Demographic composition, not demographic diversity, predicts biomass and turnover across temperate and tropical forests

Abstract

Accepted ArticleThis article is protected by copyright. All rights reservedThe growth and survival of individual trees determine the physical structure of a forest with important consequences for forest function. However, given the diversity of tree species and forest biomes, quantifying the multitude of demographic strategies within and across forests and the way that they translate into forest structure and function remains a significant challenge. Here, we quantify the demographic rates of 1,961 tree species from temperate and tropical forests and evaluate how demographic diversity (DD) and demographic composition (DC) differ across forests, and how these differences in demography relate to species richness, aboveground biomass, and carbon residence time.We find wide variation in DD and DC across forest plots, patterns that are not explained by species richness or climate variables alone. There is no evidence that DD has an effect on either aboveground biomass or carbon residence time.Rather, the DC of forests, specifically the relative abundance of large statured species, predicted both biomass and carbon residence time. Our results demonstrate the distinct demographic compositions of globally distributed forests, reflecting biogeography, recent history, and current plot conditions. Linking the demographic composition of forests to resilience or vulnerability to climate change, will improve the precision and accuracy of predictions of future forest composition, structure and function.

Major axes of variation in tree demography across global forests

“…It is worth noting that we intentionally included no fixed effects in the models, in contrast to the usual statistical approach when searching for specific mechanisms, e.g., including dbh as a predictor to estimate ontogenetic effects in vital rates (Needham et al ., 2022). In our framework, all mechanisms are considered through OPs, which represent the dimensions at which they generate variability.…”

“…Moreover, demography has been identified as critical for more accurately modelling the terrestrial component in earth system models (Fisher et al ., 2018) and projecting the future of the terrestrial carbon sink (Pan et al ., 2011). Even small changes, over space and time, in tree vital rates can affect carbon cycles (Needham et al ., 2022) and thus the extent to which climate change can be mitigated by forests (Canadell & Raupach, 2008).…”

“…For example, through natural selection species have developed different strategies to acquire and allocate resources. This has resulted in a species dimension that represents the range of phenotypes among species(Díaz et al ., 2016) and, thus, also the observed vital rates of individual species (Johnson et al ., 2018; Rüger et al ., 2018; Needham et al ., 2022). Moreover, as resource availability and stressors vary along spatial and temporal dimensions, the environmental conditions of a forest also structure the vital rates of the trees, e.g.…”

Incorporando a variabilidade: o uso de modelos de efeitos mistos em ecologia

“…These vital (i.e. demographic) rates combine to determine key features of forests, such as biomass stocks and fluxes (Needham et al, 2020), structural complexity (Kohyama, 1993;Iida et al, 2014), and diversity (Lasky et al, 2014). Improving our understanding of demography, for instance, may foster the development of ecological theories, for example in community ecology as coexistence and niche theory (Kohyama, 1993;Adler et al, 2010;Diez et al, 2014;Lasky et al, 2014).…”

“…Even small changes in tree vital rates, if consistent over space or time, can affect carbon cycles (Needham et al, 2022) and thus the extent to which climate change can be mitigated by forests (Bonan, 2008;Canadell & Raupach, 2008).…”