Eucalyptus forest shows low structural resistance and resilience to climate change-type drought

QuestionsClimate change-type drought (the combination of drought and heatwave) has become a widely documented driver of forest dieback yet, to date, limited measurement of post-event forest dynamics has been reported. Can climate change-type drought trigger structural and/or compositional changes in a forest type which is usually highly resilient to other disturbances? LocationMixed Eucalyptus forest in southwestern Australia. MethodsForest areas that were severely and minimally affected by drought were measured repeatedly at 3, 6, 16, 26 and 49mo post-event for changes in forest structure and composition of the two dominant tree species (Eucalyptus marginata/ Corymbia calophylla). Means and dispersal from pre-drought conditions were analysed for each structural variable among drought severity classes and between measurement periods. Resprouting, the predominant resilience mechanism in Eucalyptus, was assessed at 6 and 16mo, while regeneration type and density were determined at 16mo post-drought. ResultsStructural changes were observed after 49mo on plots severely but not minimally affected by drought, including a 30% increase in stem density (P<0.0001) and reduction in tree diameter (23%; P<0.01), basal area (33%; P<0.0001), canopy height (40%; P<0.0001) and live biomass (36%; P=0.01). On severely affected plots, all structural variables plateaued at levels different from pre-drought conditions. Large, old trees on severely affected plots were replaced by high densities of small stems (1-10cm DBH). Resprouting among drought affected trees (P<0.001) and tree regeneration (P=0.02) were higher on severely affected plots. No significant shifts in the proportional abundance of the two dominant species, E. marginata and C. calophylla, were observed for structural attributes or regeneration after 49mo. ConclusionClimate change-type drought can cause structural shifts in a resprouting mediterranean-type forest, providing evidence for a shift to an alternate state, particularly with repeated disturbance. While the study area showed low structural resistance and resilience, tree species composition was resilient to change, as E. marginata is likely to remain dominant in the future due to its resprouting capacity. Findings support the view that climate change-type drought will drive replacement of large trees with short, multi-stemmed individuals, transforming ecosystem structure.