Ecological significance of hydrological connectivity for wetland plant communities on a dryland floodplain river, MacIntyre River, Australia M. A. Reid, M. C. Reid & M. C. Thoms
Aquatic Sciences Research Across Boundaries ISSN 1015-1621 Aquat Sci DOI 10.1007/s00027-015-0414-7
1 23
Your article is protected by copyright and all rights are held exclusively by Springer Basel. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.
1 23
Author's personal copy Aquat Sci DOI 10.1007/s00027-015-0414-7
Aquatic Sciences
RESEARCH ARTICLE
Ecological significance of hydrological connectivity for wetland plant communities on a dryland floodplain river, MacIntyre River, Australia M. A. Reid1 • M. C. Reid1 • M. C. Thoms1
Received: 12 August 2014 / Accepted: 5 August 2015 Springer Basel 2015
Abstract Hydrological connections between river channels and their adjacent floodplains facilitate the flux of organisms and nutrients and access to increased habitat and new resources. Hydrological connections also deliver water subsidy and potentially disturb (through hydraulic forces) floodplain ecosystems. This study investigates the role of hydrological connectivity as a driver of patterns in wetland plant assemblages in billabongs on the floodplain of an Australian dryland river, exploring indirectly the relative importance of the mechanisms of flux, subsidy and disturbance. Wetland plants were surveyed in billabongs across gradients of hydrological connectivity and depth. Surveys were accompanied by experiments examining germination from the soil seed banks of each site under submerged and waterlogged conditions. The patterns in extant and germinant plant communities in relation to connectivity and depth gradients were used to infer the relative importance of the connectivity-related mechanisms of flux, subsidy and hydraulic disturbance in structuring wetland plant communities. Depth influenced both extant and germinating plant communities. Shallow billabongs supported a greater diversity and abundance of plants, and greater numbers and diversity of germinable seeds in the seed bank. Germination of seeds was greater in waterlogged soils than submerged soils. Thus, the main controls of plant abundance in wetlands appear to be availability of waterlogged soil habitat for germination and absence of
& M. A. Reid
[email protected] 1
Riverine Landscapes Research Laboratory, Geography and Planning, School of Behavioural, Cognitive and Social Sciences, University of New England, Armidale, NSW 2351, Australia
light limitation for growth. Hydrological connectivity did not influence the abundance of plants or germinable seeds, but did influence species presence-absence in growing vegetation; this effect did not extend to the germinating community. Thus, hydrological connection does not appear to influence wetland vegetation by facilitating the movement of propagules between habitats. Instead, the patterns observed are consistent with hydrological connection providing a cue for germination through the delivery of water, and by modifying hydraulic habitat. Keywords Hydrology Higher plants Floodplains Dispersal Community
Introduction Hydrological connections have an important role in influencing the structure and function of aquatic habitat patches in lowland river floodplains (Junk et al. 1989; Pringle 2001; Amoros and Bornette 2002; Thorp et al. 2006; Reid et al. 2012). Intermittent connection facilitates active and passive movement of biota and resources between habitats (Pringle 2003). In so doing, hydrological connection increases opportunities for organisms to colonise new habitats or exploit new resources. Conversely, connection may also result in the removal of organisms or introduce competitors or predators (Bornette et al. 1998; Huryn et al. 2001). Hydrological connections are frequently considered in relation to highly mobile animals such as fish and some aquatic invertebrates that can exploit the connection phase. Our understanding of the importance of hydrological connections is less for organisms that disperse passively, such as zooplankton, phytoplankton and floating vascular plants,
123
Author's personal copy M. A. Reid et al.
and for dissolved and particulate nutrients (Tockner et al. 1999; Hein et al. 2003; Jenkins and Boulton 2003; Frisch et al. 2005; Padial et al. 2014). This is also the case for sessile organisms, in particular rooted or attached plants (Bornette et al. 1994; Holzel and Otte 2001; Bornette and Arens 2002; Leyer 2006; Dos Santos and Thomaz 2007), and for organisms that do not require hydrological connection to move between aquatic habitat patches (e.g. turtles, frogs and some macroinvertebrates) (Morand and Joly 1995; Tockner et al. 1998; Leigh and Sheldon 2009). In the case of floodplain wetland plants, hydrological connectivity is important because plants must cope with environmental variations associated with the wet and dry phases. For species unable to tolerate harsh conditions (be they wet or dry), persistence in the floodplain requires recolonisation of habitats following phase transitions, which can only be achieved through dispersal or by resistant stages. For plants, the role of hydrological connectivity has thus tended to focus on hydrochory, the water mediated transport of plant propagules (Riis and Biggs 2003; Boedeltje et al. 2004; Leyer 2006; Gerard et al. 2008). The transport of propagules by flow influences the distribution of aquatic and riparian plants (Van Eck et al. 2005; Leyer 2006). However, disentangling the role of water-mediated transport of propagules as a driver of the distribution of aquatic and riparian plants is difficult, partly because other vectors such as waterbirds and other animals, as well as wind may confound the effect of water transport, but also because mechanisms other than propagule transport related to hydrological connection can influence plant communities. Hydrological connections influence floodplain river ecosystems via three principal mechanisms. The first mechanism is through the hydrological connection itself, which facilitates fluxes of material and biota between habitats (Pringle 2003). The second is by the provision of resources (water, sediment and nutrients) that create potential subsidies and, or stresses for floodplain wetland biota. Connection delivers these resources to water-limited ecosystems (Sims and Thoms 2002; Reid et al. 2011). In some circumstances, this can also create a stress, notably through light limitation (Porter et al. 2007; Reid et al. 2007), a factor of particular importance in Australian lowland floodplain rivers which are typically highly turbid, because of the natural state of the continent’s highly weathered soils, age, disturbance associated with agriculture and the resultant high levels of catchment soil erosion (Kirk 1985; Prosser et al. 2001; Olley and Wasson 2003). Stress may also arise through low oxygen concentration and low redox (Van Den Brink et al. 1995; Sims and Thoms 2002). The third mechanism is through the hydraulic character of the connection. Flow can disturb connected habitats (Thoms 2003; Thoms et al. 2005) and
123
remove sediments, nutrients and individual biota via scouring. The influence of hydrological connection on floodplain wetland plant communities is further complicated by the likelihood that the relative importance of the mechanisms of flux, subsidy/stress and hydraulic disturbance is modified by geomorphology, which controls wetland water depth as well as the size and shape of connecting channels, will thus influence hydroperiod, light, retention time and absolute volumes of through flow. For example, disturbance will be more important if a connecting channel carries large volumes through the habitat than if the channel only carries a small volume; similarly, subsidy will be more important if average water depth is low resulting in desiccation between connection events. The mechanisms of flux, subsidy/stress and hydraulic disturbance as well as the interactions of these mechanisms with depth are likely to influence different aspects of wetland plant community structure to varying degrees. For example, flux is likely to influence the species pool more than the total abundance of plants because a connection event has the potential to deliver propagules from many upstream species pools, but the recruitment success of these propagules (and hence total abundance of plants) will depend on in situ habitat conditions (see Fig. 1). Conversely, subsidy (or stress) will influence abundance more than the species pool because the subsidy can only act on the available pool, but will directly contribute to recruitment success. Hydraulic disturbance is also more likely to influence abundance more than species pool because, although different taxa may be more susceptible to losses associated with hydraulic stress (Biggs 1996), it is unlikely that species would be totally lost from a wetland (Riis and Biggs 2003). Further discrimination of the influences of flux, subsidy/ stress and hydraulic disturbance should also be possible through comparison of extant and germinant plant assemblages (Fig. 1). The diversity and abundance of germinable seeds in wetland soil seed banks will be directly influenced by flux and hydraulic disturbance (with flux delivering seeds and hydraulic disturbance potentially removing them), but indirectly influenced by subsidy/stress (via its influence on mature plant abundance and hence in situ seed production). In contrast, extant plant assemblages will be directly influenced by subsidy/stress and hydraulic disturbance and indirectly influenced by flux (via recruitment from germinable seeds). Here we examined the influence of hydrological connectivity on plant communities in floodplain wetlands (billabongs) on a dryland river floodplain in SE Australia, comparing plant communities in wetlands across hydrological connectivity and depth gradients. We assumed that the frequency of connection controls: a) connectivity
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… Fig. 1 Conceptual diagram illustrating how connectivity may influence species pools and species abundance of extant and seed bank assemblages in deep and shallow billabongs via the mechanisms of flux, subsidy/ stress and hydraulic disturbance (HD). The relative influence of each mechanism on each assemblage and each aspect (species pool or abundance) is indicated by line width. Note that extant species pools and abundances are influenced by those of the seed bank and vice versa
among billabongs and to the main channel (thus offering a pathway via which propagules move between habitats), b) the delivery of water to billabongs, and, c) the hydraulic character (e.g. flow velocity, turbulence) of the connection and disturbance of the substratum. Field surveys were supplemented by seed bank trials in which the germination of plants from wetland seed banks was compared under different wetting regimes. The study design allowed for patterns in species pool and plant abundance among extant and germinant communities in relation to connectivity and depth gradients to be revealed. These patterns, in turn, were used to infer the relative importance of the mechanisms of flux, subsidy and hydraulic disturbance based on the conceptual framework described above. We stress that the study provides only indirect measures of the importance of these mechanisms and thus does not test the relationships rigorously. However, the inferences made from this study do illustrate the value of the proposed framework as a guide for further testing.
