Quantifying the hydrological niche of swamp vegetation communities using indicator species

Indicator species are typically used to infer the presence of other biota or specific environmental conditions. Here we use indicator species to quantify the hydrological niche of their corresponding vegetation communities within Coastal Upland Swamps of the Sydney Basin Bioregion, Australia. Swamp vegetation organizes naturally into five recognised communities, thought to occupy distinct positions along a hydrological gradient. However, longwall mining reduces hydro-period, potentially impacting the observed vegetation. We modelled the hydrological niche for each community using the relative frequency of 20 vascular plant indicator species (four for each community) and time series soil moisture data from 11 unmined sites across four swamps. We then used this model to predict indicator species frequencies in 3 mine-impacted sites. Indicator species were modelled as a function of the average number of days-per-year that swamps remained saturated at the base of the root zone (30 cm below surface). Hydrological niches were well differentiated for four communities with the estimated optimal mean annual days-per-year saturated ranging from Restioid heath at 16 [<16, 29] (mean {+/-} [95% uncertainty]) to Ti-tree thicket at 352 [322, >353]. Banksia thicket 96 [65, 154] and Cyperoid heath 257 [204, 297] communities were intermediate. However, Sedgeland indicator species showed limited variation with changing saturation, and their hydrological niche remains unclear. The model under-predicted the frequency of Cyperoid heath and over-predicted Banksia thicket indicator species in mine-impacted sites, suggesting vegetation is not yet in equilibrium with hydrology. Results suggest indicator species can provide a reliable basis for determining the hydrological niche of wetland plant communities, which can in turn predict community-level impacts of hydrological change.