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.