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Understanding the impacts of multiple climatic and edaphic factors on forest diversity, structure and biomass is crucial to predicting how forests will react to global environmental change. Here, we addressed how do forest structural attributes (i.e. top 1% big, top 25% big medium and small trees; in terms of tree height, diameter, and crown), species richness, and aboveground biomass respond to temperature-related and water-related climatic factors as well as to edaphic factors. By assuming disturbance as a constant factor in the study forests, we hypothesize that water-related and temperature-related climatic factors play contrasting roles whereas edaphic factors play an additional role in shaping forest diversity, structure and aboveground biomass in species-poor and structurally-complex forests. We used forest inventory and environmental factors data from 248 forest plots (moist temperate, semi-humid, and semi-arid) across 12 sites in Iran. We developed multiple linear mixed-effect models for each response variable by using multiple climatic and edaphic factors as fixed effects whereas sites as a random effect. Top 1% big, top 25% big, medium, and small trees enhanced with mean annual temperature but declined with water-related climatic (i.e. mean annual precipitation, cloud cover, potential evapotranspiration, and wet day frequency) factors, whereas soil texture (i.e. sand content) and pH were of additional importance. Species richness increased with precipitation and cloud cover but decreased with temperature, potential evapotranspiration, soil fertility and sand content. Aboveground biomass increased along temperature gradient but decreased with potential evapotranspiration, clay and sand contents. Temperature seemed to be the main driver underlying the increase in forest structure (i.e. diameter-related attributes) and biomass whereas precipitation did so for species richness. We argue that the impacts of multiple climatic factors on forest structural attributes, diversity and biomass should be properly evaluated in order to better understand the responses of species-poor forests to climate change., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Plant diversity supports multiple ecosystem functions, including carbon sequestration. Recent shifts in plant diversity in rangelands due to increased grazing pressure and climate changes have the potential to impact the sequestration of carbon in arid to semi‐humid regions worldwide. However, plant diversity, grazing intensity and carbon storage are also influenced by environmental factors such as nutrient availability, climate and topography. The complexity of these interactions limits our ability to fully assess the impacts of grazing on biodiversity–ecosystem function (BEF) relationships.We assessed how grazing intensity modifies BEF relationships by determining the links between plant diversity and ecosystem carbon stocks (plant and soil carbon) across broad environmental gradients and different plant growth forms. To achieve this, we surveyed 1493 quadrats across 10 rangelands, covering an area of 23,756 ha in northern Iran.We show that above‐ground carbon stocks increased with plant diversity across topographic, climatic and soil fertility gradients. The relationship between above‐ground carbon stocks and plant diversity was strongest for forbs, followed by shrubs and grasses. Soil carbon stocks increased strongly with soil fertility across sites, but aridity, grazing, plant diversity and topography were also important in explaining variation in soil carbon stocks. Importantly, above‐ground and soil carbon stocks declined at high grazing intensity, and grazing modified the relationship between plant diversity and carbon stocks regardless of differences in abiotic conditions across sites.Our study demonstrates that relationships between plant diversity and ecosystem carbon stocks persist across gradients of aridity, topography and soil fertility, but the relationships are modified by grazing intensity. Our findings suggest that potential losses in plant diversity under grazing intensification could reduce ecosystem carbon storage across wide areas of arid to semi‐humid rangelands. We discuss the potential mechanisms underpinning rangeland BEF relationships to stimulate future research. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

• Ecological mechanisms underlying species diversity and productivity are not mutually exclusive. • Energy-related climatic factors increased large-diameter trees. • Ecogeographic gradients in large-diameter trees increased aboveground biomass. • Available energy promoted big-sized trees for higher forest functioning. • Sites variation shaped forest diversity – biomass relationship. 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-structure 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. [ABSTRACT FROM AUTHOR]