Forest strata-dependent effects of vegetation attributes and soil nutrients on decadal changes in aboveground net carbon stock in two temperate forests

Abstract Vegetation attributes and edaphic properties are pivotal determinants for forest ecosystem functioning. However, how these factors determine the aboveground net carbon stock change (net C change) across forest strata remains poorly understood, especially for temperate forests. Here, we used repeat forest inventories, functional trait and soil nutrient data from large permanent plots in two old-growth temperate forests in northeast China to evaluate the effects of vegetation attributes (diversity, trait composition and forest structure) and soil nutrients on the net C change of overstorey and understorey trees. Using structural equation modeling, we found that forest structure was the strongest driver of net C change across forest strata in both forest types, which was positively related to stand density in the overstorey, and basal area in the understorey stratum. Neither diversity nor trait composition had any significant effect on the net C change in the overstorey. However, in the understorey stratum, diversity was positively associated with the net C change through niche complementarity effects. Due to understorey vegetation attributes, overstorey stand density had negative direct and indirect effects on net C change in the understorey. Diversity and trait composition in the overstorey were not significantly related to net C change in the understorey, but overstorey diversity increased the diversity, trait composition and forest structure of understorey trees. The soil phosphorus concentration had positive effects on the net C change in the overstorey, but negative effects in the understorey stratum. In addition, soil phosphorus concentration increased overstorey diversity but had no effect on understorey vegetation attributes. Our results showed that vegetation attributes and soil nutrients had different effects on the net C change in the overstorey and understorey strata. Resource filtering by overstorey trees affected the ecosystem functioning of the understorey stratum as well as its responses to soil nutrients. By separately analyzing the overstorey and understorey trees in complex natural forests, this study provides a better insight into the ecological mechanisms that play key roles in the aboveground carbon sequestration of temperate forests.