length across a flat, semi-arid landscape (Fig. 2). The longterm median annual rainfall (n = 68 years) decreases from east (1100 mm in the upper catchment) to west (621 mm at Goondiwindi) across the catchment. Rainfall occurs mostly in the summer months (November–April) and is associated with tropical monsoonal activity (Thoms and Sheldon 2002). Flow in the Macintyre River is naturally highly variable, but is regulated by three headwater dams, a series of main channel weirs and numerous small weirs on tributaries and anabranch channels. These weirs serve to provide water for irrigation, urban, stock and domestic purposes. Boggabilla Weir, the main regulating structure on the lower Macintyre River (20 km upstream of Goondiwindi) controls flows during the main irrigation season from October to March. The study focuses on a ca 100 km reach of the lower Macintyre River downstream of Boggabilla (Fig. 2), a river reach that features extensive floodplains up to 20 km wide with a network of anabranch channels (Thoms and Parsons 2003). These temporary channels are disconnected from the main channel for most of the year and many retain water as ‘billabongs’ for several months.
Methods Characterising connectivity and morphology Study area The Macintyre River originates in the well-watered highlands of south-eastern Australia and flows for most of its
The connectivity of individual billabongs in the study area was calculated as a function of the Commence-To-Flow (CTF) discharges for the anabranches that connected them
123
Author's personal copy M. A. Reid et al.
Fig. 2 Study area
to the mainstream. The CTF discharge estimates (hereafter CTF thresholds) were extracted from the results of a regional survey of anabranch channels (Thoms et al. 2005). In this survey, CTF thresholds were determined from crosssectional surveys of the main channel at the entry and exit points of each anabranch channel, in combination with the elevations of the sills controlling inflow into each anabranch (Thoms et al. 2005). The discharge (at the Goondiwindi town gauge) resulting in over topping of these sills was calculated using the Manning equation (Gordon et al. 1992). Further details of this procedure can be found in Thoms et al. (2005). The estimated CTF thresholds were checked against field observations by the authors, local water agency personnel and local land holders. Based on these CTF thresholds calculated for the population of anabranches in the 100 km reach, a subset of 11 billabongs (sections of anabranch that retained water after flow ceased) were selected for the study using a random-stratified approach from among the anabranches identified as the most connected (Flow group 1 of Thoms et al. 2005) and from among those identified as the least connected (Flow groups 4 and 5 of Thoms et al. 2005).
123
This random stratified approach was taken to ensure strong contrast with respect to connectivity. The final data set included five high connectivity billabongs, and 6 low connectivity billabongs. The high connectivity billabongs all had CTF thresholds less than 7000 ML day-1 which equates to an annual return interval (ARI) of approximately 1 year at the Goondiwindi gauge (that is these billabongs are flooded once a year on average). The six low connectivity billabongs included five connected only during overbank flows, equivalent to a discharge of around 60,000 ML day-1 (ARI *4 years), and one billabong with a CTF threshold of 25,700 ML day-1 (ARI *2 years). The CTF thresholds were also used as continuous explanatory variables. Billabong depths recorded during vegetation surveys (described below) were combined with land-based differential GPS surveys to establish maximum ponding depths based on sill heights. These showed a dichotomy in the maximum depths of the study billabongs between those with maximum ponding depths of less than 1.25 m (6 billabongs) and those with maximum ponding depths of greater than 2.7 m (5 billabongs). Accordingly, the
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a…
billabongs were also classed as either shallow (\1.25 m; n = 6) or deep ([2.70 m; n = 5) to enable the effect of this potentially important factor on extant and germinating plant assemblages to be tested. As for the CTF thresholds, maximum depths were also used as continuous explanatory variables. Connectivity and depth information is provided in Table 1, which also includes information on surface area, wetted perimeter and shoreline development for the study billabongs and for the larger population of anabranches surveyed by Thoms et al. (2005). Vegetation surveys Vegetation surveys were conducted from April 4 to 12, 2006, approximately 10–11 weeks after a flow pulse that peaked at 6812 ML day-1 and hence connected all high connectivity billabongs to the main channel of the Macintyre River. All high connectivity billabongs had also been connected by a pulse peaking at 7031 ML day-1 in early December, 2005, approximately 4 months prior to sampling. Vegetation cover was assessed by point count along transects positioned randomly along the length of each billabong and extending across the billabong from high water mark on each bank (12 transects in all at each site). Points were positioned at 1 m intervals along each transect. This sampling strategy resulted in sampling points at a density of one point per 19–381 m2 and between 1.44 and 12.88 m of shoreline. At each point, water depth was recorded, as was the presence of plant taxa, woody debris (defined as woody material greater than 5 cm in diameter) and plant litter. Presence was defined as the
elements that touched a measuring staff (2 9 2 cm in crosssection) positioned vertically at the measuring point. The cover of each element in each billabong was calculated as the percentage of the total number of points assessed in each billabong where that element was recorded as present. Depth at points greater than wadeable depth were recorded from a boat and assumed to be bare. While this assumption likely led to an underestimation of litter, woody debris and plant cover in deeper billabongs, litter and woody debris were rare at depths greater than 1 m and no plants were detected at greater than 1 m. Voucher specimens of unknown plant taxa were collected and pressed for later identification. Two soil samples were collected at each billabong from edge and submerged locations for seed bank experiments to test for systematic spatial variation in seed banks within billabongs, notably in relation to potential accumulations at ‘strand lines’ at the water’s edge. Soil was collected using a circular metal tube of 9.5 cm diameter and 5 cm depth which was pressed into the sediment to extract an entire plug of soil/sediment. Deep locations were either the deepest central point or the maximum depth (approximately 50 cm) to which intact samples of soil could be extracted using the metal tube. Marginal locations were at the water level at the time of sampling. Each sample consisted of two pooled plugs of soil, each 9.5 cm in diameter and 5 cm deep (surface area of 71 cm2 and volume 352 cm3). These pooled samples were air dried and any vegetation removed prior to transport and storage. Samples were stored dry and in darkness at ambient room temperature for up to 3 weeks. A 5 g sub-sample of each
Table 1 Commence-to-flow (CTF) thresholds, maximum depths, area, perimeter length and shoreline development of the study billabongs and the relevant classes applied as factors in analyses of variance CTF (ML day-1)
Connectivity class
Depth (m)
Depth class
Site ID
Site name
Boo_A
Booberoi Billabong A
60,000
Low
3.2
Deep
Boo_B
Booberoi Billabong B
60,000
Low
0.7
Shallow
Mac_A
Macintyre Downs Billabong A
Maynes Pung
Maynes Lagoon Pungbougal Lagoon
Riv_A
Riverview Billabong A
Tar_A
Taraba Billabong A
Tar_B
Taraba Billabong B
Why_A
Whynot Billabong A
Why_B Why_C
528
High
1.0
Shallow
60,000 60,000
Low Low
3.8 8.4
Deep Deep
5731
High
0.8
25,700
Low
2.7
533
High
1730
Whynot Billabong B Whynot Billabong C
Area (m2)
Perimeter (m)
Shoreline development
27,430
1441
2.45
2328
239
1.4
25,644
1851
3.26
387,888 732,660
7024 11,467
3.18 3.78
Shallow
15,780
2614
5.87
Deep
19,760
1178
2.36
3.0
Deep
21,271
854
1.65
High
1.2
Shallow
38,964
1895
2.71
2322
High
1.2
Shallow
19,630
1798
3.62
60,000
Low
0.42
Shallow
5829
382
1.41
Mean*
30,595
2.40
117,926
2795
2.88
SD*
28,993
2.32
232,104
3411
1.30
Mean#
48,918
NA
73,861
1809
2.28
SD#
22,222
NA
133,543
1883
1.12
Mean and standard deviations (SD) are for this data set (*) and the full population of anabranches in the study reach (#; Thoms et al. 2005)
123
Author's personal copy M. A. Reid et al.
