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Aquat Ecol (2020) 54:985–999 https://doi.org/10.1007/s10452-020-09788-4(0123456789().,-volV)(0123456789().,-volV) Responses of grasses to experimental submergence in summer: implications for the management of unseasonal flows in regulated rivers . . Lyndsey M. Vivian Joe Greet Christopher S. Jones Received: 11 May 2020 / Accepted: 21 July 2020 / Published online: 29 July 2020 The Author(s) 2020 Abstract River regulation has altered the seasonal and 80% light reduction), over 8 weeks in summer and timing of flows in many rivers worldwide, impacting early autumn. All submergence treatments, including the survival and growth of riparian plants. In south- the 2-week pulse, resulted in the death of all plants of eastern Australia, demand for irrigation water in three species (Bromus catharticus, Dactylis glomerata summer often results in high river flows during a and Rytidosperma caespitosum). Lolium perenne season that would naturally experience low flows. exhibited moderate survival rates in the shorter-dura- Although unseasonal high summer flows are thought to tion unshaded submergence treatments, while Poa significantly impact waterways, their effects on vege- labillardierei largely survived all treatments. Similar tation are poorly quantified. We investigated the responses across species were observed for plant height responses of five grass species commonly occurring and biomass, although height generally increased while in riparian zones to different durations of submergence biomass growth was reduced by shading. These results in summer. We experimentally tested the response of show that even 2-week periods of summer submergence three exotic and two native grasses to four submergence can reduce growth and cause the death of some riparian treatments (4 weeks, 8 weeks, 2-week pulses and no grasses. Although some species may survive longer submergence), and two levels of shading (no shading submergence durations, impacts on other aspects of fitness, and ongoing effects of repeated unseasonal submergence, remain uncertain. Our study highlights that the impacts of unseasonal flows require further Handling Editor: Te´lesphore Sime-Ngando. investigation and careful management. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10452-020-09788-4) con- Keywords Rivers Unseasonal flows Riparian tains supplementary material, which is available to authorized plants Grasses Environmental flows Submergence users. L. M. Vivian (&) C. S. Jones Arthur Rylah Institute for Environmental Research, Department of Environment, Land, Water and Planning, Introduction PO Box 137, Heidelberg, VIC 3084, Australia e-mail: Lyndsey.Vivian@delwp.vic.gov.au Natural flow regimes sustain the structure, composi- J. Greet tion and function of riparian plant communities (Bunn School of Ecosystem and Forest Sciences, Burnley and Arthington 2002). However, river regulation has Campus, The University of Melbourne, Burnley, VIC, Australia 123 986 Aquat Ecol (2020) 54:985–999 resulted in the alteration of these natural regimes eastern Australia, with many of the region’s larger across many of the world’s rivers, including flow rivers, such as Victoria’s Campaspe and Goulburn seasonality (Nilsson and Svedmark 2002; Poff and Rivers, increasingly used to deliver flows across Zimmerman 2010; Grill et al. 2019). For riparian valleys to more-distant irrigation districts (inter-valley plants, life history stages including growth, flowering, transfers; IVTs) (Cottingham et al. 2010). Given this seed dispersal, germination and seedling establish- growing demand for unseasonal flows, it is important ment are often timed with patterns of seasonal flow to understand their impacts on riparian plants as well (Poff et al. 1997; Stromberg et al. 2007; Greet et al. as terrestrial plants invading into riparian zones and 2011). Consequently, when flow seasonality is altered, whether such flows can be delivered in a manner to declines in populations of riparian plant species and reduce or manipulate their impacts. ultimately changes in riparian plant community com- In this study, we investigated the response of five position are likely (Greet et al. 2013a, b). grass species commonly occurring in riparian zones of In flow-regulated rivers where the seasonal timing south-eastern Australia to different durations and of high flows has been altered by river regulation, frequencies of submergence in summer. Previous riparian vegetation can become partially inundated or experimental studies have shown that some grass even fully submerged at a time of year that would have species can exhibit a degree of tolerance to submer- naturally (i.e. pre-regulation) experienced low flows gence durations ranging from several days to more than 2 weeks (Imaz et al. 2012; Striker and Ploschuk (Cottingham et al. 2010; Ye et al. 2018a). This occurs in many regulated rivers in the temperate region of 2018; Kitanovic 2019). However, less is known about south-eastern Australia, where the demand for irriga- the response of grasses to different submergence tion water during the driest and warmest time of year durations during unseasonal times of the year. We has led to an increase of high flows during summer and tested four combinations of submergence durations early autumn (i.e. December to April) when rivers are and frequencies that represent realistic potential used to deliver water to irrigators (Maheshwari et al. scenarios for delivering summer flows (Fig. 1): single 1995; Humphries and Lake 1996; Greet et al. 2013a; continuous submergence periods of either 4 or Fig. 1). 