Abstract
Exploring diversity-function relationships across overstory and understory strata is crucial for understanding ecological mechanisms, biodiversity conservation and forest management. In this study, we aimed to investigate the relationships among species diversity and stand structure, environmental factors, leaf area index (LAI), and the growth rate of overstory and understory strata in a mixed-broadleaf Korean Pine Forest in Northeast China. We performed structural equation models (SEMs) to evaluate the effects of environmental factors on overstory and understory species diversity, stand structure, and growth rate; the effects of overstory and understory species diversity and stand structure (total basal area, stem density, and diameter at the breast height variation) on the LAI and their respective growth rates; and the effect of overstory stratum on understory stratum species diversity, stand structure, and growth rate. Our results showed that species diversity determined the growth rate for overstory strata, while stem density was appropriate to determine understory growth rate. The integrative SEM accounted for 13, 13, 11, 8, and 7% of the variance in overstory species diversity, understory absolute growth rate, overstory absolute growth rate, understory stem density, and LAI, respectively. The diversity of overstory species had a positive direct influence on overstory growth but a negative direct effect on understory growth. Environmental factors (i.e., soil nutrients and soil moisture) positively influenced the understory growth rate but negatively influenced the overstory growth rate. Increasing understory stem density may increase the LAI; simultaneously, the LAI supported the understory growth rate. In natural forests, an arrangement between the overstory canopy and the availability of environmental resources is ideal for controlling species diversity and maximizing the productivity of both the overstory and understory strata.