pooled sample was used to determine organic content as a function of loss on ignition (LOI) at 550 C. Organic content was measured to check for anomalies in samples, but formal statistical tests against explanatory variables are not reported here due to non-independence in relation to vegetation cover. Seed bank trial methods In preparation for seed bank experiments, soil samples were disaggregated and large root and rhizome material was removed. Soil was then placed in plastic pots (16.5 9 16.5 9 9 cm) to a depth of approximately 6 cm with the lowermost 3 cm being made up of sterilised growth medium (Vermiculite). Each sample was divided to produce three subsamples, each of which was subjected to one of three wetting treatments, resulting in a total of 66 sampling units: 11 billabongs, 2 locations within each billabong (centre and edge) and 3 treatments (water logged, submerged and submerged with weekly replacement of water). The waterlogged treatment consisted of maintaining a fully saturated soil without surface water, essentially a water depth of 0 cm, while the two submerged treatments consisted of maintaining surface water to a depth of approximately 3 cm. For one of these treatments the water was only topped up to maintain depth while for the other, water was completely replaced in an effort to limit algal growth and encourage growth of submerged aquatic plants. This approach was taken because thick algal growth was considered to be a potential experimental effect that could have prevented successful germination and growth. Additional control samples for each treatment, each consisting of pots containing only sterilised growth medium, were also maintained throughout the course of the experiment. Trials were conducted in a greenhouse commencing in April and were maintained for 16 weeks, a duration that was chosen based on past studies that suggest that periods in excess of 12 weeks are required to allow plants to establish and be identifiable (Casanova and Brock 2000; Webb et al. 2006). This duration also ensured substantial temperature change, which can also increase the diversity of germinant communities (Reid and Capon 2011; James et al. 2007). As plants matured and flowered they were identified and removed so as not to add to the seed bank (Capon 2007). Removing plants reduces competition and encourages continual germination in order to see the full extent of the seed bank, assuming germination is triggered by the imposed treatments (Reid and Capon 2011). Plants that became too large for the pots before they could be identified were repotted in deeper pots and separated from the remaining samples so as to reduce mortality and encourage flowering to aid identification (James et al. 2007).
123
Data analysis Extant and germinating plant communities from each sampling unit were compared using the response variables: total abundance of plants (as percent occurrence in the extant communities and number of seedlings in the germinating communities); total number of taxa; Shannon diversity; and Berger–Parker dominance, calculated as the abundance of the most common taxon (the highest % occurrence or highest number of seedlings per sampling unit) as a percentage of total plant abundance (Southwood and Henderson 2000). For the extant vegetation the response variables were compared across connectivity, and depth classes in two-factor ANOVA models. In all cases, response variables were transformed as appropriate to ensure the tests fulfilled the assumptions of ANOVA (approximating normal distribution, homogeneity of variance and no trends in residual values). Hydroperiod was not tested directly, rather, it is assumed that any hydroperiod effect should arise from the interaction of depth and connectivity (i.e., deep high connectivity billabongs should be most permanent; shallow, low connectivity billabongs should be least permanent). This assumption is based on studies of similar systems that have shown hydroperiod to be a function of potential evaporation rates and the frequency of connection due to limited groundwater interaction (Hamilton et al. 2005). In addition, stable isotope analysis of surface and groundwater pools in the study region also suggested that groundwater interaction was not significant (Reid et al. 2012). Evidence for spatial structure in the extant plant assemblages was tested for using the Moran’s I test calculated for total abundance and total number of taxa. For the plant assemblages that germinated in the seed bank trials, response variables were compared across connectivity and depth classes and against sampling location and watering treatments using a mixed model ANOVA with connectivity, depth, location (edge vs submerged) and treatment as fixed factors and site as a random factor. In all cases, response variables were transformed as appropriate to ensure the tests fulfilled the assumptions of ANOVA. As for the extant plant assemblages, evidence for spatial structure in the germinating plant assemblages was tested for using the Moran’s I test calculated for total abundance and total number of taxa. Differences in extant and germinating plant community structure in relation to connectivity and depth classes were tested using two-factor permutational multivariate analysis of variance (PERMANOVA; Anderson 2001), based on Bray–Curtis resemblance matrices calculated from square root transformed abundance data as well as presence-absence data. The use of both abundance and presence-absence data enabled the relative importance of connectivity
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a…
and depth to the available species pool (presence-absence) and recruitment success (abundance) to be explored. The contributions of individual plant taxa to within group similarity and between group dissimilarity based on square-root transformed abundance data were assessed using the SIMilarity PERcentage (SIMPER) procedure. Correlations between potential explanatory environmental variables: CTF, depth, woody debris (WD) recorded in the surveys, billabong surface area (area), shoreline development (SLD) and perimeter length (PL) and the positions of plant assemblages in multivariate space were explored using distance-based linear models (dbLM). Distancebased redundancy analysis (dbRDA), a non-parametric ordination in which axes are constrained as functions of the explanatory variables was used to provide visual illustrations of the relationships between plant assemblages and explanatory variables. For the germinant assemblages, the cover of extant vegetation at each site (Veg cover) was also included as an explanatory variable. Environmental variables were added step-wise beginning with the variable explaining the most variation. The selection criterion was that additional variables should result in an increase in the proportion of variance explained, adjusted for the number of parameters (Adjusted R2). The amount of variation attributed to each variable is thus the contribution not accounted for by variables already included in the model. Differences between extant and germinant plant assemblages were also tested using a three-factor PERMANOVA including assemblage (extant vs seed bank), connectivity and depth classes. These relationships were visually represented in non-metric multi-dimensional scaling (MDS) ordination plots. Comparisons of extant and germinant assemblages were also made using twoway ANOVA comparing the Bray–Curtis similarity values calculated for each billabong’s assemblage pairs (extant vs germinant) across depth and connectivity classes. ANOVAS were carried out in SPSS v20 (IBM SPSS Statistics 2011). Moran’s I test was carried out in R using the APE package. Multivariate statistical analyses were carried out in PRIMER 6 with the PERMANOVA? add on (v 6.1.5; PRIMER-E Ltd 2006).
Results Extant vegetation assemblages A total of 47 taxa belonging to 22 families were identified in the field survey (Table 2). The most abundant species were Pseudoraphis spinescens (Spiny mud-grass), Persicaria hydropiper (Water Pepper), Cyperus exaltatus (Giant Sedge) and Lemna minor (Common Duckweed). The most
widespread species were Pseudoraphis spinescens (Spiny mud-grass) and Ludwigia peploides (Water Primrose). Although there was substantial within class variation in several of the response variables, particularly among shallow billabongs, strong differences were also evident between classes, especially in relation to depth. Plant abundance (log10 plant abundance) was significantly higher in shallow billabongs (F = 8.354, p = 0.023; Fig. 3a). Diversity of plant communities was also slightly higher in shallow billabongs (F = 4.984, p = 0.061; Fig. 3b) and log10 BergerParker dominance was greater in deep billabongs (F = 5.318, p = 0.054; Fig. 3c). The number of taxa did not vary between deep and shallow billabongs. Plant abundance, number of taxa, diversity and Berger-Parker dominance did not vary between high and low connectivity classes. No interaction effects were apparent. None of the response variables showed evidence of strong spatial patterning (Moran’s I ranging from 0.2 to -0.2; all p [ 0.1). Extant plant assemblages of billabongs varied across both depth and connectivity classes. For the comparisons based on abundance data, differences were greatest between depth classes (Table 3). The differences between the extant plant assemblages of deep and shallow billabongs reflect the greater abundance of Pseudoraphis spinescens, Cyperus exaltatus, Persicaria hydropiper, Juncus spp., Ludwigia peploides, Lemna minor, Alternanthera denticulata and Persicaria orientale in shallow billabongs (Table 4). Although the difference in abundance data between low and high connectivity billabongs was not significant, a similar suite of plant species drive the existing differences between high and low connectivity billabongs, with Pseudoraphis spinescens, Juncus spp., Persicaria hydropiper, Cyperus exaltatus, Lemna minor and Ludwigia peploides all more abundant in high connectivity billabongs (Table 4). Comparison of depth and connectivity classes based on presence-absence data showed greater differences between connectivity classes than between depth classes (Table 3). No interaction effects were apparent for either square-root or presenceabsence data (Table 3). Correlations between extant plant assemblages and environmental variables in the distance-based linear models using abundance data showed that Depth explained the most variation in the assemblage (24.2 %; Table 5; Fig. 4). With sequential addition of variables, PL was added next (17.5 %) followed by CTF (11.9 %) (Table 5; Fig. 4). The addition of Area, SLD and WD did not result in an increase in Adjusted R2 and thus the variables were not included in the model. In keeping with the univariate findings, the presenceabsence extant assemblage data showed a stronger relationship with CTF, with this variable explaining 28.4 % of the variation in the assemblage (Table 5; Fig. 4). With sequential addition of variables, Area was the second
123
Author's personal copy M. A. Reid et al. Table 2 Plant taxa recorded in field surveys and seed bank experiments
123
Taxon
Family
Life form
Extant
Seed bank
Glinus lotoides
Aizoaceae
Forb
Glinus oppositifolius
Aizoaceae
Forb
Damasonium minus
Alismataceae
Aquatic forb
Yes
Yes
Alternanthera denticulata
Amaranthaceae
Forb
Yes
Yes
Centipeda sp.
