Multiple abiotic and biotic pathways shape biomass demographic processes in temperate forests

Abstract

Forests play a key role in regulating the global carbon cycle, and yet the abiotic and biotic conditions that drive the demographic processes that underpin forest carbon dynamics remain poorly understood in natural ecosystems. To address this knowledge gap, we used repeat forest inventory data from 92,285 trees across four large permanent plots (4–25 ha in size) in temperate mixed forests in northeast China to ask the following questions: (1) How do soil conditions and stand age drive biomass demographic processes? (2) How do vegetation quality (i.e., functional trait diversity and composition) and quantity (i.e., initial biomass stocks) influence biomass demographic processes independently from soil conditions and stand age? (3) What is the relative contribution of growth, recruitment, and mortality to net biomass change? Using structural equation modeling, we showed that all three demographic processes were jointly constrained by multiple abiotic and biotic factors and that mortality was the strongest determinant on net biomass change over time. Growth and mortality, as well as functional trait diversity and the community‐weighted mean of specific leaf area (CWMSLA), declined with stand age. By contrast, high soil phosphorous concentrations were associated with greater functional diversity and faster dynamics (i.e., high growth and mortality rates), but associated with lower CWMSLA and initial biomass stock. More functionally diverse communities also had higher recruitment rates, but did not exhibit faster growth and mortality. Instead, initial biomass stocks and CWMSLA were stronger predictors of biomass growth and mortality, respectively. By integrating the full spectrum of abiotic and biotic drivers of forest biomass dynamics, our study provides critical system‐level insights needed to predict the possible consequences of regional changes in forest diversity, composition, structure and function in the context of global change.

Big‐sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests

“…Moreover, such type of a positive relationship between vegetation quantity (i.e., initial biomass stocks, stand basal area, mean DBH) and aboveground biomass productivity has been widely reported for oldgrowth and secondary natural forests across the globe (Jucker et al, 2016;Lohbeck et al, 2015;Paquette & Messier, 2011;Vilà et al, 2013;Yuan et al, 2019Yuan et al, , 2018.…”

“…These results further confirm that the "big-sized trees effect" overrules the effects of tree crown complementarity and niche differentiation of the remaining trees on aboveground biomass. In this case, it is understandable that the intra-and interspecific competition of the 99% remaining trees lead to the community assembly processes, which in turn might impose differential effects on forest functioning probably due to the differential functional strategies (i.e., fast-growing acquisitive and slow-growing conservative) of the component species and interacting individuals within a community (Ali, Lohbeck, et al, 2018;Lohbeck et al, 2015;Reich, 2014;Yuan et al, 2019). In addition, the negative relationships between species richness and the 99% remaining trees attributes might be attributable to the fact that high biomass stand excludes weak competitors (Ali et al, 2016), because 99% remaining trees still contain top 10%-25% remaining big-sized trees (Bastin et al, 2015).…”

“…For example, it would be very interesting to explore the underlying ecological mechanisms (in one integrative SEM) for the "big-sized trees effect" on initial biomass stock, and then on biomass growth, recruitment, and mortality (or considering net biomass change only), while accounting for the effects of abiotic factors as well as multiple metrics of species diversity and stand structure (Poorter et al, 2017;Rozendaal et al, 2017;Yuan et al, 2019). For example, it would be very interesting to explore the underlying ecological mechanisms (in one integrative SEM) for the "big-sized trees effect" on initial biomass stock, and then on biomass growth, recruitment, and mortality (or considering net biomass change only), while accounting for the effects of abiotic factors as well as multiple metrics of species diversity and stand structure (Poorter et al, 2017;Rozendaal et al, 2017;Yuan et al, 2019).…”

Above‐ and below‐ground biodiversity jointly regulate temperate forest multifunctionality along a local‐scale environmental gradient

“…However, the forest functions we selected may be more directly related to plant attributes such as the stand structure and trait composition than to soil microbial diversity (Ali et al, 2019;Prado-Junior et al, 2016;van der Sande et al, 2017;Yuan et al, 2019).…”

“…More specifically, the single indirect effect was calculated by multiplying the standardized direct effects of a given predictor on FM via mediator in one route, and then we summed the multiple indirect effects of a given predictor to quantify the total indirect effect. To quantify the relative contribution of different predictors to FM, we calculated the relative importance for each predictor of FM using the ratio between the beta coefficient of a given predictor and the sum of the absolute value of beta coefficients of all predictors (Yuan et al, 2019).…”