Wetland vegetation – hydrology co-evolution in response to rainfall variability

Abstract

Nonlinear interactions between physical, chemical and biological factors determine the spatio- temporal extent of flooding, the level of salinisation and the vegetation dynamics in wetlands subjected to a certain climate signal. These interactions were studied using an ecohydrological model of a wetland in a semi-arid climate designed to account for the tolerance of vegetation functional groups to salinity and water availability. In particular, the model represents Melaleuca strobophylla and Casuarina obesa, as well as terrestrial short-rooted grasses, to represent those typically found in Lake Toolibin, a Ramsar appointed wetland of south-west Western Australian (SWWA). In a previous study, the model showed a good agreement when compared against available field data from Lake Toolibin. In this study, we explored specifically how variability in rainfall delivery can affect salt mobilization and subsequent vegetation abundance and assemblage. In order to test this, the model was tested under a range of rainfall intra-annual distribution with the same annual depth. Being particularly interested in semi-arid regions, the rainfall realisations were synthetically generated by a model previously calibrated to represent the precipitation typical of SWWA. The model demonstrated the co-evolution between hydrology and vegetation, as well as the non-linear responses of vegetation dynamics to climate forcing, both being strongly influenced by salinisation. A higher rainfall intensity enhanced runoff, raised the water table level and decreased salt leaching, intensifying accumulation of salt in the root zone. This altered salt mobilization affected vegetation abundance, water uptake and significantly changed to the vegetation assemblage. The short-rooted, terrestrial-adapted C. obesa benefited from a rainfall signal that was more evenly distributed over the year, while M. strobophylla benefited from more intense rainfall events that cause water to pond for prolonged periods. This exercise highlighted the fact that salinity amplifies the impact of climate variability, significantly affecting both the overall vegetation density and assemblage. This fact reinforces the need to include salinisation processes within ecohydrological models used to study vegetation dynamics in semi-arid regions.