Asteraceae
Forb
Yes
Yes
Yes Yes
Centipeda spp.
Asteraceae
Forb
Conyza bonariensis
Asteraceae
Forb
Pseudognaphalium luteoalbum
Asteraceae
Forb
Azolla filiculoides
Azollaceae
Floating fern
Azolla pinnata
Azollaceae
Floating fern
Yes
Rorippa eustylis
Brassicaceae
Forb
Yes
Rorippa laciniata
Brassicaceae
Forb
Yes
Wahlenbergia fluminalis
Campanulaceae
Forb
Yes
Charophyte
Characeae
Charophyte
Chenopodium desertorum Chenopodium pumilio
Chenopodiaceae Chenopodiaceae
Subshrub Forb
Yes Yes
Carex appressa
Cyperaceae
Sedge
Yes
Cyperaceae (unident.)
Cyperaceae
Sedge
Yes
Cyperus difformis
Cyperaceae
Sedge
Cyperus exaltatus
Cyperaceae
Sedge
Yes
Cyperus iria
Cyperaceae
Sedge
Yes Yes
Yes Yes Yes
Yes Yes
Yes
Cyperus pygmaeus
Cyperaceae
Sedge
Cyperus sp.
Cyperaceae
Sedge
Yes
Eleocharis acuta
Cyperaceae
Sedge
Yes
Eleocharis plana
Cyperaceae
Sedge
Yes
Eleocharis sphacelata
Cyperaceae
Sedge
Yes
Bergia trimera
Elatinaceae
Forb
Elatine gratioloides
Elatinaceae
Forb
Chamaesyce drummondii
Euphorbiaceae
Forb
Aeschynomene indica
Fabaceae
Forb
Yes
Medicago polymorpha Sesbania cannabina
Fabaceae Fabaceae
Forb Forb
Yes Yes
Yes Yes Yes Yes Yes
Swainsona sp.
Fabaceae
Forb
Trigonella suavissima
Fabaceae
Forb
Geranium sp.
Geraniaceae
Forb
Ottelia ovalifolia
Hydrocharitaceae
Aquatic forb
Yes
Juncus sp.
Juncaceae
Rush
Yes
Yes
Yes Yes Yes
Triglochin sp.
Juncaginaceae
Aquatic monocot
Lemna minor
Lemnaceae
Floating forb
Yes
Ammannia multiflora
Lythraceae
Forb
Yes
Marsilea drummondii
Marsileaceae
Fern
Yes
Marsilea mutica
Marsileaceae
Fern
Nymphoides crenata
Menyanthaceae
Aquatic forb
Acacia stenophylla
Mimosoideae
Tree
Yes
Eucalyptus camaldulensis
Myrtaceae
Tree
Yes
Eucalyptus seedling Melaleuca trichostachya
Myrtaceae Myrtaceae
Tree Tree
Yes Yes
Boerhavia dominii
Nyctaginaceae
Forb
Yes
Ludwigia peploides
Onagraceae
Aquatic forb
Yes
Chloris pectinalisa
Poaceae
grass
Yes
Yes Yes Yes Yes Yes
Yes
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… Table 2 continued Taxon Dicanthium tenue Eriochloa crebra
a
a
Family
Life form
Extant
Poaceae
Grass
Yes
Seed bank
Poaceae
Grass
Yes
Echinochloa colona
Poaceae
Grass
Yes
Yes
Eragrostis parviflora
Poaceae
Grass
Yes
Yes
Eragrostis type
Poaceae
Grass
Yes
Eriochloa crebra
Poaceae
Grass
Yes
Panicum effusum
Poaceae
Grass
Yes
Paspalidium jubiflorum
Poaceae
Grass
Yes
Paspalum distichum
Poaceae
Grass
Yes
Pseudoraphis spinescens
Poaceae
Grass
Yes
Muehlenbeckia florulenta
Polygonaceae
Shrub
Yes
Persicaria hydropiper Persicaria lapathifolium
Polygonaceae Polygonaceae
Forb Forb
Yes Yes
Persicaria orientale
Polygonaceae
Forb
Yes
Polygonum plebium
Polygonaceae
Forb
Yes
Potamogeton tricarinatus
Potamogetonaceae
Aquatic forb
Yes
Ranunculus sp.
Ranunculaceae
Forb
Limosella australis
Scrophulariaceae
Forb
Solanum nigrum
Solonaceae
Subshrub
Verbena sp.
Verbenaceae
Forb
a
Yes
Yes Yes Yes
Yes Yes
Taxa not confirmed from herbarium samples because individuals were immature
variable added to the model, explaining a further 14.7 % of the variation, followed by depth (12.2 %) (Table 5). The addition of PL, SLD and WD did not result in an increase in Adjusted R2 and thus the variables were not included in the model. Seed bank assemblages A total of 36 taxa belonging to 25 families were identified in the germination experiments (Table 2). The most abundant taxa are Centipeda sp., Ammannia multiflora, Juncus sp., Cyperus difformis and Elatine gratioloides. The same taxa were also the most widespread. As for the extant vegetation, patterns in seedling germination showed substantial variation within depth, location and connectivity classes and within treatments (Fig. 5). Despite this variation, several significant class and treatment effects were evident. The number of seedlings germinating from sediment was strongly influenced by treatment (F = 33.824, p \ 0.001), with the greatest number germinating from the waterlogged treatment and substantially fewer seedlings germinating from the two submerged treatments (Fig. 5a). The size of the difference in seedlings with respect to treatment suggests that the pattern was not influenced by seedling removal during the course of the experiment. More seedlings also germinated
from shallow billabongs than from deep billabongs (F = 6.359, p = 0.04; Fig. 5a). However, there was also a significant three-way interaction evident between treatment, depth class and connectivity (F = 3.829, p = 0.034), whereby the strength of the treatment effect was strongest in shallow, high connectivity billabongs and in deep, low connectivity billabongs (Fig. 5a). This interaction suggests that the main effects reflect complex interactions between depth, treatment and connectivity. The number of taxa germinating from sediment samples also varied with treatment and depth class (Fig. 5b), with higher numbers of taxa germinating from water logged treatments (F = 34.45, p \ 0.001) and from shallow billabongs (F = 7.325, p = 0.03). Germinant assemblage diversity followed the same pattern, with higher values in waterlogged treatments (F = 26.466, p \ 0.001) and in shallow billabongs (F = 8.634, p = 0.022). Neither connectivity nor sampling location on their own influenced germinant assemblage diversity; however, there was evidence of an interaction between location and connectivity, whereby germinant assemblages from sediment sampled from submerged areas were more diverse in high connectivity billabongs (Fig. 5d), but germinant assemblages from sediment sampled from submerged areas were less diverse in low connectivity billabongs, although this effect was not significant (F = 4.154, p = 0.081).
