Linking a decade of forest decline in the south-west of Western Australia to bioclimatic change

The south-west of Western Australia has experienced severe and prolonged drought over the last three decades. This has coincided with forest declines and more recently (following the summer of 2010–2011) sudden stand mortality in the Northern jarrah forest. Over the same period the Southern jarrah and Southern karri forests remained unaffected. The bioclimatic linkage between these localised climatic events and forest responses is key to developing a predictive capability that permits timely interventionist management strategies. We looked at the temporal dynamics of three accessible bioclimatic parameters (monthly mean diurnal temperature range, monthly mean precipitation and an aridity index derived from evaporation data) that were spatially registered with forested areas known to have been affected by this shift towards dryer and hotter conditions. Changes in forest condition were determined by accessing the vegetation fractional-cover data set, freely available from the high temporal resolution satellite MODIS. This data set provided estimates of three vegetation-related indices, namely photosynthetic vegetation, non-photosynthetic vegetation and bare soil cover. Both the climatic variables and the vegetative response variables were spatially co-registered over each of the three selected forest areas and a time series analysis undertaken for each variable. From the observed trends, we identify a set of threshold values for each bioclimatic metric and the approximate time lag associated with observed notable deterioration in the vegetation cover metrics.