8 weeks; 2-week submergence pulses interspersed Submergence causes stress to plants primarily with 2-week dry periods; and no submergence (sim- because gas exchange is impeded by the slow diffu- ulating a typical pre-regulation summer low flow). sion rates of gases in water inhibiting respiration These three submergence treatments reflect both our (Jackson and Colmer 2005; Colmer and Voesenek preconceptions of tolerance thresholds and the approx- 2009). Plants submerged by unseasonal summer flows imate previously delivered scenarios in regulated are likely to face additional challenges. For example, rivers in Victoria. There has been increasing aware- summer flows may submerge plants during the peak ness that the delivery of these summer flows as a single growing season when their metabolic demands are stable long-duration event can negatively impact greatest (Crawford 2004; Greet et al. 2011; Fig. 1). vegetation that becomes submerged, as well as cause Water temperatures during summer and early autumn notching and erosion of riverbanks, leading to con- flows are also likely to be higher than during natural certed efforts to vary levels by delivering flows as peak flows in late winter and spring. Warmer water multiple pulsed events (Cottingham et al. 2018; Webb temperatures can exacerbate submergence stress for et al. 2019). However, the impact on vegetation of plants, particularly in the growing season, resulting in pulsed flows of shorter durations compared to single increased loss of biomass and faster mortality rates longer events is not well understood. We aimed to test compared to submergence in colder water (van Eck which of these scenarios of submergence duration et al. 2005, 2006; Ye et al. 2018a). Summer and would result in the lowest plant mortality and reduc- autumn flows may also be associated with increased tion in plant growth. Each submergence scenario was water turbidity and reduced light, which can also also tested under unshaded and shaded conditions. exacerbate the impact of submergence on plants (Das Shade was included as a factor to investigate the et al. 2009). effects of lower light availability that often occurs in In recent years, the demand for water during turbid waters (e.g. Colmer and Voesenek 2009). summer and autumn has increased across south- 123 Aquat Ecol (2020) 54:985–999 987 Fig. 1 a Conceptual diagram adapted from Greet et al. (2011) typical perennial tufted grass, including growth and reproduc- illustrating typical changes in flow regimes in many reaches of tion, seed release and germination, in relation to river flows. regulated rivers in south-eastern Australia. Diagram shows pre- Inset (b–e) represent the four submergence treatments imposed regulation peak flows in winter-spring and low flows in summer- on the study species in our study, including b no submergence; autumn (solid line) and post-regulation higher summer-autumn c 2-week pulses interspersed by 2-week dry periods; d 4-week flows due to irrigation water demands and lower winter-spring continuous submergence; and e 8-week continuous submer- flows due to capture and storage of water in dams and weirs gence (see Table 1 for detailed description of treatments) (dotted line). Also indicated are idealised life history stages of a We hypothesised that: (1) plant survival and growth of submerged plants will be reduced in shaded will be greatest in response to no submergence compared to unshaded conditions. compared to submerged conditions; (2) for submerged plants, their survival and growth will be greater in response to the 2-week submergence pulses compared to continual submergence; and (3) survival and growth 123 988 Aquat Ecol (2020) 54:985–999 Table 1 Description of experimental submergence and shading treatments imposed on the study species Treatment factors and levels Description of submergence treatments Submergence Shade 0-week Unshaded/ Water levels maintained 2–3 cm above base of pots shaded 2-week Unshaded/ Plants completely submerged for 2 weeks, followed by water levels maintained 2–3 cm above base shaded of pots for 2 weeks, repeated twice 4-week Unshaded/ Plants completely submerged for 4 weeks, followed by water levels maintained 2–3 cm above base shaded of pots for 4 weeks 8-week Unshaded/ Plants completely submerged for 8 weeks shaded Methods dominant species of lowland temperate grasslands and grassy woodlands, and subalpine grasslands (Birch Study species et al. 2015; Clarke 2015; VicFlora 2019). The five species were chosen as a representative set We experimentally tested our hypotheses on five grass of common grasses with tufted growth forms that species commonly found in riparian zones of south- includes both exotic and native species and which are eastern Australia, particularly northern Victoria. This also likely to show a gradation of tolerance to region encompasses the southernmost river systems of submergence duration. Three of the species, B. the Murray-Darling Basin. Major waterways include catharticus, D. glomerata and R. caespitosum, are the Goulburn, Campaspe and Loddon Rivers, which considered to be terrestrial dry species, indicating that all flow into the Murray River. Rainfall is variable they occur more widely in the landscape but can across the region, broadly ranging from 200 to invade or persist in riparian zones (Casanova 2011). 