123
Author's personal copy M. A. Reid et al.
influenced by connectivity, such that the difference was strongest in high connectivity billabongs (F = 4.237, p = 0.047). Moreover, a three-way interaction shows that both patterns were driven by the low Berger–Parker dominance of germinant assemblages in sediments sampled from the edge of deep, high connectivity billabongs (F = 4.478, p = 0.04) (Fig. 5e). None of the response variables showed evidence of strong spatial patterning (Moran’s I p [ 0.05). The germinant assemblages of deep and shallow billabongs differed for both square-root transformed abundance and presence-absence data (Table 3). The differences between the germinant assemblages of deep and shallow billabongs reflected the greater abundance of Limosella australis, Juncus spp., Elatine gratioloides, Centipeda spp. Cyperus difformis and Cyperus spp. germinating from the soils of shallow billabongs (Table 6). Differences between the germinant assemblages of high and low connectivity classes were not significant, although this is only marginally so for presence-absence data (Table 3). Germinant assemblages of high connectivity billabongs were characterized by higher abundances of Juncus spp. Centipeda spp., Elatine gratioloides, Ammannia multiflora and Cyperus spp. and lower abundance of Limosella australis compared to low connectivity billabongs (Table 6). No interaction effects were apparent for either square-root or presence-absence data (Table 3). The distance-based linear models showed that Depth explained 26.0 % of the variation in the germinant assemblages (abundance data) (Table 5). With step-wise addition of variables, Area and PL were the second and third variables added to the model, explaining a further 11.3 and 10.1 % of the variation respectively (Table 5). WD, CTF and SLD were the next variables added, explaining a further 8.3, 8.7 and 8.0 % of the variation respectively (Table 5; Fig. 6). The inclusion of Veg cover did not result in an increase in Adjusted R2 and thus the variable was not included in the model. For presence-absence data, Area explained the most variation (34.2 %) in the assemblage data. With step-wise addition of variables, Depth was the second variable added, explaining a further 14.1 % of the variation in the germinant assemblage, followed by CTF (11.1 %) and WD (8.6 %)(Table 5; Fig. 6). The inclusion of Veg cover did not result in an increase in Adjusted R2 and thus the variable was not included in the model. Fig. 3 Extant vegetation abundance (a), diversity (b) and dominance (c) by depth and connectivity (error bars are 95 % confidence intervals; L10 log 10 transformed)
Berger–Parker dominance of germinant assemblages was higher in sediments sampled from submerged locations (F = 4.369, p = 0.044); however, this pattern was
123
Comparison of extant and seed bank assemblages Relatively few taxa are present in both extant and seed bank assemblages (9 taxa; Table 2). This pattern is reflected in the clear differences between the extant plant and germinant assemblages (Fig. 7; Table 7). Significant
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… Table 3 PERMANOVA results comparing extant and germinant billabong plant assemblages across depth and connectivity classes using abundance (square-root transformed; Abund) and presence–absence (P–A) data. Statistically significant results are in bold
Assemblage
Transformation
Source
df
Pseudo-F
P (permutational)
Permutations
Extant
Abund
Connectivity
1
1.6557
0.0957
8635
Depth
1
2.4895
0.0077
8642
Co 9 De
1
1.1515
0.2695
7478
P–A
Germinant
Abund
P–A
differences were also evident in the combined data between the plant assemblages of deep and shallow billabongs and of low and high connectivity billabongs. A significant interaction was also evident between assemblage type (extant vs germinant) and connectivity. Two-way ANOVA comparing Bray–Curtis similarity measures returned for individual billabongs showed no difference by depth or connectivity, so it is likely that the interaction detected in PERMANOVA reflects the higher variation in the extant plant assemblages of low connectivity billabongs compared with that of germinant assemblages and the extant plant assemblages of high connectivity billabongs (Fig. 7).
Discussion The conceptual framework applied in this study proposed that hydrological connections influence billabong plant communities via three principal mechanisms. These were the capacity of connections to facilitate flux of propagules between habitats; the capacity of connections to deliver a water subsidy or stress; and, the capacity of connections to alter hydraulic conditions (in the sense that connection may introduce high flow velocities and turbulence). The framework further proposed that the influence of these mechanisms is modified by the depth of individual billabongs. We argue that these two factors potentially determine whether water acts as a subsidy or a stress; whether propagules are delivered to billabongs during connection phases; and, whether hydraulic conditions during connection phases are benign or harsh (Peckarsky 1983) with respect to the capacity of plants and propagules to persist and recruit. Based on this framework, connectivity should contribute to both the diversity and abundance of wetland plants. Moreover, the relative strength of relationships observed for extant and germinant communities should illustrate the
Connectivity
1
3.1929
0.0106
8625
Depth
1
2.2543
0.0563
8620
Co 9 De
1
0.87925
0.5126
7561
Connectivity
1
1.3514
0.1978
8661
Depth
1
3.2374
0.0041
8546
Co 9 De
1
1.2356
0.2319
7543
Connectivity
1
2.4719
0.0676
8624
Depth
1
2.8585
0.0336
8630
Co 9 De
1
0.92711
0.459
7530
respective influences of the mechanisms of propagule flux, resource subsidy and hydraulic disturbance. Results show that abundance of extant and germinant plants is not clearly influenced by frequency of connection, but some influence by connectivity on community composition is evident, particularly for extant communities (Tables 3, 5; Fig. 4). These findings suggest that while hydrological connection does not strongly influence the abundance of plants or propagules in these systems, it does influence the available species pool. In addition, the finding that diversity is higher among high connectivity billabongs in extant communities, but not germinant assemblages, suggests that the higher diversity and distinct composition of extant plant communities in high connectivity billabongs does not arise from connection-facilitated flux of propagules (hydrochory) (Bornette et al. 1998). This is because, if flux were an important mechanism, its effects would be expected to be most clearly manifested in germinant assemblages. This study did not distinguish functional groups of wetland plants (Brock and Casanova 1997). Therefore it is possible that the distribution of some functional groups across floodplain wetland habitats is influenced by watermediated flux of propagules. For example, the floating plants Lemna minor and Azolla were found in the extant and germinant assemblages of high connectivity billabongs, but not those of low connectivity billabongs (Table 4), suggesting that these plants do rely on hydrological connection to disperse. Our findings provide indirect evidence that the diversity and distinctiveness of plant communities in high connectivity billabongs reflect the effects of hydrological connectivity via the mechanisms of resource delivery and creation of hydraulic habitat. This is despite the likely importance of flux for floating plants. All high connectivity billabongs in the study were connected during the flow pulses that occurred 4 months and 10 weeks prior to sampling. These pulses thus delivered water subsidies to all
123
Author's personal copy M. A. Reid et al. Table 4 Results of SIMPER analysis of plant taxa contributions to the similarity within and between connectivity (low and high) and depth (deep and shallow) classes for extant plant assemblage abundance data (square-root transformed)
Av. abund Low
High
Deep
Shallow
Low v high
Deep v shallow
Av. sim
Sim/SD
Cum. %
Pseudoraphis spinescens
1.69
6.02
0.75
33.01
Ludwigia peploides
1.30
5.22
0.88
61.61
Paspalum distichum
0.52
4.69
0.42
87.30
Centipeda spp.
1.00
1.14
0.47
93.55
Pseudoraphis spinescens
3.91
12.00
2.80
32.88
Juncus spp.
2.16
6.62
1.78
51.01
Persicaria hydropiper
2.88
4.19
0.93
62.50
Cyperus exaltatus
2.18
2.35
1.02
68.95
Lemna minor
2.29
2.20
0.61
74.99
Pseudoraphis spinescens
0.95
6.93
0.49
37.11
Paspalum distichum
0.51
5.70
0.41
67.62
Ludwigia peploides
1.03
3.17
0.48
84.60
Juncus spp.
0.47
2.35
0.47
97.17
Pseudoraphis spinescens
4.16
15.14
3.72
41.20
Cyperus exaltatus Juncus spp.
2.22 1.73
3.73 3.55
1.05 0.74
51.34 61.00
Persicaria hydropiper
2.40
2.79
0.68
68.60
Ludwigia peploides
1.53
2.60
0.91
75.68
Persicaria orientale
1.21
2.11
0.77
81.43
Species
Low/deep
High/shallow
Av. Diss
Diss/SD
Cum. %
Pseudoraphis spinescens
1.69
3.91
10.46
1.36
12.74
Juncus spp.
