamoides DC. SeneTamo Asteraceae Climber 18 0.49 Canthium ciliatum (D.Dietr.) Kuntze CantCili Rubiaceae Tree 19 0.47 Kiggelaria africana L. KiggAfri Achariaceae Tree 21 0.46 Heteropyxis natalensis Harv. HeteNata Myrtaceae Tree 14 0.45 Zehneria scabra (L.f.) Sond. ZehnScab Curcubitaceae Climber 21 0.45 Combretum molle R.Br. ex G.Don CombMoll Combretaceae Tree 20 0.43 Canthium inerme (L.f.) Kuntze CantIner Rubiaceae Tree 19 0.38 Tabernaemontana ventricosa Hochst. ex A.DC. TabeVetr Apocynaceae Tree 14 0.36 Baphia racemose (Hochst.) Baker BaphRace Fabaceae Tree 11 0.30 Bridelia micrantha (Hochst.) Baill. BridMicr Phyllanthaceae Tree 17 0.30 Ekebergia capensis Sparrm. EkebCape Meliaceae Tree 13 0.19 Dregea floribunda E.Mey. DregFlor Apocynaceae Climber 10 0.19 Trema orientalis (L.) Blume TremOrie Cannabaceae Tree 10 0.16 b) Invasive Chromolaena odorata (L.) R.M.King ChrmOdor Asteraceae Shrub 38 1.09 Lantana camara L. LantCama Verbenaceae Shrub 35 0.93 Cardiospermum grandiflorum Sw. CardGran Sapindaceae Climber 13 0.65 Litsea glutinosa (Lour.) C.B. Rob. LitsSebi Lauraceae Tree 15 0.57 Melia azedarach L. MeliAzed Meliaceae Tree 16 0.42 Ipomoea purpurea (L) Roth IpomPurp Convolvulaceae Climber 10 0.31 Parthenocissus quinquefolia (L.) Planch. PartQuin Vitaceae Climber 18 0.30 Passiflora suberosa L. PassSube Passifloraceae Climber 14 0.30 Solanum mauritianum Scop. SolaMaur Solanaceae Shrub 12 0.27 environmental variables of patch size and patch connect- variables are positively correlated to similarity indices of ivity index are strong drivers of species composition at forest and grass lands and all show a positive influence the patch scale. Furthermore, these environmental on the establishment of the tree IAP Melia azedarach. Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 8 of 14 Fig. 2 Dendrogram showing the results of cluster analysis based on Ward-Euclidean for the species occurring in the plots. Community clusters delimited at 15% dissimilarity are labelled with letters from A to M, and the red line cuts clusters dominated by similar species. Codes on x-axis refer to all plot numbers of patches where the purple are from scarp forest and black from lowland forests The environmental variables land similarity indices of lowland coastal forests, whereby the IAP is correlated to residential and industrial area show a positive influence elevation and canopy cover. The H community also on the establishment of the IAPs Solanum mauritianum, occur in both scarp and lowland coastal forests and the Cardiospermum grandiflorum and Litsea glutinosa. Both vine grows in association with both tree and shrub IAPs sets of environmental factors have weaker correlation communities. The influence of the factors of patch size, with the remaining five IAPs as shown in Fig. 6. patch connectivity index, similarity indices of forest and The Buffelsdraai Conservancy and Stainbank Nature grass lands are prominent in lowland coastal forests. Reserve have a high occurrence of community E plots as shown in Fig. 7. These are situated in lowland coastal Discussion forest, with large canopy gaps. Some of the properties of The environmental variables of the three different func- Buffelsdraai Conservancy are large patch size and high tional types (tree IAPs, shrub IAPs and climber IAPs) patch connectivity in relation to the landscape index. are not correlated, which means that they have different Stainbank Nature Reserve has an average patch size and environmental niches, as illustrated in Figs. 3 and 4.The average connectivity in relation to the landscape index tree IAPs are distributed around the environmental vari- (Table 1). The K community occurs in both scarp and able of canopy cover while the climber and shrub IAPs Table 4 Details of plant communities observed across the coastal lowland and scarp forests, and the number of their representative plots and forest patches each community occurred in Community label Dominant species Abbreviation Plots Patches A Albizia adianthifolia Aa Com 5 1 B Protorhus longifolia Pl Com 3 1 C Dalbergia obovata - Englerophytum natalense Do-En Com 4 3 D Dalbergia armata Da Com 8 4 E Chromolaena odorata Co Com 6 3 F Dalbergia obovata Do Com 7 2 G Isoglossa woodii Iw Com 7 4 H Cardiospermum grandiflorum Cg Com 5 5 I Englerophytum natalense En Com 3 1 J Psychotria capensis Pc Com 7 6 K Litsea glutinosa Ls Com 5 4 L Trimeria grandifolia Tg Com 4 3 M Strelitzia nicolai Sn Com 10 6 Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 9 of 14 Fig. 4 Constrained ordination CCA of species distribution across the environmental factors studied (see Table 2 for details). Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the environmental factors, while red labels are IAPs and black labels are native species Fig. 3 Constrained ordination CCA for vegetation communities across the fifteen forest patches along the Durban Metropolitan Open Space System. Blue arrows and corresponding labels represent the direction and degree of various environmental variables (See Table 2 for details) are distributed around the environmental variable of canopy gaps. The highly invaded communities E, H and K are important because each represents one of the troublesome functional types occurring in forests which are tree, shrub and climber IAPs. Plot-scale environmental effects The environmental factors of high elevation and medium canopy cover are positively correlated and are strong determinants for tree IAPs (Fig. 5). These factors have a negative influence on diversity of IAPs and native species. Small and scattered canopy gaps found in higher elevation scarp forests explain the limited establishment of pioneer native species there (Everard et al. 1995). The IAPs that are most likely to become problem plants in higher elevation scarp forests are shade-tolerant and bird-dispersed species (Geldenhuys 2004, 2013). These IAPs grow in closed forest ecosystems and are ranked top in the hierarchy of species competitiveness, making Fig. 5 The plot scale (fine-scale) spatial variables from the Durban them important species for control (Everard et al. 1995). Metropolitan Open Space System. Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the In canopy gaps, species diversity is high for both alien environmental factors, while red labels are IAPs and black labels and native species. This indicates that forest habitat are native species openness increases the establishment of alien species Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 10 of 14 Fig. 6 The patch scale (broader-scale) spatial variables from the Durban Metropolitan Open Space System. Species codes are detailed in Table 3. Blue arrows and corresponding labels represent the environmental factors, while red labels are IAPs and black labels are native species Fig. 7 Envelop mapping of the three important invasive species communities found in association with native species communities (Charbonneau and Fahrig 2004), which are generally that occur in 15 forest patches found in the Durban Metropolitan shade intolerant (Geldenhuys 2013). In this study, shrub Open Space System. Blue arrows and corresponding labels represent and climber IAPs were found in canopy gaps (Fig. 5), the environmental factors. Community codes are detailed in Table 4 meaning that the success of invasion by these IAPs is at- tributed to canopy gaps, species attributes and ecosys- tem properties (Deckers et al. 2005). Coastal and scarp coastal forests. This finding implies that strong wind ex- forests in Durban and nearby areas are occasionally ex- posure is more severe in the lowland coastal forest than posed to strong winds that can blow down adult trees the scarp forest (Everard et al. 1995). growing on generally shallow soils (Everard et al. 1995; Botzat et al. 2015; Vecchio et al. 2015). The lowland Patch-scale environmental effects coastal forests are characterised by communities of early The size and types of matrices surrounding forest successional species and are thus dominated by patches have an influence on the establishment of inva- shade-intolerant species (Everard et al. 1995). The phys- sive species (Hansen and Clevenger 2005; Bartuszevige ical disruption of the land surface facilitates invasion due et al. 2006; Botzat 2012; Nitoslawski and Duinker 2016). to reduced cover and vigour of native species (Davis et The niches that have high numbers of IAPs correspond al. 2000), and the exposure to light and greater with the areas that have high values for the correlated temperature fluctuations which increases nitrogen min- similarity indices of residence and industry (Fig. 6). eralisation (D'Antonio and Meyerson 2002). Forest gaps Spatial factors such as the close proximity of forest that are created in this manner assist invaders to suc- patches to propagule sources could be the reason why cessfully fill unoccupied niches because of the reduction urban forests that have high residential and industrial of native species due to degradation (Mack and D'Anto- matrix values