600 mm per annum (Bureau of Meteorology 2020). However, experimental studies have shown that these Grasses are a dominant structural component of many species can exhibit a degree of tolerance to complete riparian communities of the waterways in this region, submergence. For example, Striker and Ploschuk particularly on mid to upper banks (Roberts and (2018) found B. catharticus and D. glomerata sur- Ludwig 2006; Cottingham et al. 2010). vived five days of complete submergence, but growth Five grass species were selected for this study: declined during both the submergence and subsequent Bromus catharticus (prairie grass), Dactylis glomerata recovery periods. Similarly, Kitanovic (2019) found (cocksfoot), Lolium perenne (perennial rye grass), D. glomerata and R. caespitosum survived submer- Rytidosperma caespitosum (common wallaby grass) gence for 35 days during late winter and early spring and Poa labillardierei (common tussock grass). These with 100% survival, while B. catharticus had a 50% species are frequently recorded in riparian surveys survival rate, although plant growth declined with along major rivers in the region, particularly down- increasing submergence duration. Lolium perenne has stream of dams (Greet et al. 2012, 2013a; Jones and been previously classified as a terrestrial damp species Mole 2018). The first three species are undesirable in northern Victorian wetlands (Reid and Quinn 2004) exotic tufted pasture grasses widespread throughout and can grow and survive under waterlogged condi- south-eastern Australia, particularly in disturbed tions (McFarlane et al. 2003). In addition, Banach areas, while the latter two are desirable native species. et al. (2009) found it exhibited a 100% survival rate Bromus catharticus is an annual or short-lived peren- and increased leaf biomass after 3 weeks of submer- nial and the remaining four species are perennial. gence, with survival rates dropping by 20–30% after Although all species occur both within and outside of 6 weeks of submergence. Poa labillardierei has been riparian areas, P. labillardierei is particularly common previously classified as a terrestrial damp species along streams and alluvial flats, as well as being a (Gehrig and Nicol 2010), indicating it can germinate 123 Aquat Ecol (2020) 54:985–999 989 and establish on saturated or damp ground but cannot water. Full descriptions of each treatment are provided tolerate flooding in a vegetative state (Casanova in Table 1. Tanks in the shaded treatments were 2011). Recent experiments have shown that it can covered with a shade cloth that reduced light penetra- survive 35 days of submergence in experimental tion by 80%. conditions (Kitanovic 2019). For each species, 54 plants of a similar size were selected for the experiment, with six randomly chosen Plant material for pre-treatment biomass harvesting (see below) and the remaining 48 allocated to one of the eight Seeds for the experiment were either sourced from a experimental treatments. Each treatment therefore commercial seed supplier (R. caespitosum, L. perenne comprised six replicate plants per species, split across and D. glomerata) or collected in the field (P. two tanks, with each tank containing fifteen plants labillardierei and B. catharticus) next to the Cam- (three of each species). The exception was L. perenne, paspe River in northern Victoria, near the township of where only 50 plants were available; for this species, Rochester (- 36.380974, 144.708964). Soil texture four plants (rather than six) were selected for pre- analysis conducted within two separate studies at treatment biomass harvesting while two treatments riparian sites within the southern Murray-Darling (the unshaded 0-week and shaded 2-week treatments) Basin showed that soils were dominated by sand and received one fewer plant each (i.e. five rather than six replicate plants). Plants were placed on a platform sandy loam to a depth of 0.9 m (Hao et al. 2017;Hu et al. 2017). In April 2018, seeds were germinated in inside each tank, which was positioned approximately trays containing seed raising mix in a heated glass- halfway up the tank. house with a temperature range of 18–25 C and were The experiment commenced on February 15, 2019, kept moist via mist irrigation. After 1 month, 80 when tanks were filled to the appropriate levels with seedlings of each species were planted into individual tap water and ran for 8 weeks. Water levels were 1.9 L pots containing a 7:1 sand to topsoil mix with a approximately 45 cm above the level of the substrate 3–5 cm base layer of pine bark. The pots were placed within the pots, resulting in plants being submerged by in an unheated polytunnel for 7 months to allow the an average of approximately 18 cm, depending on plants to establish. Plants were watered for four initial height. At approximately 2-week intervals, each minutes three times a day and fertilised with 0.5 g/L of plant was measured for height, defined as the longest fertiliser (N/P/K = 20:8.7:16.6) once a month for the green section of leaf. Once green leaf material was no first 3 months and then once every 2 months for the longer evident, plants were recorded as dead. At the remaining period. end of the experiment, plants recorded as dead were placed back in the polytunnel, watered regularly and Experimental design monitored for recovery (none recovered). The remain- ing live plants were harvested for biomass assessment. The experiment was conducted in a covered outdoor For each plant, leaf and root material was separated, area in Burnley, Melbourne, Victoria (- 37.828299, gently washed of soil, dried for at least 72 h at 60 C 145.020861). The experimental design comprised 16 and weighed. water tanks measuring 102 cm tall and 97.5 cm The mean daily maximum air temperature during diameter. Each tank was randomly allocated to one the 8 weeks of the experiment was 24.4 C (range 15.6–38.1 of eight experimental treatments, resulting in two C) while the mean daily minimum was tanks per treatment. These treatments consisted of four 14.6 C (range 7.4–25.1 C), measured at the Mel- levels of submergence: (1) no submergence (0 weeks), bourne (Olympic Park) weather station (ID086338) simulating a low summer flow more typical of pre- located 3.5 km to the west (Bureau of Meteorology regulation conditions; (2) 2-week submergence pulses 2020). Water temperature, pH, electrical conductivity interspersed with 2-week dry periods; (3) 4 weeks of and dissolved oxygen were measured in each tank on continual submergence; and (4) 8 weeks of continual two occasions