0.32
2.16
7.40
1.22
21.75
Persicaria hydropiper
0.00
2.88
7.39
1.48
30.75
Cyperus exaltatus
0.40
2.18
5.79
1.11
37.81
Lemna minor
0.00
2.29
5.54
0.98
44.55
Ludwigia peploides
1.30
1.32
5.40
1.09
51.12
Pseudoraphis spinescens
0.95
4.16
12.64
2.02
14.79
Cyperus exaltatus
0.00
2.22
7.65
1.20
23.74
Persicaria hydropiper
0.00
2.40
6.81
1.20
31.71
Juncus spp. Ludwigia peploides
0.47 1.03
1.73 1.53
6.51 5.60
0.96 1.19
39.33 45.88
Lemna minor
0.00
1.91
5.05
0.84
51.79
Alternanthera denticulata
0.43
1.27
4.99
0.90
57.63
Persicaria orientale
0.00
1.21
4.47
1.09
62.86
Av. abund average abundance of taxon, Av. sim/diss average similarity within or dissimilarity between classes, Sim/SD and Diss/SD average similarity or dissimilarity divided by the standard deviation in similarity and dissimilarity values, Cum. % cumulative % contribution to total similarity or dissimilarity
high connectivity billabongs and created a range of habitats including submerged areas as well as waterlogged and damp marginal sediments suitable for germination of seeds as well as vegetative propagation from dormant mature plants and rhizomes. These synchronous connection events may also account for the relatively high similarity of the extant plant assemblages of high connectivity billabongs (Fig. 7). Having been stimulated by the connection events, the subsequent abundance of plants would then be more strongly influenced by factors mediated by depth (such as light availability, sediment chemistry and availability of
123
suitable shallow, marginal habitat), which may account for the apparently weaker relationship between connectivity and extant plant assemblages when abundance is considered (Table 3). The proposed framework assumed that any influence of connectivity would be modified by depth. The findings of this study suggest that this framework may need revision, at least in emphasis, to acknowledge a stronger influence of floodplain habitat water depth on both extant plant communities and seed banks. Plants are more abundant in shallower billabongs than deeper billabongs and this
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… Table 5 Results of nonparametric multiple regression of extant and germinant assemblage data using abundance (square-root transformed; abund) and presence–absence (P–A) data on individual environmental variables
Adjusted R2
Pseudo-F
P
Prop.
Cumul.
Assemblage
Transformation
Variable
Extant
Abund
?Depth
0.158
2.872
0.002
0.242
0.242
?PL
0.272
2.410
0.006
0.175
0.417
P–A
Germinant
Abund
P–A
?CTF
0.338
1.810
0.062
0.119
0.537
?CTF
0.204
3.566
0.006
0.284
0.284
?Area
0.288
2.066
0.101
0.147
0.431
?Depth
0.361
1.909
0.127
0.122
0.553
?Depth
0.177
3.156
0.002
0.260
0.260
?Area
0.217
1.453
0.123
0.114
0.373
?PL
0.249
1.341
0.204
0.101
0.474
?WD
0.263
1.132
0.347
0.083
0.558
?CTF
0.289
1.223
0.333
0.087
0.645
?SLD
0.310
1.154
0.373
0.080
0.724
?Area
0.269
4.673
0.01
0.342
0.342
?Depth
0.354
2.189
0.045
0.141
0.483
?CTF ?WD
0.420 0.466
1.907 1.609
0.114 0.201
0.111 0.086
0.594 0.680
Values reflect sequential, stepwise addition of variables, where variables are added according to the amount of variation explained (greatest first) and amount explained by each variable added to model is conditional on variables already in the model (i.e. those variables listed above it) Prop. proportion of variance in assemblage data explained by that variable, Cumul. cumulative proportion of variance explained
influence appears to extend to the abundance of germinable propagules in the sediments of these systems. Interestingly, depth appears to have less influence on diversity and the total species pool, particularly for the extant plant community, suggesting that the patterns associated with depth reflect differential recruitment success under shallow versus deep conditions rather than differences in the species pool of available propagules. Moreover, the finding that seed banks of shallower billabongs also appear to contain a greater number and diversity of germinable seeds than the seed banks of deeper billabongs, suggests that the recruitment success of extant communities in shallower billabongs results in the delivery of a greater diversity and abundance of seeds to the seed bank. The finding that water depth influences the recruitment success of wetland plants is not new (Bornette et al. 1994; Casanova and Brock 2000). Given the natural propensity of Australian lowland floodplain rivers to be highly turbid, (Kirk, 1985; Prosser, et al. 2001; Olley and Wasson 2003), it is likely that depth influences plant abundance principally via light limitation (Bornette et al. 1998; Porter et al. 2007). However, other mechanisms are also likely. As has also been shown in other studies of seed bank germination, the largest number and diversity of germinations occurred in this study under water logged conditions (Warwick and Brock 2003; Webb et al. 2006; James et al. 2007; Reid and Capon 2011). Germinations from the submerged treatments were few, and this result cannot be attributed to light
limitation, particularly given that germination was lowest in the water replacement treatment which removed algae that were potentially shading new germinants. Therefore, the lack of marginal, shallow habitat suitable for germination from the seed bank may also contribute to the lower plant abundance in deeper billabongs. The area of marginal habitat within the study billabongs was not directly measured in this study, so this interpretation needs to be tested in further studies examining the influence of physical habitat (for example, total area, shallow marginal area, slope and shoreline complexity) on extant and germinant assemblages in greater detail. Low plant and germinant abundances and diversity were observed at a single high connectivity billabong (Tar_B); however it is likely that this reflects the lack of suitable habitat at this site, which is deep with steep banks and likely to not contain abundant shallow, marginal habitat suitable for the establishment of plants or the accumulation of a germinable seed bank due to light limitation and scouring during connection phases (Bornette et al. 1998). Hydroperiod has previously been shown to be an important driver of wetland plant community composition (Brock and Casanova 1997; Casanova and Brock 2000). As noted above, the influence of hydroperiod was not tested directly, rather, it was assumed that any hydroperiod effect would arise from the interaction of depth and connectivity. Surprisingly, apart from the three-way interactions between connection and depth and treatment (number of seedlings)
123
Author's personal copy M. A. Reid et al. Fig. 4 Distance-based RDA ordination relating environmental variables to extant plant assemblage data, showing biplot projections for environmental variables. Analysis was performed on principal coordinate axes obtained from Bray–Curtis resemblance matrices of squareroot transformed (top) and presence-absence data (bottom). % of fitted and total variation refers to the percentage of the variability in the original data explained by the axis and the percentage of the variation in the fitted matrix (i.e. the fitted relationship between y and the explanatory variables) explained by the axis
and between connection, depth and location (Berger-Parker Dominance), no other depth-connectivity interactions were detected in the study, suggesting that either hydroperiod was not a strong driver of the observed patterns, or that hydroperiod itself is not a simple function of depth and connectivity. With regard to the latter, it is possible that local runoff during high intensity rainfall events may influence hydroperiod; however, the available topographic information is not sufficiently detailed to quantify this. Nevertheless, it seems unlikely that local runoff would exert a systematic influence sufficient to counter the
123
influence of dry phase duration and frequency of replenishment via connection events on hydroperiod (that is, there being higher rates of local runoff to shallow billabongs and/or less frequently connected billabongs). Accordingly, we suggest that the range in hydroperiod (or the depth 9 connectivity interaction) tested in the study was not large enough for it to exert a strong influence over and above that of depth and connection frequency. Although Depth appeared to exert the strongest influence over both extant and germinant assemblages, the distance-based linear modelling showed billabong area and
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a…
Fig. 5 Estimated marginal means for total number of seedlings (a), number of taxa (b), diversity (c, d) and dominance (e) by connectivity, depth, location and treatment. Error bars are 95 % confidence intervals
to a lesser extent perimeter length and woody debris each showed some relationship with billabong plant assemblages. In the case of Area and Perimeter Length the
relationship appears to reflect the similarity in the assemblages of the two very large billabongs, Pungbougal Lagoon and Maynes Lagoon (Fig. 7), which were each
123
Author's personal copy M. A. Reid et al. Table 6 Results of SIMPER analysis of plant taxa contributions to the similarity within and dissimilarity between connectivity (low and high) and depth (deep and shallow) classes for germinant assemblage abundance data (square-root transformed)
Class
Species
Low
Juncus spp.
3.83
6.08
1.18
19.32
Centipeda spp.
3.25
6.05
0.85
38.54 54.31
High
Deep
Av. abund
Sim/SD
Cum. %
Chamaesyce drummondii
1.31
4.96
1.42
Polygonum plebium
1.23
4.30
1.00
67.98
Cyperus spp.
1.44
1.93
0.67
74.13
Ammannia multiflora
1.74
1.84
0.58
79.97
Centipeda spp.
7.04
11.46
2.49
34.83
Juncus spp.
9.40
9.81
1.66
64.63
Cyperus difformis
2.80
2.10
1.08
71.01
Ammannia multiflora
1.84
2.07
0.96
77.29
Alternanthera denticulata
1.39
1.77
0.89
82.67
Centipeda spp.
