Neighbourhood effects influence drought-induced mortality of savanna trees in Australia

Questions The following hypotheses of neighbourhood effects on drought-induced mortality are evaluated: (A) drought-induced stem death is randomly distributed in space, (B) stems are predisposed to drought-induced death through negative density-dependent effects and (C) stems are predisposed to drought-induced death due to local deficits in plant available resources. Location Central Queensland, Australia. Methods Recent mass mortality of woody stems was surveyed and mapped in three 1.21-ha quadrats within Eucalyptus melanophloia-dominated savanna. A multi-faceted analytical approach was adopted including spatial pattern analyses, two logistic regressions of neighbourhood density effects on survival and spatial autocorrelation analyses of model residuals. Results Mortality was concentrated in stems < 15-cm diameter at breast height (DBH). Survival was aggregated or random in quadrats 1 and 3 and random o regular in quadrat 2. Small neighbour density had a negative effect on survival in all quadrats. In addition, the second model identified a positive relationship between survival and living neighbour density in quadrat 3 (indicating a resource patch effect), but a negative relationship in quadrat 2 (density effect). Analysis of model residuals showed that neighbour density explained mortality equally well across quadrat 2, but not across quadrats 1 and 3. Conclusions There was evidence in support of hypotheses B (neighbour density) and C (resource heterogeneity). We found strong support for an interaction between microsite quality and neighbourhood stem densities, and suggest that this interaction is driven by plant available water.