Including tree spatial extension in the evaluation of neighborhood competition effects in Bornean rain forest.

David M Newbery,Peter Stoll

doi:10.1002/ece3.7452

Abstract

Classical tree neighborhood models use size variables acting at point distances. In a new approach here, trees were spatially extended as a function of their crown sizes, represented impressionistically as points within crown areas. Extension was accompanied by plasticity in the form of crown removal or relocation under the overlap of taller trees. Root systems were supposedly extended in a similar manner. For the 38 most abundant species in the focal size class (10–<100 cm stem girth) in two 4‐ha plots at Danum (Sabah), for periods P1 (1986–1996) and P2 (1996–2007), stem growth rate and tree survival were individually regressed against stem size, and neighborhood conspecific (CON) and heterospecific (HET) basal areas within incremented steps in radius. Model parameters were critically assessed, and statistical robustness in the modeling was set by randomization testing. Classical and extended models differed importantly in their outcomes. Crown extension weakened the relationship of CON effect on growth versus plot species’ abundance, showing that models without plasticity overestimated negative density dependence. A significant negative trend of difference in CON effects on growth (P2−P1) versus CON or HET effect on survival in P1 was strongest with crown extension. Model outcomes did not then support an explanation of CON and HET effects being due to (asymmetric) competition for light alone. An alternative hypothesis is that changes in CON effects on small trees, largely incurred by a drought phase (relaxing light limitation) in P2, and following the more shaded (suppressing) conditions in P1, were likely due to species‐specific (symmetric) root competition and mycorrhizal processes. The very high variation in neighborhood composition and abundances led to a strong “neighborhood stochasticity” and hence to largely idiosyncratic species’ responses. A need to much better understand the roles of rooting structure and processes at the individual tree level was highlighted.

Highlights

NEWBERY and STOLL1.1 | Current tree neighborhood modelOne of most important advances in estimating and understanding dynamics of trees within forest communities was made when statistical analysis and population modeling moved away from the application of species or guild parameter averages and replaced them with spatially explicit estimates (DeAngelis & Mooij, 2005; DeAngelis & Yurek, 2017)
Given that competition is a driving process of change in tree species abundance locally, differences in biomass, architecture, and ecophysiological traits between focal trees and neighbors will in part be determining forest dynamics (Chen et al, 2018, 2019)
This third concluding paper on the role of neighborhood effects on tree growth and survival in the lowland rain forest at Danum in Sabah builds directly on Stoll and Newbery (2005) and Newbery and Stoll (2013) by incorporating spatial extension to trees. It attempts to (a) reject the “default hypothesis” for conspecific effects in favor of a resource-based competition one and, where successful (b), reject the hypothesis that conspecific competition is largely for light in favor of the alternative that it is more for nutrients. This leads to a revision in how negative density dependence is seen to operate in tropical forests and its role in tree community dynamics, as well as a reconsideration of neighborhood stochasticity

Summary

| INTRODUCTION

One of most important advances in estimating and understanding dynamics of trees within forest communities was made when statistical analysis and population modeling moved away from the application of species or guild parameter averages and replaced them with spatially explicit estimates (DeAngelis & Mooij, 2005; DeAngelis & Yurek, 2017). Unraveling the causal nexus of system interactions (direct and indirect effects, reciprocation and feedback, time-lagged) is very complicated if the aim is to reduce a phenomenon such as the average conspecific effect of a species at population and community levels to a set of understandable mechanisms operating between individuals in space and time (Clark, 2007; Clark et al, 2010, 2011) Conspecific effects, if they are real, and not “by default,” might play a role in determining species composition in forests, but they do not necessarily need to be competitive or facilitative if they come about from a combination of spatial clustering (caused by dispersal) and stochastic environmental (climatic) variability (Newbery & Stoll, 2013). This leads to a revision in how negative density dependence is seen to operate in tropical forests and its role in tree community dynamics, as well as a reconsideration of neighborhood stochasticity

| MATERIALS AND METHODS

| DISCUSSION

Findings

| CONCLUSION