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

Authors


  • Co-ordinating Editor: Dr. Ingolf Kühn.

Dwyer, J. M. (corresponding author: j.dwyer2@uq.edu.au), Fensham, R. J. (rod.fensham@derm.qld.gov.au) & Buckley, Y. M. (y.buckley@uq.edu.au): The University of Queensland, School of Biological Sciences, Queensland 4072, Australia.
Fensham, R. J. & Fairfax, R. J. (russell.fairfax@derm.qld.gov.au): The Queensland Herbarium, Mt Coot-tha Road, Toowong, Qld 4066, Australia.
Buckley, Y. M.: CSIRO Sustainable Ecosystems, 306 Carmody Rd, Queensland 4067, Australia.

Abstract

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.

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