The importance of water regimes operating at small spatial scales for the diversity and structure of wetland vegetation

Abstract

1. In most cases, the most important determinant of wetland vegetation is the water regime. Although water regime is usually described and managed at the scale of whole wetlands, the patterning of vegetation is likely to be determined by water regimes that are experienced at much finer spatial scales. In this study, we assess the significance of internal heterogeneity in water regimes and the role that this heterogeneity plays in vegetation patterning. 2. The effects of water regime on wetland plant species richness and vegetation structure were studied at Dowd Morass, a 1500 ha, Ramsar-listed wetland in south-eastern Australia that is topographically heterogeneous. Data on plant variables and water depth were collected along 45 (50 m) transects throughout the wetland and related to water regimes assigned individually for each transect. Wetland plants were assigned to plant functional groups (PFG) that describe the response of plants to the presence or absence of water at different life stages. 3. The classification of water depth data indicated four distinct water regimes in the wetland that were differentiated primarily by the duration of the dry period. Representatives of all PFGs co-existed over small spatial scales where topographical variation was present, and the richness and cover of understorey species declined as transects became more deeply and permanently flooded. Some PFGs (e.g. amphibious fluctuation tolerator-low growing and amphibious fluctuation responder-morphologically plastic) were eliminated by extended periods of flooding, which increased the cover but not richness of submerged plants. Species richness and foliage projective cover declined as water regimes shifted from shallow and frequently exposed conditions to regimes typified by deeper and longer inundation. Cover of the structurally dominant woody species was compromised by deeply flooded conditions but vegetative regeneration occurred despite high water levels. 4. Internal topographical variation generates mosaics of water regimes at fine spatial scales that allow plant species with different water regime requirements to co-exist over small distances. Deep water and an absence of dry periods result in decreased cover of plants and an overall loss of species richness in the understorey. Water regimes are described that promote regeneration and cover of structurally dominant taxa and increased species richness in the understorey. The study demonstrates a strong association between vegetation and the diverse water regimes that exist within a single wetland, a pattern that will be useful for modelling the effects of modified water regimes on wetland vegetation.

Topographically controlled soil moisture is the primary driver of local vegetation patterns across a lowland region

“…It consistently influenced the plant's average preference for moisture (EIV moist ), resembling previous studies, which have observed strong relationships between local topography and the soil moisture affinity of different plant species (Vivian-Smith 1997, Silvertown et al 1999, Øk-land et al 2008, Raulings et al 2010. Topography was also strongly and consistently linked to the most important local floristic and functional gradients (DCA and PCA), as expected from niche-based species sorting (Leibold et al 2004).…”

“…These results demonstrate that topography structures local plant species richness patterns not only in saline (Moeslund et al 2011) and freshwater wetland plant communities (Silvertown et al 1999, Øk-land et al 2008, Raulings et al 2010), but also in other habitat types. The present study is the first to identify a link between local species richness and topography in dry habitats generally.…”

“…Most studies addressing the local effects of elevation on plant communities in lowland areas have focused on wet habitats (Vivian-Smith 1997, Silvertown et al 1999, Økland et al 2008, Raulings et al 2010, Moeslund et al 2011. In these settings, even cm-scale topographic heterogeneity can cause major differences in water availability, salinity and flooding frequency that may in turn impact species richness and composition (Vivian-Smith 1997, Silvertown et al 1999, Økland et al 2008, Moeslund et al 2011.…”

Hydroecological model predictions indicate wetter and more diverse soil water regimes and vegetation types following floodplain restoration

“…[32] The generation of mosaics of water regimes leading to a higher diversity of plant community types has been documented in other wetlands [Bornette et al, 1998;Courtwright and Findlay, 2011;Raulings et al, 2010;Tabacchi et al, 1998] and experimentally [Vivian-Smith, 1997]. Riparian zones increase regional biological diversity by supporting different species pools compared to uplands [Sabo et al, 2005] and are generally more diverse at the site scale .…”

“…Specifically, development of these types of predictive ecological tools is seen as critical for filling in knowledge gaps and reducing uncertainty associated with stream-floodplain restoration activities [Palmer and Bernhardt, 2006;Wohl et al, 2005]. The ability to predict vegetation composition at the spatial scales offered by hydrological models is especially helpful in wetlands where diverse water regimes lead to heterogeneity in vegetation patterning over small spatial scales [Raulings et al, 2010;Tabacchi et al, 1998]. Methods associated with predictive habitat distribution modeling that relates field observations of vegetation to environmental predictor variables have substantially improved in recent decades [Guisan and Zimmermann, 2000;Guisan and Thuiller, 2005] and may provide an opportunity to predict the impacts of various restoration or conservation measures.…”