Organisms of all species must balance their allocation to growth, survival and recruitment. Among tree species, evolution has resulted in different life‐history strategies for partitioning resources ...to these key demographic processes. Life‐history strategies in tropical forests have often been shown to align along a trade‐off between fast growth and high survival, that is, the well‐known fast–slow continuum. In addition, an orthogonal trade‐off has been proposed between tall stature—resulting from fast growth and high survival—and recruitment success, that is, a stature−recruitment trade‐off. However, it is not clear whether these two independent dimensions of life‐history variation structure tropical forests worldwide.
We used data from 13 large‐scale and long‐term tropical forest monitoring plots in three continents to explore the principal trade‐offs in annual growth, survival and recruitment as well as tree stature. These forests included relatively undisturbed forests as well as typhoon‐disturbed forests. Life‐history variation in 12 forests was structured by two orthogonal trade‐offs, the growth−survival trade‐off and the stature−recruitment trade‐off. Pairwise Procrustes analysis revealed a high similarity of demographic relationships among forests. The small deviations were related to differences between African and Asian plots.
Synthesis. The fast–slow continuum and tree stature are two independent dimensions structuring many, but not all tropical tree communities. Our discovery of the consistency of demographic trade‐offs and life‐history strategies across different forest types from three continents substantially improves our ability to predict tropical forest dynamics worldwide.
Zusammenfassung
Individuen aller Arten müssen ihrer Ressourcen zwischen Wachstum, Überleben und Nachwuchsrekrutierung allozieren. Baumarten haben, evolutionär bedingt, verschiedene biologische Strategien entwickelt, wie sie ihre Ressourcen auf diese wichtigen demografischen Prozesse verteilen. In tropischen Wäldern lassen sich die biologischen Strategien der Bäume oft entlang eines Gradienten anordnen, welcher Arten mit schnellem Wachstum von Arten mit langem Überleben trennt, i.e. das bekannte Fast‐Slow‐Kontinuum. Ein weiterer orthogonaler Trade‐off welcher Arten die durch schnelles Wachstum und langes Überleben eine hohe Statur erreichen können von solchen Arten trennt, die eine hohe Rekrutierungsrate vorweisen (i.e. ein Statur‐Rekrutierungs‐Trade‐off) wurde bisher zwar postuliert, aber es wurde noch nicht geklärt, ob die tropischen Wälder auch global von diesen beiden unabhängigen Dimensionen der biologischen Strategien strukturiert werden.
In dieser Studie haben wir die Daten von 13 großflächigen und langfristigen Plots in tropischen Wäldern über drei Kontinente analysiert und die vorherrschenden Trade‐offs zwischen den jährlichen Wachstums‐, Überlebens‐ und Rekrutierungsraten sowie der Statur der lokalen Baumarten bestimmt. Die untersuchten Flächen umfassten dabei relative ungestörte, sowie Taifun‐gestörte Wälder. In zwölf dieser Wälder ordneten sich die biologischen Strategien der lokalen Baumarten entlang zweier orthogonaler Trade‐offs an, einem Trade‐off zwischen Wachstum und Überleben und einem Trade‐off zwischen Statur und Rekrutierung. Anschließende paarweise Procrustes‐Analysen zeigten eine hohe Ähnlichkeit in den Korrelationen zwischen den artspezifischen demografischen Raten zwischen diesen unterschiedlichen Wäldern. Wir fanden auch kleinere, aber signifikante Unterschiede, zwischen den Wäldern in Afrika und Asien.
Synthesis. Das Fast‐Slow‐Kontinuum und die Statur der Baumarten stellen zwei unabhängige Dimensionen dar, welche viele, aber nicht alle tropischen Baumgemeinschaften strukturieren. Unsere Ergebnisse zur Übereinstimmung der demographischen Trade‐offs und den resultierenden biologischen Strategien über Wälder und Kontinente hinweg, ermöglicht es uns in Zukunft die Entwicklung von tropischen Wäldern weltweit besser vorherzusagen.