are subjected to high levels of invasion by nio 1998; Goodall 2000; Wilson et al. 2007; Gaertner et alien species (Vidra 2004). Residential areas also influ- al. 2009). The light in canopy gaps limits late succes- ence invasion of forest patches due to high population, sional species (mostly natives), and the resultant increase pollution (Grunewald et al. 2018) and by the growing of in mineral resource levels will favour fast-growing early exotic plants for aesthetic values by property occupiers successional species and this may include invader spe- (Nitoslawski and Duinker 2016). This explains why cies (D'Antonio and Meyerson 2002). Canopy gaps are human-derived land uses increase propagule pressure more prevalent in the forest patches found in lowland from transformed land matrices into natural forest Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 11 of 14 habitats (González-Moreno et al. 2013). In the current potential propagule sources of IAPs regeneration in study, those forest patches characterised by high patch nearby forest reserves (Botzat 2012). size and patch connectivity index (and, simultaneously, The E community is more pronounced in patches medium land similarity indices of forest and grassland) of lowland coastal forest, and the K community is have poor diversity of invasive species (Fig. 6). More iso- more pronounced in patches of high-elevation scarp lated patches with a high level of land-use change affect forest. Additionally, the H community is found in ecosystem integrity, which provides an opportunity for patches of both types of coastal forests (Fig. 7). A IAPs to establish (Borgmann and Rodewald 2005). study on IAPs ranked the effects of species functional Therefore, the proportion of pioneer invasive species on types and found that the shrub and climber IAPs site are an indicator that some form of disturbance oc- have high ecological impact on species diversity and curs in that forest community, but resilience to invasion distribution extent when compared with the tree is shown by a greater patch size (Hernández-Ruedas et IAPs. Furthermore, the two functional types are diffi- al. 2014). There is a weak influence of patch size, patch cult to control (Macdonald and Jarman, 1985). The connectivity index, and similarity indices of forest and patterns of forest community assembly relate directly grass lands on shrub IAPs distribution at the patch scale to the forest dynamics and to the species adapting to in variation partitioning (Fig. 6). This indicates that the specific environmental factors or disturbance regimes interaction of canopy gap with these variables is crucial (Geldenhuys 2013). Factors that determine the levels for species displacement. The level of connectivity and of invasion are propagule pressure and suitable traits the spatial arrangement of forest corridors do affect the for establishment (Matthews et al. 2009). Further- rate of invasion (Bartuszevige et al. 2006). In the current more, species establishment is determined by suitable study, the poor invasion of forest patches characterised traits for establishment traits and the site condition. by these factors indicates that the more connected forest A few lowland coastal forest patches had occurrence patches are, the more resilient they are to the risks of in- of all three IAP functional type clusters. Invasive spe- vasion due to a lower ratio of edge area to patch size cies that co-exist are likely to vary in their impact which limits the establishment of IAPs. The more edge and their niche preference (Kuebbing et al. 2014). habitat a forest patch has, the more vulnerable it is to in- Multiple invasions lead to high risk of transformation vasion by alien invasive plants (Hansen and Clevenger to novel communities. The K community is associated 2005). Solar radiation changes along forest edges and with canopy cover in both forest types in an eleva- often influences the state of soil moisture and tional range of 80–580 m asl. The species of the K temperature, which creates a micro-climate which pro- vegetation community are shade-tolerant which makes motes the germination of IAP seeds in the seedbank them a great threat to the integrity of natural forest (Bonanomi et al. 2018). (Geldenhuys 2004). This is because shade-tolerant IAPs exhibit advanced regeneration and establishment properties under closed canopies, by taking advantage The distribution of communities of the small canopy gaps to grow and reach the can- The E (Chromolaena odorata) community