during the fourth and fifth weeks of the submergence (Fig. 1). Each treatment also had two experiment. Dissolved oxygen ranged between 2.98 levels of shading (unshaded/shaded) to simulate lower and 8.04 mg/L, electrical conductivity ranged light availability that may be experienced in turbid between 0.065 and 0.010 mS/cm and pH ranged 123 990 Aquat Ecol (2020) 54:985–999 between 5.91 and 6.95. Dissolved oxygen was lower in species that survived the first 4 weeks of inundation: the shaded compared to unshaded 4-week and 8-week L. perenne, R. caespitosum and P. labillardierei. submergence treatments. Mean water temperatures Thirdly, maximum green leaf height and the change in across all tanks measured on the two occasions were total biomass at 8 weeks was analysed for P. labil- 17.1 C and 23.1 C. As a comparison, in rivers of lardierei only, as most individuals of the other species south-eastern Australia water temperatures generally did not survive in the submergence treatments. show a marked difference between winter (generally Linear mixed effects models were used to evaluate less than 10 C) and summer (22—26 C) (Online the relationship of plant height and biomass with Resource 1). treatment. The data were explored visually to check for assumptions of normality with subsequent trans- Analyses formations applied where necessary (outlined below). All model covariates were binary or factor variables Survival rates were assessed as the percentage of and were therefore not scaled prior to analyses. plants showing evidence of green leaf material. The Treatments (shaded and submerged) and their inter- log-rank test was used to test the null hypothesis that actions were included as fixed effects. We constructed survival curves differed between treatments. Log-rank linear mixed-effects models in R using the lmer tests were constructed using the Kaplan–Meier function in package lme4 version 1.1–21 (Bates et al. 2015). The plant height models were constructed at method in R version 3.6.0 (R Core Team 2019), with the survival package version 2.38 (Therneau 2015). different survey periods (2, 4 and 8 weeks) to compare This approach takes into account uncensored data, heights at different stages during the experiment for where an experiment ends prior to the death of all those species with the majority of plants assessed as individuals, so as to avoid underestimating the lifes- still alive. For comparisons at two weeks, the pans of these individuals (Pyke and Thompson 1986). submergence treatment was treated as a two-level However, due to the small number of replicate plants factor (i.e. 0 weeks vs. 2 weeks of submergence), with for each species in each treatment combination, the 2-week, 4-week and 8-week treatments pooled comparisons were made separately between levels of because they were identical after 2 weeks. Similarly, shade and levels of submergence. for comparisons at 4 weeks, the submergence treat- Growth was assessed using two variables: (1) ment was treated as a three-level factor (i.e. 0 weeks height, defined as the longest green section of leaf vs. 2 weeks vs. 4 weeks of submergence), with the (i.e. maximum green leaf height), measured at 4-week and 8-week treatments pooled because they approximately 2-week intervals (days 0, 14, 29, 43 were identical after 4 weeks. Treatment tank identity and 56 of the experiment), and (2) change in total was included as a random effect to account for biomass (roots and leaves combined), assessed at the variation between tanks. Fixed effect coefficients for end of the experiment on living plants. Trends in the models indicated effect sizes for each of the growth responses were also examined using relative treatment levels compared to controls, i.e. 0-week height growth and rates of growth (cm per week) with submergence and unshaded. Interactions were both metrics showing very similar responses as height. included for submergence and shade treatments. For Change in total biomass for each plant was calculated the biomass model, the response variable data were as the final biomass less the species mean biomass of log transformed to allow for model assumptions of the initially harvested plants (as described above). The normality. For this log-linear model, the exponenti- height and biomass data were zero-inflated due to the ated fixed effect coefficients provided the percentage high numbers of plants that died during the experi- increase in the response for that treatment compared to coeff ment, resulting in insufficient numbers of replicates the control (percentage calculated as 10 - 1*100). for most species and treatments. As such, the analyses Confidence intervals for each model were calculated of growth (height and biomass) were performed on using the confint function within the stats package. living plants only. Firstly, maximum green leaf height Post hoc contrasts to assess effects and significance was compared, for living plants, between treatments at between treatment factors were conducted on models day 14. Secondly, maximum green leaf height was using the emmeans function in the emmeans package compared between treatments at day 29 for the three 123 Aquat Ecol (2020) 54:985–999 991 Fig. 2 Survival rates of plants by species during the experi- treatment combination was six, with the exception of Lolium ment. Grey shading in each panel indicates periods of perenne in the unshaded 0-week and shaded 2-week pulse submergence, while white (no shading) indicates periods of treatments (five plants) emergence. The initial number of plants per species for each version 1.4 (Lenth 2019), with significance level of 100% survival rates after 8 weeks in the 0-week 0.05. treatment (both shaded and unshaded), compared to All graphs were produced in R version 3.6.0 (R the survival rates after 8 weeks when submerged, Core Team 2019), using