Drought-associated heatwaves: Consequences for vegetation of the Northern Jarrah Forest

Worldwide, extreme climatic events such as drought and heatwaves are associated with forest mortality. While many studies address the immediate impacts of forest die-off events, information about the ongoing structural and compositional consequences is lacking. The Northern Jarrah (Eucalyptus marginata) Forest of southwestern Australia experienced a severe tree die-off event during a drought-associated heatwave. Using transects spanning a gradient of drought impacts (severe, transitional, minimal), tree species mortality in relation to soil depth and stand condition (stand basal area, stem density) was quantified. Differential mortality was exhibited by the two dominant overstorey species and two midstorey tree species, which occurred in distinct zones along the transects. The dominant, structural forest species Eucalyptus marginata (jarrah), had higher mortality than the co-dominant Corymbia calophylla and midstorey species Allocasuarina fraseriana. The midstorey tree (Banksia grandis) exhibited the highest stem mortality and a substantial shift in structure in response to the drought-associated heatwave in relation to tree size, soil depth and stand basal area. Banksia grandis represents a drought intolerant species. Regeneration of tree species was surveyed over three years and was greater in areas of high drought-impact than in healthy forest. To determine the response of understorey communities to the droughtassociated heatwave and subsequent changes in forest canopy, the abundance, composition and structure of understorey species was compared among drought-impact zones. In areas of high drought-impact, understorey communities showed different species assemblages and higher richness and cover than in healthy forest. The recovery of tree canopy following the event was studied by examining the temporal response of Eucalyptus marginata (jarrah) trees in zones of drought-impact and healthy forest using dendrometer bands. Trees growing in the high drought-impact zone had greater stem-diameter growth than trees in adjacent healthy forest. This study provides valuable insights into the consequences of a droughtassociated heatwave in driving structural and compositional changes in jarrah forest vegetation and highlights the importance of studying all components of the vegetation following forest mortality events