Neighbourhood abundance and small-tree survival in a lowland Bornean rainforest

Abstract

Conspecific effects of neighbours on small-tree survival may have a role in tree population dynamics and community composition of tropical forests. This notion was tested with data from two 4-ha plots in lowland forest at Danum, Sabah (Borneo), for a 21-year interval (censuses at 1986, 1996, 2001, 2007). Species with ≥45 focal trees 10 to <100 cm stem girth per plot in 1986 were selected. Logistic regressions fitted mean focal tree size and mean inverse-distance-weighted basal area abundance of neighbours (within 20 m), for the periods over which each focus tree was alive. Coefficients of variation of neighbourhood basal area abundance, both spatially and temporally, quantified the changing environment of each focus tree. Fits were critically and individually evaluated, with corrections for spatial autocorrelation. Conspecific effects at Danum was generally very weak or non-existent: species’ mortality rates varied also across plots. The main reasons appear to be that (1) species were not dense enough to interact despite frequent although weak spatial aggregation, and their neighbourhoods were highly differing in species composition; and (2) these neighbourhoods were highly variable temporally, meaning that focus trees experienced stochastically fluctuating neighbourhood environments. Only one species, Dimorphocalyx muricatus, showed strong conspecific effects (varying between plots) which can be explained by its distinct ecology. This understorey species is highly aggregated on ridges and is drought-tolerant. That this functionally and habitat-specialized species, has implied intraspecific density-dependent feedback in its dynamics is a remarkable indication of the overall processes maintaining stability of the Danum forest.