either the base package or the which were either very low (L. perenne) or zero (B. ggplot2 package version 3.2.1 (Wickham 2016). catharticus, D. glomerata and R. caespitosum) irre- spective of submergence duration (Fig. 2). Lolium perenne was the only species to significantly differ in Results survival between levels of shade (Online Resource 2). Survival rates for this species were 67% and 50% after Survival 8 weeks in the unshaded 2-week pulse and unshaded 4-week treatments, respectively, whereas survival was There was no significant difference in the survival of zero in the equivalent shaded treatments. However, in P. labillardierei between shade or submergence the 8-week submergence treatments, survival rates of levels, as survival rates after 8 weeks were 100% L. perenne were zero irrespective of shading (Fig. 2). across all treatments, except the shaded 4-week In the unshaded and shaded 2-week pulse treat- treatment where one plant died (Fig. 2; Online ments, survival rates for all species were 100% by the Resource 2). For the remaining four species, survival end of the first 2 weeks of submergence. However, was significantly different between treatments, with survival rates declined for all species, except P. 123 992 Aquat Ecol (2020) 54:985–999 Fig. 3 Heights of plants assessed as alive (measured as the height of the longest section of green leaf) during the experiment; values are means (excluding dead plants) ± 95% CI. Grey shading indicates periods of submergence labillardierei, during the following 2 weeks of non- positive effect of shading for these three species submergence/emergence (Fig. 2). By the end of this (Fig. 4a). In contrast, the maximum green leaf height period, all B. catharticus plants in the 2-week pulse of P. labillardierei remained similar across all treat- treatment had died, while for D. glomerata 100% of ments within the first 2 weeks (Figs. 3, 4a). unshaded plants and 66% of shaded plants had died in After 4 weeks, the majority of B. catharticus and D. the 2-week pulse treatment. glomerata plants had died. Submergence continued to have a strong negative effect on the maximum green Growth leaf height of R. caespitosum and L. perenne plants recorded as alive (Fig. 3). However, there were no There was an overall negative effect of submergence significant differences in the effect sizes between the on maximum green leaf height of B. catharticus, D. 2-week pulse and the 4-week submergence treatments glomerata, R. caespitosum and L. perenne by the end (Fig. 4b). Both L. perenne and P. labillardierei of the first 2 weeks (Fig. 3). This was particularly increased in height in the 0-week shaded treatment evident for B. catharticus and D. glomerata, while (Figs. 3, 4b). there was a relatively smaller negative effect for R. By the end of the experiment, P. labillardierei was caespitosum and L. perenne (Fig. 4a). In the 0-week the only species to exhibit a relatively stable maximum treatment, B. catharticus, D. glomerata and L. perenne green leaf height across all submergence treatments increased in height during the first 2 weeks when (Fig. 3). While submergence, regardless of duration, shaded compared to unshaded (Fig. 3), with a strong was estimated to reduce growth, these differences 123 Aquat Ecol (2020) 54:985–999 993 Fig. 4 Effect sizes from linear mixed-effects models of 0 weeks vs. 2 weeks vs. 4 weeks of submergence with the treatment covariates (and interactions) on plant height growth 4-week and 8-week treatment pooled); and (c) 8 weeks of after: a 2 weeks of treatment for all species, with submergence treatment for P. labillardierei with all submergence treatments treated as a two-level binary factor (i.e. 0 weeks of submergence and shading included. Interactions included are denoted by ‘*’. vs. 2 weeks of submergence with the 2-week, 4-week and Levels of treatment on y-axis are in comparison to 0-week (no) 8-week treatments pooled); b 4 weeks of treatment for submergence and unshaded treatments. Error bars represent Rytidosperma caespitosum, Lolium perenne and Poa labil- 95% CI. The vertical dashed line occurs at effect size of zero (no lardierei, with submergence included as a three-level factor (i.e. effect). Sub = submergence treatment Fig. 5 The modelled effect of treatments on percentage submergence and unshaded treatments, with interactions difference in total biomass of Poa labillardierei plants after included (denoted by *). Error bars indicate 95% CI. The 8 weeks: a excluding interactions and b including interactions. vertical dashed line occurs at difference of zero (no effect). Levels of treatments on y-axis are in comparison to 0-week (no) Sub = submergence treatment 123 994 Aquat Ecol (2020) 54:985–999 were uncertain (Fig. 4c). In the 2-week pulse and zones along waterways in south-eastern Australia. As 4-week treatments, heights were higher when shaded, such, their tolerance to submergence is likely to be although these differences were not significant. Poa lower than riparian and wetland specialists, many of labillardierei was also taller in the 0-week treatment which can tolerate submergence durations ranging when shaded compared to unshaded (Fig. 3). from weeks to months, including at various times of After 8 weeks, there was a decrease in the relative the year (Mommer et al. 2006; Vivian et al. 2014; biomass of P. labillardierei when shaded (compared Nicol et al. 2018). However, based on the reported to unshaded) and when submerged (compared to tolerances of the study species in other experiments, 0-week submergence) (Fig. 5a). There were no sig- which range from days to multiple weeks (Banach nificant differences in the decline in biomass between et al. 2009; Striker and Ploschuk 2018; Kitanovic the 2-week, 4-week and 8-week submergence treat- 2019), we expected a higher degree of survival of the