Phreatophytic vegetation responses to groundwater depth in a drying mediterranean-type landscape
Does the plant hydrotype composition of phreatophytic vegetation alter with drawdown of the water table? Can alternative states of phreatophytic vegetation be identified, and do they persist despite a multi-decadal drying of a mediterranean-type landscape?
Field study, coastal sandy plain, Gnangara Groundwater System, south-western Australia.
We used a data set spanning 35 yr to determine alternative states of phreatophytic vegetation and how they relate to spatial variation and temporal change in hydrological habitat. Multivariate regression trees (MRT) were used to identify the range in depth to groundwater associated with vegetation states (characteristic hydrotype compositions). Logistic regression analyses were conducted to predict the proportional representation of each hydrotype. Hydrotype compositions of recent observations were correlated against modelled values derived from historic observations to test whether the relationship between vegetation states and supporting hydrological habitats remains valid after significant decline in the water table.
The MRT analyses for the historic and recent observations produced a main split criterion at 2.0 and 2.9 m groundwater depth, respectively, essentially dividing records into those dominated by hydrophytes or xerophytes. No hydrophytes were present at groundwater depths >10.5 m (>8.5 m for recent observations), where xerophytes and generalists were dominant. Only extreme groundwater depths were dominated by a single hydrotype class (hydrophytes or xerophytes), while intermediate clusters represented a transition across the hydrotype classes. The logistic regressions of the proportion of hydrotypes as a function of groundwater depth resulted in good fits for hydrophytes and xerophytes only.
Vegetation states were consistent over 35 yr of progressive increase in depth to water table. Distribution of alternative states along a gradient in groundwater depths was exponential and was reflected in the distribution of hydrophytes and mesophytes. The ranges of groundwater depths at which individual states occur suggest that as groundwater depth increases, the degree of habitat specificity decreases. The narrow range of groundwater depths within which hydrophytes and mesophytes are dominant suggests that small changes in depth could lead to transitions between states. With ongoing drying of the landscape, persistence of these vegetation states can be expected to diminish.