3.05
10.23
1.07
29.29
Juncus spp.
1.99
8.40
0.86
53.34
Polygonum plebium
1.28
4.80
0.94
67.08
Chamaesyce drummondii Ammannia multiflora
1.09 1.72
4.66 2.43
0.96 0.59
80.43 87.39 93.96
Pseudognaphalium luteoalbum Shallow
Av. sim
1.53
2.29
0.60
10.00
11.39
2.30
28.43
Centipeda spp.
6.58
8.10
1.53
48.64
Elatine gratioloides
5.49
4.46
1.06
59.79
Cyperus difformis
3.00
3.37
3.31
68.19
2.81
2.50
1.07
74.42
Juncus spp.
Cyperus spp. Class
Species
Low v high
Juncus spp.
3.83
9.40
10.39
1.37
15.47
Limosella australis
9.78
3.24
10.28
0.64
30.77
Centipeda spp.
3.25
7.04
7.88
1.29
42.50
Elatine gratioloides
2.53
3.75
5.57
1.09
50.79
Ammannia multiflora
1.74
1.84
3.43
0.98
55.90
Cyperus spp.
1.44
2.28
3.42
1.08
60.99
Limosella australis Juncus spp.
0.20 1.99
12.32 10.00
12.16 12.12
0.63 1.50
16.84 33.64
Deep v shallow
Low/deep
High/shallow
Av. diss
Diss/SD
Cum. %
Elatine gratioloides
0.20
5.49
7.64
1.37
44.23
Centipeda spp.
3.05
6.58
7.38
1.21
54.45
Cyperus difformis
0.48
3.00
3.96
1.33
59.94
Cyperus spp.
0.63
2.81
3.70
1.24
65.07
Av. abund average abundance of taxon, Av. sim/Diss average similarity within or dissimilarity between classes, Sim/SD and Diss/SD average similarity or dissimilarity divided by the standard deviation in similarity and dissimilarity values, Cum. % cumulative % contribution to total similarity or dissimilarity
characterised by relatively depauperate extant and germinating assemblages, itself likely a reflection of a relative absence of suitable littoral habitat due to high turbidity caused by wind-driven re-suspension of sediment in these waterbodies with a large fetch. In the case of woody debris, the relationship with plant assemblages is not a strong one and is restricted to germinant assemblages (Table 5). Accordingly, the
123
relationship may reflect an effect of woody debris on the hydraulic environment, which, in turn may influence retention of seeds due to lower flow velocities at sites where woody debris is abundant (Pettit et al. 2005; Parsons et al. 2006). While other results suggest that fluxes of propagules between billabongs are not facilitated by hydrological connection to a significant degree; the relationship between WD and the germinant assemblages may
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… Fig. 6 Distance-based RDA ordinations relating environmental variables to germinant assemblage data, showing biplot projections for environmental variables. Analysis was performed on principal coordinate axes obtained from Bray–Curtis resemblance matrices of squareroot transformed (top) and presence-absence data (bottom). % of fitted and total variation refers to the percentage of the variability in the original data explained by the axis and the percentage of the variation in the fitted matrix (i.e. the fitted relationship between y and the explanatory variables) explained by the axis
reflect local, within billabong, water-mediated transport of propagules. Previous studies have noted the high levels of spatial variability in seed banks at similar spatial scales and attributed this variability to local hydraulic variation and its effect on seed movement (Webb et al. 2006; James et al. 2007). The variation in relation to location in germinant assemblages (Fig. 5), as well as the high levels of within-
class variation in seed banks and extant assemblages (Figs. 3, 5) also suggest that within billabong transport of propagules may be an important factor influencing the distribution of propagules in billabongs. The sampling design used in this study did not focus on within-site spatial variation in seed banks, so these interpretations are highly speculative and there is a clear need to explore within-site spatial variation in seed banks and the role of local
123
Author's personal copy M. A. Reid et al. Fig. 7 MDS ordination calculated from a Bray-Curtis resemblance matrix of combined extant and germinant assemblages using presenceabsence data
Table 7 PERMANOVA results comparing billabong plant assemblages (presence-absence data) across assemblage, depth and connectivity classes Source
df
Pseudo-F
P (permutational)
Permutations
Connectivity
1
3.0439
0.0121
9943
Depth
1
3.9225
0.0025
9937
Assemblage
1
9.2188
0.0001
9932
Co 9 De
1
0.46797
0.8573
9945
Co 9 As
1
2.3198
0.0401
9950
De 9 As
1
0.74386
0.618
9932
Co 9 De 9 As
1
1.2565
0.2692
9955
Statistically significant results are in bold
transport of propagules as a driver of within-site variation in future studies.
Conclusions The results of this study demonstrate that hydrological connection does seem to play an important role in structuring the billabong plant communities, influencing the number of taxa and the species pool of extant communities; however, this influence appears to occur largely via the mechanisms of water subsidy/stress and control of hydraulic conditions within the billabongs rather than hydrological connection-facilitated fluxes between habitats, which are not an important mechanism controlling wetland plant communities in the studied billabongs. The study also shows that depth appears to be an important structuring driver of plant communities in the billabongs possibly via its capacity to control light availability and influence germination cues.
123
These findings do not imply that connection between habitats is not important for floodplain plant communities. Rather the temporal variability of these systems means that habitats typically experience distinct wet and dry phases, both of which can impose stresses that are beyond individual species’ capacity to survive. Thus, plant communities in floodplain river ecosystems must rely heavily on connectivity to recolonise habitats once conditions become suitable. However, for billabong plant communities, it is likely that biological connectivity is provided by the temporal connectivity afforded by a persistent seed bank or other dormant propagules such as rhizomes and by spatial connectivity afforded by other vectors such as wind or waterfowl (Porter et al. 2007; Green et al. 2008) rather than by hydrological connection. The interpretations made in relation to the importance of flux, subsidy and disturbance as mechanisms by which hydrological connection influences floodplain wetland plant communities are based on indirect inference, so there is clearly a need to conduct further more detailed studies that are designed to test these mechanisms as well as other vectors directly. Nevertheless, we believe the results of this study highlight the need to consider the underlying mechanisms by which hydrological connectivity may influence plant and other biotic communities in floodplain ecosystems. Better understanding of the role of these mechanisms will advance our understanding of the role of connectivity generally, the effects of reductions in hydrological connectivity and our capacity to use control of hydrological connectivity as a tool to manage and restore floodplain wetlands. Acknowledgments This research was supported by a grant from Land and Water Australia to MCT. We thank all the landholders who allowed access to their properties for this research to be undertaken.
Author's personal copy Ecological significance of hydrological connectivity for wetland plant communities on a… We also thank Cassandra James, Erin Lennon, Rohan Rehwinkel, Katherine Holland and Joanne Boulos for assistance provided during many stimulating days in the field. Particular thanks also to Greg Falzon for his assistance in statistical analysis. Comments and advice from anonymous reviewers were much appreciated and greatly improved the manuscript.