ments. Interactions between the shading and submer- study species in the 2-week pulse treatment. For gence treatments indicated that the increase in P. example, a study investigating the response of riparian labillardierei biomass when unshaded compared to grasses to experimental submergence in late winter shaded was only apparent in the 0-week treatment and early spring, including three of the species (Fig. 5b). investigated here (B. catharticus, P. labillardierei and R. caespitosum) and using the same experimental tank set-up, found that all species had a 100% survival Discussion rate under the maximum submergence period of 35 days, with the exception of a low number of B. Our study shows that complete submergence in catharticus plants (Kitanovic 2019). As such, the high summer and early autumn under experimental condi- mortality rates of four of the five species in this study tions ultimately resulted in the death of almost all of after 2 weeks of submergence appears to reflect a the grass species tested after 8 weeks, irrespective of lower tolerance than reported in other experiments. whether the submergence occurred as 2-week pulses Our study differs from many other experimental or as longer continuous periods. The exception was P. studies on plant responses to submergence in that labillardierei, which largely survived all submergence plants were submerged during summer. One possible durations. As such, for four out of the five species interactive effect on plants with summer submergence studied, these results support our first hypothesis that is the potential for higher water temperatures. In the plant survival and growth would be greatest in current study, mean temperatures measured on two response to no submergence compared to submerged occasions were 17.1 C and 23.1 C, in contrast to the conditions. However, there was little support for our lower mean water temperature of 12.9 C reported by second hypothesis, which predicted that for plants Kitanovic (2019). The primary cause of stress for exposed to submergence, survival and growth would submerged plants is the low diffusion of oxygen and be greater in response to the 2-week submergence carbon dioxide in water, which substantially reduces pulses compared to continual submergence. Instead, gas exchange to the plant; however, with increasing plant survival and height was similarly low for four of temperatures, plant survival and growth rates can the five species across all submergence treatments. We decline further due to higher respiration rates, accel- found only minimal support for our third hypothesis erated carbohydrate utilisation, and more rapid leaf that survival and growth of submerged plants would be decay (van Eck et al. 2005; Bailey-Serres and reduced in shaded compared to unshaded conditions, Voesenek 2008; Das et al. 2009; Ye et al. 2018a, b). with L. perenne surviving better in the 2-week pulse For example, survival rates of ten grassland species and 4-week continual submergence treatments when from riparian zones of the River Rhine, including four unshaded compared to shaded. perennial grasses, were lower in response to simulated Four of the five study species had a high overall summer submergence (water temperatures of 20 C) mortality rate, including in the 2-week pulse treat- compared to winter submergence (maximum water ment. These four species are not considered riparian temperature of 6 C), although the magnitude of the specialists; rather, they are terrestrial species that difference varied between species (van Eck et al. occur widely, but are also often recorded in riparian 2006). Experiments on submergence tolerance of 123 Aquat Ecol (2020) 54:985–999 995 Oryza sativa (rice) have also shown declines in plant of unseasonal submergence may also depend on plant survival with increasing water temperatures (Das et al. age (e.g. Denton and Ganf 1994), with the tolerance of 2009). However, differences in survival and growth at smaller or younger plants (e.g. germinants) likely to be different times of the year may also be related to other lower. factors, such as seasonal growth patterns, with plants Light is generally limited for submerged plants potentially more sensitive to submergence during the because light is diffused in water and can be further growing season (van Eck et al. 2006). Untangling the reduced by turbidity, with studies showing that relative importance of water temperature on riparian survival and growth underwater can be improved with plant survival and growth compared to other poten- increased light intensity (Bailey-Serres and Voesenek tially influential factors, such as submergence during 2008; Das et al. 2009; Li et al. 2011). In this study, the growing season, photoperiod, and interactions with only L. perenne showed some degree of increasing water quality and light availability, will assist in survival rates under unshaded conditions when sub- understanding and predicting plant responses. merged, but only in the two shorter submergence Of the five species, P. labillardierei was the only periods (2-week pulse and 4-week continuous sub- one to survive submergence while also maintaining mergence). However, after 8 weeks, the maximum relatively constant leaf height and biomass. This green leaf height of P. labillardierei was marginally suggests an ‘acquiescent’ submergence response taller in both the shaded 2-week and shaded 4-week treatments compared to the shaded 8-week treatments. strategy, where submergence is tolerated by limiting underwater growth and downregulating processes to This suggests that if submergence of this species conserve energy, in contrast to an ‘escape’ strategy, occurs under low light conditions, shorter durations where species rapidly elongate stems or leaves to the may allow more opportunity for leaf elongation, water surface to re-establish contact