All species must balance their allocation to growth, survival and recruitment, resulting in different life‐history strategies for partitioning resources to these demographic processes. Across 13 (sub)tropical forests, the diversity of life‐history strategies in tropical tree communities is often, but not always, structured along two independent dimensions that are related to the fast–slow continuum and to a gradient in tree stature.
Editor's Choice
Though substantial research has been conducted on possible historical, physiological, and symbiotic mechanisms that permit monodominance to occur within tropical lowland rainforests, less is known ...about the successional rates at which monodominance exerts itself on surrounding forest structures. Here we extend efforts to evaluate the longitudinal dynamics of Gilbertiodendron dewevrei-dominated forest in Central Africa by considering this species' spatial dynamics. Using three 10-ha censused field plots measured across three time periods, we present the first quantitative estimates of the spatial propagation of Gilbertiodendron into adjacent mixed species forest. Using three analytical strategies, we demonstrate that Gilbertiodendron is increasing in dominance and that monodominant forest patches are expanding into the surrounding forest at a statistically significant rate. The rates of successional advance vary by patch and direction, but average 0.31 m year-1, with speeds greatest in the direction of the prevailing winds. We show that the advancement of Gilbertiodendron is significantly slower than documented rates from other forest ecotones across Central Africa. When paired with stress tolerance traits and ectomycorrhizal associations, these findings help to clarify the means by which Gilbertiodendron dewevrei gains dominance in otherwise species-diverse regions.
Long‐term surveys of entire communities of species are needed to measure fluctuations in natural populations and elucidate the mechanisms driving population dynamics and community assembly. We ...analysed changes in abundance of over 4000 tree species in 12 forests across the world over periods of 6–28 years. Abundance fluctuations in all forests are large and consistent with population dynamics models in which temporal environmental variance plays a central role. At some sites we identify clear environmental drivers, such as fire and drought, that could underlie these patterns, but at other sites there is a need for further research to identify drivers. In addition, cross‐site comparisons showed that abundance fluctuations were smaller at species‐rich sites, consistent with the idea that stable environmental conditions promote higher diversity. Much community ecology theory emphasises demographic variance and niche stabilisation; we encourage the development of theory in which temporal environmental variance plays a central role.
The 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 1961 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 (AGB), 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 AGB 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 DCs of globally distributed forests, reflecting biogeography, recent history, and current plot conditions. Linking the DC of forests to resilience or vulnerability to climate change, will improve the precision and accuracy of predictions of future forest composition, structure, and function.
(a) We fit growth and survival models to 1,961 species across 20 tropical and temperate forest plots. (b) Species were clustered into Growth‐Survival‐Stature Modes (GSSMs) based on demographic rates. At each plot we calculated demographic diversity (DD) as the area occupied by species in demographic space, and demographic composition (DC) as the relative abundance of each GSSM. (c) DD peaks at intermediate levels of species richness. (d) Aboveground biomass (AGB) and carbon residence time are not related to DD across plots, but are related to DC, specifically the relative abundance of high‐survival, large‐statured GSSMs 5 and 6.
Among the local processes that determine species diversity in ecological communities, fluctuation‐dependent mechanisms that are mediated by temporal variability in the abundances of species ...populations have received significant attention. Higher temporal variability in the abundances of species populations can increase the strength of temporal niche partitioning but can also increase the risk of species extinctions, such that the net effect on species coexistence is not clear. We quantified this temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. A fitted mechanistic model showed that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal variability in the abundances of species populations has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
We quantified temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. In addition, we used a fitted mechanistic model to show that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal population variability has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
Resource allocation within trees is a zero-sum game. Unavoidable trade-offs dictate that allocation to growth-promoting functions curtails other functions, generating a gradient of investment in ...growth versus survival along which tree species align, known as the interspecific growth-mortality trade-off. This paradigm is widely accepted but not well established. Using demographic data for 1,111 tree species across ten tropical forests, we tested the generality of the growth-mortality trade-off and evaluated its underlying drivers using two species-specific parameters describing resource allocation strategies: tolerance of resource limitation and responsiveness of allocation to resource access. Globally, a canonical growth-mortality trade-off emerged, but the trade-off was strongly observed only in less disturbance-prone forests, which contained diverse resource allocation strategies. Only half of disturbance-prone forests, which lacked tolerant species, exhibited the trade-off. Supported by a theoretical model, our findings raise questions about whether the growth-mortality trade-off is a universally applicable organizing framework for understanding tropical forest community structure.
