Relaxation of species‐specific neighborhood effects in Bornean rain forest under climatic perturbation

Abstract

Abstract. Evidence of negative conspecific density dependence (NDD) operating on seedling survival and sapling recruitment has accumulated recently. In contrast, evidence of NDD operating on growth of trees has been circumstantial at best. Whether or not local NDD at the level of individual trees leads to NDD at the level of the community is still an open question. Moreover, whether and how perturbations interfere with these processes have rarely been investigated. We applied neighborhood models to permanent plot data from a Bornean dipterocarp forest censused over two 10-11 year periods. Although the first period was only lightly perturbed, a moderately strong El Nin˜o event causing severe drought occurred in the first half of the second period. Such events are an important component of the environmental stochasticity affecting the region. We show that local NDD on growth of small-to-mediumsized trees may indeed translate to NDD at the level of the community. This interpretation is based on increasingly negative effects of bigger conspecific neighbors on absolute growth rates of individual trees with increasing basal area across the 18 most abundant overstory species in the first period. However, this relationship was much weaker in the second period. We interpreted this relaxation of local and community-level NDD as a consequence of increased light levels at the forest floor due to temporary leaf and twig loss of large trees in response to the drought event. Mitigation of NDD under climatic perturbation acts to decrease species richness, especially in forest overstory and therefore has an important role in determining species relative abundances at the site.

Sapling growth rates reveal conspecific negative density dependence in a temperate forest

“…In a later study at BCI, Condit, Hubbell, and Foster () examined an abundant understory tree species, Faramea occidentalis , and found that small individuals grew more slowly when large conspecific neighbors were nearby. In a mapped tropical forest plot in Borneo, Newbery and Stoll () found that most overstory species showed strong negative effects on conspecific stem growth rates, but these patterns were weaker in a second census following an El Nino ENSO event. Finally, in a study of Microberlinia bisulcata from a tropical forest in Cameroon, Norghauer and Newbery () found that growth rates decreased with increasing conspecific adult basal area for some but not all juvenile classes.…”

Effect sizes and standardization in neighbourhood models of forest stands: potential biases and misinterpretations

“…By using neighbourhood models without distance decay and unstandardized input variables, in single‐species analyses, a negative relationship between CNDD and forest‐level abundance was found, at least for the first of the two 10‐year periods analysed (Newbery & Stoll ). Using no distance decay, yet standardizing before fitting their models, Lin et al .…”

“…It might then support the notion that fundamentally different density‐dependent processes are likely operating at the seedling as opposed to the small‐tree stage in tropical forest dynamics (Uriarte et al . ,b; Newbery & Stoll ).…”

“…In both cases, a positive correlation of b with abundance implies that less abundant, rare species have stronger CON effects – β is more negative – (as in Comita et al . ), whereas a negative relationship implies more abundant species have stronger CON effects (as in Newbery & Stoll ). Note, however, that possible correlations of b with abundance may be biased due to correlations of SD X (or additionally SD Y ) with abundance.…”

“…In an empirical study, Newbery & Stoll () showed negative effects of conspecific neighbours on absolute growth rate of medium‐sized trees. The argument was that reduced growth of an individual tree will – other factors being equal – translate into survivorship and fecundity reductions and hence affect species abundances.…”