Abstract
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•Tree-size dimension inequality boosted overstorey AGC stock better than species diversity.•Overstorey drivers overruled understorey drivers for predicting understorey AGC stock.•High AGC stock of overstorey rather than understorey was found on a gentle slope at high elevation.•High species diversity across strata was found on steep slopes at low elevation.
It is generally well-explored that overstorey stratum regulates understorey stratum through the dominant role over the available resources in forests. However, the relationships amongst topography, soil nutrients, species diversity, tree-size dimension inequality and aboveground carbon (AGC) stock across forest strata remain debated in forest ecosystems. Here, we assumed that soil nutrients, species diversity and tree-size dimension inequality promote AGC stock in overstorey stratum, which in turn regulate AGC stock in understorey stratum in natural temperate forests. To do so, using several structural equation models (SEMs) on the data from 104 temperate forest plots in Northern Iran, we assessed the effects of tree-size dimension inequality as well as the individual effects of either inequality in tree diameter, height, basal area, crown volume, or crown area, species diversity, soil nutrients and topographic factors on AGC in each forest stratum and at whole-community level as well as the effects of overstorey on understorey (i.e., joint SEMs). Results show that AGC stock in overstorey stratum was mainly determined by tree-size dimension inequality rather than by species diversity on nutrient-rich soils but controlled by topographic factors related to light availability. In understorey stratum, only basal area inequality was the strong driver of AGC stock, but overstorey overruled the effects of understorey drivers on understorey AGC stock. Specifically, overstorey tree-size dimension inequality increased but AGC stock decreased the AGC stock of understorey. The observed mechanism and effects at overstorey stratum were nearly similar to whole-community level. This study shows that overstorey stratum regulates understorey through dominant occupancy on the light availability and soil nutrients. We suggest that integrative modeling of overstorey and understorey strata based on tree height, diameter and crown inequality may better enhance our understanding and extend the existing debate based on multiple drivers of forest functioning, which may help to provide specific guidelines for forest management.