Resilience of phreatophytic vegetation to groundwater drawdown: is recovery possible under a drying climate?

Banksia (Proteaceae) woodlands are one of a number of groundwater-dependent ecosystems in southwestern Western Australia that are threatened by groundwater abstraction. In addition to this threat is an ongoing decline in regional water tables due to a drying climate. We used ecological resilience theory to analyse and interpret a long-term vegetation monitoring dataset from a site that has experienced an abstraction-induced acute groundwater drawdown in the late 1980s and early 1990s. Despite reduced plant abundance, all dominant over-and understorey species were still found on all transect plots in which they were recorded pre-drawdown. This suggests a notional resilience and a strong likelihood of recovery, in the event that pre-drawdown ecohydrological habitat conditions were to return. However, since the drawdown event, the regional water table continued to decline, with the vegetation responding through progressive and uni-directional change in abundance and composition. The change in composition was primarily manifested as a shift towards non-woody, shallow-rooted species not dependent on specific hydrological conditions. This slow, progressive change in hydrology associated with reduced rainfall and land use changes has continued to force a transition in the floristics towards an alternative ecohydrological state. Despite the absence of an acute drawdown event, the same progressive floristic response was also observed at two reference sites that were not under the immediate influence of production bores. The challenge for adaptive water resource management will be to enhance the capacity for resilience in these groundwater-dependent ecosystems in a drying environment through appropriate regulation of groundwater abstraction. Copyright (C) 2010 John Wiley & Sons, Ltd.