Abstract
The macro-ecological mechanisms, underlying species diversity and productivity are not mutually exclusive, but our understanding is limited to the climatic variation in species richness and productivity along ecogeographical gradients. Yet, the ecogeographical gradients in a few large-diameter trees are not clearly tested on the linkage amongst forest diversity-structure attributes and aboveground biomass in natural forests. Here, we hypothesized that energy-related climatic factors (i.e. available energy) promote few big-sized trees compared to species or functional diversity or remaining-diameter trees, thereby explains the linkage between forest diversity-scructure attributes and aboveground biomass (i.e. the 'big-trees - energy hypothesis'). We used a piecewise structural equation modelling on 238 forest plots from three different forest or vegetation types, i.e., moist, temperate, semi-humid, and semi-arid, across 12 sites in Iran, in order to test the hypothesized causal relationships amongst geo-topographic, climatic factors, edaphic factors, top 1% large-diameter trees, forest diversity-structure attributes (stand density, species richness or functional diversity or 99% remaining-diameter trees, and functional dominance) and aboveground biomass. Energy-related climatic factors increased large-diameter trees and functional dominance, but not species richness, functional dispersion, remaining-diameter trees, and stand density. Climatic factors generally increased species richness, functional dispersion, stand density, and remaining-diameter trees via ecogeographic gradients in large-diameter trees. Ecogeographic gradients in large-diameter trees and functional dominance overruled the effects of species richness, functional dispersion and remaining-diameter trees on aboveground biomass, and hence providing support to the big-sized trees effect. Our results suggest that positive effects of energy-related climatic factors (i.e. available energy) promote big-sized trees which in turn enhance aboveground biomass, and hence forest functioning. However, all these underlying ecological mechanisms and drivers were highly dependent on sites variation, and hence context-dependent. We argue that the ecogeographic gradients and sites variation in big-sized trees should be included into the integrative ecological modeling for better understanding the species - energy mechanism as well as the diversity -functioning relationship in large-scale natural forests.