Environmental filtering, predominance of strong competitor trees and exclusion of moderate-weak competitor trees shape species richness and biomass

Stand density of co‐existing species regulates above‐ground biomass along a local‐scale elevational gradient in tropical forests

“…In this study, we consider the elevational gradient in species richness resulted in a decline in AGB, and hence, we did not strongly support the niche complementarity effect. We found support for selection or the environmental filtering effect because increasing elevation resulted in an increase in AGB indirectly via species richness (Jucker et al, 2018; Ali et al, 2020; Chun et al, 2020). However, we argue that higher species richness resulted in an increase in AGB at low elevation, and hence, provided support for the niche complementarity effect, but the mechanism was not very clear along this elevational gradient.…”

“…In addition, stand density and species richness were found to be higher at the lower elevations whereas the opposite was true for tree height variation, indicating the role of local‐scale abiotic conditions in regulating forest structure, diversity and AGB. More specifically, the indirect effect of elevation on AGB via stand density was negative but this indirect effect was positive via species richness, indicating the role of environmental filtering along a local‐scale elevational gradient (Toledo et al, 2012; Ali et al, 2020; Rodrigues et al, 2020). The direct negative effect of elevation on AGB is consistent with the findings of several previous studies which showed that AGB tends to decline along increasing gradients of elevation in tropical forests (Raich et al, 1997; Kitayama & Aiba, 2002; Álvarez‐Yépiz & Dovčiak, 2015).…”

“…CFI = 1.000, GFI = 0.999, SRMR = 0.022, Chi-square = 0.315, p-value = 0.574. Comparison of direct (dark bars) and indirect (grey bars) effects (b), derived from structural equation model, of elevation, stand density, Simpson's dominance, tree diameter variation, and tree height variation on AGB found that higher stand density but lower species diversity resulted in higher AGB which can be attributed to the selection effect or competitive exclusion mechanism rather than the niche complementarity effect (i.e., the increase in species co-existence can increase ecosystem functioning; Grime, 1973;Tilman, 1997;Poorter et al, 2015;Ali et al, 2020). More, the presence of highly functioning or productive species, e.g., Shorea robusta, in this tropical forest whereas the presence of Pinus roxburghii in our subtropical coniferous forest might be due to selection effects.…”

“…In this study, we F I G U R E 4 Structural equation model (a) for linking elevation, stand density, Shannon species diversity, tree diameter variation, tree height variation, and AGB. See Appendix S1: Table S4 for found that higher stand density but lower species diversity resulted in higher AGB which can be attributed to the selection effect or competitive exclusion mechanism rather than the niche complementarity effect (i.e., the increase in species co-existence can increase ecosystem functioning; Grime, 1973;Tilman, 1997;Poorter et al, 2015;Ali et al, 2020). More, the presence of highly functioning or productive species, e.g., Shorea robusta, in this tropical forest whereas the presence of Pinus roxburghii in our subtropical coniferous forest might be due to selection effects.…”

“…In such cases, strong competitors may exclude weak competitors in high-AGB stands, and hence negative relationships between species diversity and AGB are expected under the assumption of a competitive exclusion mechanism (Grime, 1973;Ali et al, 2016). However, the effects of niche complementarity and competitive exclusion on AGB can be moderated by stand density (i.e., canopy packing) and large-diameter trees, because higher stand density is related to both higher species diversity and greater AGB in relation to large-diameter trees (Chiang et al, 2016;Ali et al, 2020). Species diversity and stand structural complexity are not only the predictors for explaining variation in AGB, but they also respond to abiotic factors and hence play the role of mediators for linking AGB to abiotic factors (Jucker et al, 2018;Chun et al, 2020).…”