with the air although there was no difference in biomass increase. (Bailey-Serres and Voesenek 2008; Colmer and It is possible that shading—or low light—impacts may Voesenek 2009). Species exhibiting the acquiescent be more important for less flood-tolerant species (e.g. strategy tend to have fitness advantages in environ- B. catharticus, D. glomerata and R. caespitosum) and ments exposed to short duration or deep flooding, i.e. only during shorter (non-lethal) periods of submer- situations where rapid shoot elongation becomes gence. Alternatively, reductions in plant survival and overly costly (Voesenek et al. 2004; Colmer and growth may be more affected by other characteristics Voesenek 2009). For P. labillardierei, such a strategy of turbid flows rather than simply light reduction, such likely reflects its distribution on mid to upper river- as damage from sediment deposition (e.g. Lowe et al. banks, as well as in terrestrial habitats, where any 2010; Catford and Jansson 2014), that we did not flooding is likely to be of short duration. The examine in this study. maximum duration submergence that P. labillardierei An important finding of our study was that plant can survive is currently undocumented, including mortality in the pulse treatment only became apparent under more typical seasonal flooding. However, given during the 2-week dry period while plants were the differential response of other species to submer- emergent. Other experimental studies have also found gence during different seasons (e.g. van Eck et al. that mortality of plants can become evident during a 2006), it might be expected that its tolerance to winter/ recovery period following submergence (Denton and spring submergence is even greater, such as in Ganf 1994). This highlights the importance of con- Kitanovic (2019). Although we found survival and sidering the capacity for plants to recover after the biomass maintenance of P. labillardierei was rela- removal of stress (van Eck et al. 2004; Striker and tively high after 8 weeks of submergence, it is possible Ploschuk 2018) and that some indicators of plant that other aspects of its fitness may be affected by health may not be indicative of longer-term survival. prolonged unseasonal submergence. For example, Thus, any investigation of the tolerance of minimum changes to seasonal timing of flows can negatively submergence periods should include monitoring of impact other critical plant life cycle stages, such as plant survival for a period following plant re-emer- flowering, seed production and germination (Poff et al. gence. This is likely to be particularly important for 1997; Greet et al. 2013b), which in turn could lead to plants recovering from summer submergence, as they reduced population size or vigour over time. Survival may face additional impacts of tissue desiccation from 123 996 Aquat Ecol (2020) 54:985–999 exposure to hot and dry conditions (Jensen et al. 2008; species. For example, warm (20 C and 30 C) water Greet et al. 2013b). temperatures induced rapid stem elongation in the flood-tolerant species Alternanthera philoxeroides Management implications compared to cold (10 C) temperatures (Ye et al. 2018b). Our findings have important implications for the Three of the grasses included in this study are management of unseasonal summer flows. Our results introduced to south-eastern Australia and are therefore suggest that the delivery of flows in pulses of 2 weeks generally undesired components of riparian vegetation with a 2-week break to allow plants to recover in communities. Summer flows could potentially be used between flow events, rather a continuous single to control undesired species such as these by exceeding 4-week block, will not necessarily reduce plant their tolerance to submergence when they are most mortality. We chose a 2-week flow period for our susceptible. However, there are risks and uncertainties minimum period of submergence, as the reported in adopting this strategy. For example, summer flows tolerances of a broad range of riparian and wetland are likely to also impact non-target (e.g. native) species species, and terrestrial grasses found in riparian zones, such as the similarly intolerant native R. caespitosum, is generally in the order of weeks to months (e.g. Lowe which despite being a terrestrial grass can still serve the et al. 2010; Nicol et al. 2018; Kitanovic 2019). function of providing cover and habitat during dry and hot months on higher elevation parts of a bank. Thus, However, for four of the five species examined, this duration still resulted in high plant mortality. As such, using summer flows to control undesirable species if a pulse strategy was to be adopted, even shorter could result in an overall decline in vegetation cover durations are likely to be required if the goal is to and increased bare ground, especially in areas domi- reduce the mortality of plants with similar tolerances nated by introduced terrestrial plants and where tolerant to those in our study. However, the minimum duration species such as P. labillardierei are lacking. Further- of submergence in summer that many plants in more, while short summer flows may not kill some riparian zones can tolerate requires further investiga- riparian species (e.g. P. labillardierei), they may still tion. Additional experimental studies would also be suffer from reduced growth and/or vigour with impli- useful for determining the mechanisms by which cations for reproduction (e.g. reduced flowering) and summer flows can result in rapid mortality, particu- population persistence. The impacts of repeated sum- larly isolating the relative importance of high water mer submergence on these other aspects of fitness, temperatures in comparison to other effects of season particularly over the long term, are uncertain and and interactions with