is influenced opy (West et al. 2000). These species usually form by many environmental factors that are positively cor- dense populations of saplings in the forest understory related with canopy gaps (Fig. 7). Canopy gaps are (Geldenhuys 2013). found mostly in lowland coastal forest patches (Table 1), The ordination association of the IAP community E and the three IAP functional types do occur in them with A, I, D and F communities is an indication that (Fig. 7). The high proximity to non-managed forest these species are resilient to the prevailing forest dynam- patches found outside boundaries of nature reserves act ics of their niche. The ordination association of the IAP as corridors that could be occupied by plants with community K with J, L and M communities is an indica- wind-dispersed invasive seeds. The configuration of a tor that these native species can deal with the same landscape and the proximity to propagule sources in- environmental factors that are promoting the establish- creases dispersal towards nature reserves which in- ment of IAPs. Species that co-occur in these environ- creases invasion risk (Borgmann and Rodewald 2005; mental conditions are those that have developed Botzat 2012). Management of IAPs outside the bound- strategies for coping and survival (Gallien et al. 2014). aries of forest reserves might be poor; therefore, adja- Therefore, these species can be used to pre-empt invasion cent forest patches that are invaded may be propagule in forest restoration projects. The climber IAPs occur in sources of wind-dispersed IAPs as suggested by Mac- canopy gaps and grow to the level of the tree canopy. donald and Jarman (1985). So, such forest patches oc- Climber IAPs grow well in damp environments with a curring outside of nature reserves, even when they have preference for forest margins and canopy gaps, where they low transformation and resembling natural habitats, are get adequate light levels for growth. Furthermore, the Mavimbela et al. New Zealand Journal of Forestry Science (2018) 48:19 Page 12 of 14 tendrils of the vines and lianas help them to twirl around Authors’ contributions LZM designed the study, collected, organised the data for analysis and wrote other plants allowing them to climb over 10 m in height, the manuscript. EJJS is the main supervisor on the study design, data where they then form large canopy cover to outcompete collection, data analysis and manuscript write-up. ŞP co-supervised on other plants for light (Simelane et al. 2011). Competitive the study design, data collection, data analysis and manuscript write-up. All authors read and approved the final manuscript. species replace non-competitive ones in niches that have resources manipulated by disturbance. Native tree species Authors’ information of the coastal forest of KwaZulu-Natal that have shown re- LZM, currently PhD Candidate at the University of KwaZulu-Natal, School of Agricultural, Earth and Environmental Sciences. silience in canopy gaps include Croton syivaticus (Everard EJJS, Senior Lecturer at the University of KwaZulu-Natal, School of Agricultural, et al. 1995), Canthium inerme, Trema orientalis, Albizia Earth and Environmental Sciences. adianthifolia, Bridelia micrantha and Ekebergia capensis ŞP, Professor at the University of KwaZulu-Natal, School of Agricultural, Earth and Environmental Sciences. (Grainger 2012). Forest site restoration is most likely to be successful when these native species are planted to occupy Ethics approval and consent to participate any empty niches. This suggests that those native species Not applicable that remain in invaded environments are likely to persist Consent for publication (Schulten et al. 2014). Not applicable Conclusion Competing interests The authors declare that they have no competing interests. The higher elevations in scarp coastal forests and small canopy gaps favour the occurrence of tree IAPs while Publisher’sNote limiting other alien invasions. In scarp forests, early con- Springer Nature remains neutral with regard to jurisdictional claims in published trol of tree IAPs under canopy can avoid the loss of bio- maps and institutional affiliations. diversity and economic costs before the species can Received: 8 August 2018 Accepted: 30 November 2018 establish and reach canopy level. 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New Zealand Journal of Forestry Science – Springer Journals

Published: Dec 28, 2018

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