Patterns of structural change associated with monodominant tropical forest complexes have remained enigmatic for decades. Here, we extend previous efforts in presenting a longitudinal, local-scale ...analysis of forest dynamics in central Africa. Using four 10-ha census plots measured across three time periods (959,312 stems ≥1 cm DBH), we analyzed changes in a number of biometrical attributes for four distinct forest types capturing the developmental gradient from mixed species forest to Gilbertiodendron dewevrei-dominated forest. We modeled above-ground biomass (AGB), basal area (BA), and stem density across all species, and diameter at breast height (DBH), recruitment, and mortality for Gilbertiodendron dewevrei. We hypothesized that trends in these attributes are consistent with a slow spread of Gilbertiodendron dewevrei into adjacent mixed species forest. We identified statistically significant increases in AGB and BA across sites and positive, though nonsignificant, increases in AGB and BA for most forest types. DBH and relative recruitment increased significantly for Gilbertiodendron dewevrei stems, while relative mortality did not. When looking from mixed species to transitional to monodominant forest types, we found a statistically significant pattern of developmental aggradation and net expansion of monodominant forest. We do not attribute this to atmospheric forcing but to a combination of (a) landscape-scale recovery or response to widespread disturbance (primarily historical fires), (b) Gilbertiodendron dewevrei’s ectomycorrhizal association, and (c) Gilbertiodendron dewevrei’s exceptional stress tolerance traits.
The future trajectory of global forests is closely intertwined with tree demography, and a major fundamental goal in ecology is to understand the key mechanisms governing spatio‐temporal patterns in ...tree population dynamics. While previous research has made substantial progress in identifying the mechanisms individually, their relative importance among forests remains unclear mainly due to practical limitations. One approach to overcome these limitations is to group mechanisms according to their shared effects on the variability of tree vital rates and quantify patterns therein. We developed a conceptual and statistical framework (variance partitioning of Bayesian multilevel models) that attributes the variability in tree growth, mortality, and recruitment to variation in species, space, and time, and their interactions – categories we refer to as organising principles (OPs). We applied the framework to data from 21 forest plots covering more than 2.9 million trees of approximately 6500 species. We found that differences among species, the species OP, proved a major source of variability in tree vital rates, explaining 28–33% of demographic variance alone, and 14–17% in interaction with space, totalling 40–43%. Our results support the hypothesis that the range of vital rates is similar across global forests. However, the average variability among species declined with species richness, indicating that diverse forests featured smaller interspecific differences in vital rates. Moreover, decomposing the variance in vital rates into the proposed OPs showed the importance of unexplained variability, which includes individual variation, in tree demography. A focus on how demographic variance is organized in forests can facilitate the construction of more targeted models with clearer expectations of which covariates might drive a vital rate. This study therefore highlights the most promising avenues for future research, both in terms of understanding the relative contributions of groups of mechanisms to forest demography and diversity, and for improving projections of forest ecosystems.
We test the hypotheses proposed by Gentry and Schnitzer that liana density and basal area in tropical forests vary negatively with mean annual precipitation (MAP) and positively with seasonality. ...Previous studies correlating liana abundance with these climatic variables have produced conflicting results, warranting a new analysis of drivers of liana abundance based on a different dataset. We compiled a pan-tropical dataset containing 28,953 lianas (> 2.5 cm diam.) from studies conducted at 13 Neotropical and 11 Paleotropical dry to wet lowland tropical forests. The ranges in MAP and dry season length (DSL) (number of months with mean rainfall < 100 mm) represented by these datasets were 860-7250 mm/yr and 0-7 mo, respectively. Pan-tropically, liana density and basal area decreased significantly with increasing annual rainfall and increased with increasing DSL, supporting the hypotheses of Gentry and Schnitzer. Our results suggest that much of the variation in liana density and basal area in the tropics can be accounted for by the relatively simple metrics of MAP and DSL.
PDF