plant life cycles. requires further research. Other risks of such a strategy A reduction in the discharge of unseasonal summer include the potential for benefiting introduced riparian flows would reduce the risk of complete submergence species commonly found in the region that have been of vegetation at mid to higher elevations on the bank, found to be favoured by summer submergence, includ- such as the group of terrestrial grasses examined in this ing the semi-aquatic rhizomatous grass Paspalum study. This may allow plants to be partially exposed to distichum and the herb Ludwigia palustris (Greet the air, likely resulting in significant increases in et al. 2013b). Summer flows may also promote the survival capacity (e.g. Grimoldi et al. 1999; Vivian dispersal of exotic plant species, potentially shifting the et al. 2014). However, the tolerance of riparian species composition of riparian plant communities towards typical of lower bank elevations in the region, such as increasing dominance by exotics (Greet et al. Alternanthera denticulata and Persicaria decipiens,to 2012, 2013b). Thus the use of summer flows for summer submergence also remains largely unknown reducing the extent of undesirable plant species in the and requires further investigation, including any long- riparian zone is a risky strategy. term impacts of pulsed submergence. Future experi- mental studies are also needed to examine the response to unseasonal submergence of species that exhibit an Conclusions escape response via stem elongation, as submergence at different times of year and/or in different water With ever-increasing demands on our river systems, temperatures may induce a different response in such river managers must continually balance the flow 123 Aquat Ecol (2020) 54:985–999 997 Ann Bot 103:341–351. https://doi.org/10.1093/aob/ regime requirements of riverine environments with mcn183 society’s water needs and demands. Given that Bates D, Machler M, Bolker B, Walker S (2015) Fitting lin- demands for unseasonal flows are also increasing, ear mixed-effects models using lme4. J Stat Softw 67. particularly in south-eastern Australia, it is important https://doi.org/10.18637/jss.v067.i01 Birch JL, Berwick FB, Walsh N et al (2015) Distribution of to understand how these flows may impact different morphological diversity within widespread Australian biota and be best managed. By demonstrating exper- species of Poa (Poaceae, tribe Poeae) and implications for imentally that even short, 2-week periods of unsea- taxonomy of the genus. Aust Syst Bot 27:333–354. https:// sonal summer submergence can cause high mortality doi.org/10.1071/SB14028 Bunn SE, Arthington AH (2002) Basic principles and ecological of some terrestrial grasses commonly found in riparian consequences of altered flow regimes for aquatic biodi- habitats, our study suggests that the impacts of versity. Environ Manag 30:492–507. https://doi.org/10. unseasonal flows require careful consideration and 1007/s00267-002-2737-0 management. Bureau of Meteorology (2020) Climate data online. https:// www.bom.gov.au/climate/data/. Accessed 28 Apr 2020 Casanova MT (2011) Using water plant functional groups to Acknowledgements We thank Scott McKendrick, Bryan investigate environmental water requirements. Freshw Mole and Marjorie Pereira for assistance with data collection; Biol 56:2637–2652. https://doi.org/10.1111/j.1365-2427. and the assistance of Burnley nursery staff Nicholas Osborne, 2011.02680.x Sascha Andrusiak and Rowan Berry. We also thank Vanja Catford JA, Jansson R (2014) Drowned, buried and carried Kitanovic for seed collection and plant potting. We gratefully away: effects of plant traits on the distribution of native and acknowledge Andrew Boulton, Andrew Bennett, Jemma Cripps, alien species in riparian ecosystems. New Phytol Geoff Sutter, Sally Kenny and two anonymous reviewers for 204:19–36. https://doi.org/10.1111/nph.12951 feedback on earlier versions of the manuscript, and Ben Fanson Clarke I (2015) Name those grasses. Royal Botanic Gardens for advice on data analyses. This work was funded by the Victoria, South Yarra Victorian Government through the Victorian Environmental Colmer TD, Voesenek LACJ (2009) Flooding tolerance: Suites Flows Monitoring and Assessment Program. The production of of plant traits in variable environments. Funct Plant Biol this paper was supported by the Applied Aquatic Ecology 36:665–681. https://doi.org/10.1071/FP09144 writing retreat, which in turn was supported by the Capability Cottingham P, Koster W, Roberts J, Vietz GJ (2018) Assess- Fund of the Arthur Rylah Institute. ment of potential inter-valley transfers (IVT) of water from the Goulburn River. Report prepared for the Goulburn- Open Access This article is licensed under a Creative Com- Broken Catchment Management Authority. mons Attribution 4.0 International License, which permits use, Cottingham PD, Stewardson MJ, Roberts J et al (2010) sharing, adaptation, distribution and reproduction in any med- Ecosystem response modelling in the Goulburn River: how ium or format, as long as you give appropriate credit to the much water is too much? In: Saintilan N, Overton IC (eds) original author(s) and the source, provide a link to the Creative Ecosystem response modelling in the Murray-Darling Commons licence, and indicate if changes were made. The Basin. CSIRO Publishing, Canberra, pp 391–408 images or other third party material in this article are included in Crawford RM (2004) Seasonal differences in plant responses to the article’s Creative Commons licence, unless indicated flooding and anoxia. Can J Bot 81:1224–1246. https://doi. otherwise in a credit line to the material. 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Aquatic Ecology – Springer Journals
Published: Dec 29, 2020
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