Effects of drought induced canopy collapse on vegetation structure and composition in a biodiversity hotspot

Mediterranean biomes represent five biodiversity hotspots worldwide and cover just 2% of the Earth’s land area; however, they support 20% of the Earth’s known plant diversity. The Jarrah forest of South West, Western Australia, represents one of these valuable biomes. Documented climate change in Western Australia shows 35 years of consistently less rainfall and higher temperatures, with an extreme drought event in 2011 resulting in mortality of 1.7% or 19,000 ha of Northern Jarrah forest. To assess drought impact on vegetation of the Northern Jarrah forest, sites were chosen with a steep gradient from areas of canopy collapse to healthy forest. This provided an opportunity to assess the forest vegetation across an ecotone using space for time substitution. Our objectives were: 1) to determine factors affecting drought /heatwave induced vegetation mortality and 2) to assess differences in species response to this event. Results from this study show mortality of species occurs in zones on sites of drought induced canopy collapse in the Northern Jarrah Forest. Midstorey mortality is determined by soil depth. Results, indicate a complex interaction between plants with differing root functional traits and soil depth, determines the collapse response in an extreme drought event. There is an ongoing shift in the species composition and structure of the forest in and around areas of drought induced canopy collapse. We anticipate the use of presence /absence of midstorey, as a proxy for soil depth, and stand composition to predict areas of forest likely to collapse in the future.