References Amoros C, Bornette G (2002) Connectivity and biocomplexity in waterbodies of riverine floodplains. Freshw Biol 47:761–776 Anderson MJ (2001) A new method for non-parametric multivariate analysis of variance. Austral Ecol 26:32–46 Biggs BJF (1996) Hydraulic habitat of plants in streams. Regul Rivers: Res Manag 12:131–144 Boedeltje G, Bakker JP, Ten Brinke A, Van Groenendael JM, Soesbergen M (2004) Dispersal phenology of hydrochorous plants in relation to discharge, seed release time and buoyancy of seeds: the flood pulse concept supported. J Ecol 92:786–796 Bornette G, Arens MF (2002) Charophyte communities in cut-off river channels: the role of connectivity. Aquat Bot 73:149–162 Bornette G, Henry C, Barrat MH, Amoros C (1994) Theoretical habitat templets, species traits, and species richness—aquatic macrophytes in the Upper Rhone River and its floodplain. Freshw Biol 31:487–505 Bornette G, Amoros C, Lamouroux N (1998) Aquatic plant diversity in riverine wetlands: the role of connectivity. Freshw Biol 39:267–283 Brock MA, Casanova MT (1997) Plant life at the edge of wetlands: ecological responses to wetting and drying patterns. In: Klomp N, Lunt I (eds) Land and water: links and barriers. Frontiers in ecology: building the links. Elsevier Science, Oxford, pp 181–192 Casanova MT, Brock MA (2000) How do depth, duration and frequency of flooding influence the establishment of wetland plant communities? Plant Ecol 147:237–250 Dos Santos AM, Thomaz SM (2007) Aquatic macrophytes diversity in lagoons of a tropical floodplain: the role of connectivity and water level. Austral Ecol 32:177–190 Frisch D, Libman BS, D’Surney SJ, Threlkeld ST (2005) Diversity of floodplain copepods (Crustacea) modified by flooding: species richness, diapause strategies and population genetics. Arch Fur Hydrobiol 162:1–17 Gerard M, El Kahloun M, Mertens W, Verhagen B, Meire P (2008) Impact of flooding on potential and realised grassland species richness. Plant Ecol 194:85–98 Gordon ND, McMahon TA, Finlayson BL (1992) Stream hydrology—an introduction for ecologists. Wiley, Brisbane Green AJ, Jenkins KM, Bell D, Morris PJ, Kingsford RT (2008) The potential role of waterbirds in dispersing invertebrates and plants in arid Australia. Freshw Biol 53(2):380–392 Hamilton SK, Bunn SE, Thoms MC, Marshall JC (2005) Persistence of aquatic refugia between flow pulses in a dryland river system (Cooper Creek, Australia). Limnol Oceanogr 50:743–754 Hein T, Baranyi C, Herndl GJ, Wanek W, Schiemer F (2003) Allochthonous and autochthonous particulate organic matter in floodplains of the River Danube: the importance of hydrological connectivity. Freshw Biol 48:220–232 Holzel N, Otte A (2001) The impact of flooding regime on the soil seed bank of flood-meadows. J Veg Sci 12:209–218 Huryn AD, Riley RH, Young RG, Arbuckle CJ, Peacock K, Lyon G (2001) Temporal shift in contribution of terrestrial organic matter to consumer production in a grassland river. Freshw Biol 46:213–226
James CS, Capon SJ, White MG, Rayburg SC, Thoms MC (2007) Spatial variability of the soil seed bank in a heterogeneous ephemeral wetland system in semi-arid Australia. Plant Ecol 190:205–217 Jenkins KM, Boulton AJ (2003) Connectivity in a dryland river: short-term aquatic microinvertebrate recruitment following floodplain inundation. Ecology 84:2708–2723 Junk WJ, Bayley PB, Sparks RE (1989) The flood pulse concept in river-floodplain systems. In: Proceedings of the international large river symposium. Canadian special publication of fisheries and aquatic science, vol 106, pp 110–127 Kirk JT (1985) Effects of suspensoids (turbidity) on penetration of solar radiation in aquatic ecosystems. In: Davies BR, Walmsley RD (eds) Perspectives in southern hemisphere limnology. Springer, Berlin, pp 195–208 Leigh C, Sheldon F (2009) Hydrological connectivity drives patterns of macroinvertebrate biodiversity in floodplain rivers of the Australian wet dry tropics. Freshw Biol 54:549–571 Leyer I (2006) Dispersal, diversity and distribution patterns in pioneer vegetation: the role of river-floodplain connectivity. J Veg Sci 17:407–416 Morand A, Joly P (1995) Habitat variability and space utilization by the amphibian communities of the French Upper-Rhone floodplain. Hydrobiologia 300–301:249–257 Olley JM, Wasson RJ (2003) Changes in the flux of sediment in the Upper Murrumbidgee catchment, Southeastern Australia, since European settlement. Hydrol Process 17:3307–3320 Padial AA, Ceschin F, Declerck SAJ, De Meester L, Bonecker CC, Lansac-Toˆha FA, Rodrigues L, Rodrigues L, Train S, Velho LFM, Bini LM (2014) Dispersal ability determines the role of environmental, spatial and temporal drivers of metacommunity structure. PLoS One 9(10):e111227. doi:10.1371/journal.pone. 0111227 Parsons M, McLoughlin CA, Rountree MW, Rogers KH (2006) The biotic and abiotic legacy of a large infrequent flood disturbance in the Sabie River, South Africa. River Res Appl 22:187–201 Peckarsky BL (1983) Biotic interactions or abiotic limitations? A model of lotic community structure. In: Fontaine TD, Bartell SM (eds) Dynamics of lotic ecosystems. Ann Arbor Science, Ann Arbor, pp 303–323 Pettit NE, Naiman RJ, Rogers KH, Little JE (2005) Post-flooding distribution and characteristics of large woody debris piles along the semi-arid Sabie River, South Africa. River Res Appl 21:27–38 Porter JL, Kingsford RT, Brock MA (2007) Seed banks in arid wetlands with contrasting flooding, salinity and turbidity regimes. Plant Ecol 188:215–234 Pringle CM (2001) Hydrologic connectivity and the management of biological reserves: a global perspective. Ecol Appl 11:981–998 Pringle C (2003) What is hydrologic connectivity and why is it ecologically important? Hydrol Process 17:2685–2689 Prosser IP, Rutherfurd ID, Olley JM, Young WJ, Wallbrink PJ, Moran CJ (2001) Large-scale patterns of erosion and sediment transport in river networks, with examples from Australia. Mar Freshw Res 52:81–99 Reid MA, Capon S (2011) The role of the soil seed bank in vegetation response to environmental flows on a drought-affected floodplain. River Syst 19:249–259 Reid MA, Sayer CD, Kershaw AP, Heijnis H (2007) Palaeolimnological evidence for submerged plant loss in a floodplain lake associated with accelerated catchment soil erosion (Murray River, Australia). J Paleolimn 38:191–208 Reid MA, Ogden R, Thoms MC (2011) The influence of flood frequency, geomorphic setting and grazing on plant communities and plant biomass on a large dryland floodplain. J Arid Env 75:815–826
123
Author's personal copy M. A. Reid et al. Reid MA, Delong MD, Thoms MC (2012) The influence of hydrological connectivity on food web structure in floodplain lakes. River Res Appl 28:827–844 Riis T, Biggs BJF (2003) Hydrologic and hydraulic control of macrophyte establishment and performance in streams. Limnol Oceanogr 48:1488–1497 Sims NC, Thoms MC (2002) What happens when flood plains wet themselves: vegetation response to inundation on the lower Balonne flood plain. In: Dyer FJ, Thoms MC, Olley JM (eds) The structure, function and management implications of fluvial sedimentary systems IAHS publication no. 276. IAHS Press, Wallingford, pp 195–202 Southwood TRE, Henderson PA (2000) Ecological methods. Blackwell Science Publishers, Oxford Thoms MC (2003) Floodplain-river ecosystems: lateral connections and the implications of human interference. Geomorphology 56(3–4):335–349 Thoms MC, Parsons M (2003) Identifying spatial and temporal patterns in the hydrological character of the Condamine-Balonne River, Australia, using multivariate statistics. River Res Appl 19:443–457 Thoms MC, Sheldon F (2002) An ecosystem approach for determining environmental water allocations in Australian dryland river systems: the role of geomorphology. Geomorphology 47:153–168 Thoms MC, Southwell M, McGinness HM (2005) Floodplain-river ecosystems: fragmentation and water resources development. Geomorphology 71:126–138
123
Thorp JH, Thoms MC, Delong MD (2006) The Riverine Ecosystem Synthesis: biocomplexity in river networks across space and time. River Res Appl 22:123–147 Tockner K, Schiemer F, Ward JV (1998) Conservation by restoration: the management concept for a riverfloodplain system on the Danube River in Austria. Aquat Conserv 8:71–86 Tockner K, Pennetzdorfer D, Reiner N, Schiemer F, Ward JV (1999) Hydrological connectivity, and the exchange of organic matter and nutrients in a dynamic river-floodplain system (Danube, Austria). Freshw Biol 41:521–535 Van Den Brink FWB, Van Der Velde G, Bosman WW, Coops H (1995) Effects of substrate parameters on growth responses of eight helophyte species in relation to flooding. Aquat Bot 50:79–97 Van Eck W, Van De Steeg HM, Blom C, De Kroon H (2005) Recruitment limitation along disturbance gradients in river floodplains. J Veg Sci 16:103–110 Warwick NWM, Brock MA (2003) Plant reproduction in temporary wetlands: the effects of seasonal timing, depth, and duration of flooding. Aquat Bot 77:153–167 Webb M, Reid M, Capon S, Thoms M, Rayburg S, James C (2006) Are floodplain-wetland plant communities determined by seed bank composition or inundation periods? In: Rowan JS, Duck RW, Werritty A (eds) Sediment dynamics and the hydromorphology of fluvial systems. IAHS Press, Wallingford, pp 241–248
