Increasing stand stature weakens the positive effects of tree richness and structural imbalance on aboveground biomass in temperate forests: The stand stature hypothesis

Abstract

Irrespective of whether niche complementarity or selection effects enhance aboveground biomass (AGB) due to tree diversity in natural forests, these effects are confounding due to the roles of tree structural imbalance (i.e., either individual tree height or diameter inequality). Here, we hypothesize that increasing stand stature weakens the positive effects of tree richness and structural imbalance on AGB (i.e., we term ‘the stand stature hypothesis’). Using data from 365 plots of natural temperate forest of the Karakoram mountains in the Gilgit-Baltistan region of Pakistan, we tested several piecewise structural equation models (pSEMs) to disentangle the relative impacts of topographic elevation, stand density and tree richness or structural imbalance on AGB across short-statured (mean tree height < 15 m at stand level) and tall-statured (height ≥ 15 m) forests, and across species mixture (i.e., dominated by either single- or multi-species). We found that tree richness and structural imbalance had significant positive effects on AGB in short-statured (β = 0.30 to 0.33, P < 0.001) as compared to tall-statured (β = 0.02 to −0.06, P = 0.635 to 0.088) stands, irrespective of species mixture. In contrast, tree structural imbalance (β = -0.11 to −0.12, P = 0.015 to < 0.001) rather than tree richness (β = 0.01, P = 0.949) declined AGB in multi-species tall-statured stands but these effects were positive (β = 0.01 to 0.32, P = 0.866 to < 0.001) in single-species stands. Meanwhile, increasing topographic elevation declined AGB (β = -0.09 to −0.42, P = 0.151 to < 0.001). We show that increasing stand stature declines the positive effects of tree richness and structural imbalance on AGB, irrespective of the ubiquitous positive influence of stand density on AGB (i.e., the stand stature effect). Thus, we suggest that tall-statured and short-statured forests should be treated separately during the work plan of forest management and biodiversity conservation.