Stand structure determines aboveground biomass across temperate forest types and species mixture along a local-scale elevational gradient

Greater variation in stand structure (i.e., individual tree size variation and stand density) is thought to promote aboveground biomass (AGB) better than species richness due to the canopy packing by different sized trees in the forest community. Here, we hypothesized that the relationships amongst species richness, stand density, individual tree size variation and AGB are dependent on forest types but single-species dominated forest stands matter much for higher AGB through higher stand density and individual tree size variation. To do this, we analyzed the forest inventory data (i.e., tree diameter, tree height, and species names) from 365 forest plots (each 0.1 ha) which was taken at different elevational gradients of temperate forest types (i.e., dense pure conifer, dense mixed forests, sparse mixed forests, sparse pure broadleaved, sparse pure conifer, and dense pure broadleaved) as well as species mixture groups (single-species and multi-species stands) in the northern area (namely Gilgit-Baltistan) of Pakistan. We mainly used piecewise structural equation modeling for testing the fixed effects of elevation, species richness, individual tree size variation and stand density as well as the random effect of either forest types or species mixture on AGB, and then we used linear structural equation modeling to test the models across forest types and species mixtures. We found the consistent positive effect of stand density on AGB across whole data, forest types and species mixtures, albeit species richness also showed a positive effect in some of the forest types. Individual tree size variation had a greater positive effect on AGB in single-species dominated stands compared to multi-species dominated stands. Elevation declined AGB directly and indirectly via stand density. We conclude that variation in stand structures (stand density and individual tree size variation of single-species dominated stands) are key to higher AGB in most of the temperate forest types. The positive effects of stand density and species richness on AGB may provide support to the niche complementarity effect through stand packing, whereas the greater effect of individual tree size variation on AGB in single-species rather than multi-species dominated stands may also confirm the selection or competitive exclusion effect. Therefore, we argue that both of the niche complementarity and selection effects are acting simultaneously to determine the relationships amongst species richness, stand structure and AGB in the studied temperate forests.