Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest ...carbon cycle--particularly net primary productivity and carbon storage--increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.
Advances in forest carbon mapping have the potential to greatly reduce uncertainties in the global carbon budget and to facilitate effective emissions mitigation strategies such as REDD+ (Reducing ...Emissions from Deforestation and Forest Degradation). Though broad-scale mapping is based primarily on remote sensing data, the accuracy of resulting forest carbon stock estimates depends critically on the quality of field measurements and calibration procedures. The mismatch in spatial scales between field inventory plots and larger pixels of current and planned remote sensing products for forest biomass mapping is of particular concern, as it has the potential to introduce errors, especially if forest biomass shows strong local spatial variation. Here, we used 30 large (8-50 ha) globally distributed permanent forest plots to quantify the spatial variability in aboveground biomass density (AGBD in Mg ha-1) at spatial scales ranging from 5 to 250 m (0.025-6.25 ha), and to evaluate the implications of this variability for calibrating remote sensing products using simulated remote sensing footprints. We found that local spatial variability in AGBD is large for standard plot sizes, averaging 46.3% for replicate 0.1 ha subplots within a single large plot, and 16.6% for 1 ha subplots. AGBD showed weak spatial autocorrelation at distances of 20-400 m, with autocorrelation higher in sites with higher topographic variability and statistically significant in half of the sites. We further show that when field calibration plots are smaller than the remote sensing pixels, the high local spatial variability in AGBD leads to a substantial "dilution" bias in calibration parameters, a bias that cannot be removed with standard statistical methods. Our results suggest that topography should be explicitly accounted for in future sampling strategies and that much care must be taken in designing calibration schemes if remote sensing of forest carbon is to achieve its promise.
Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations (1). Our ability to understand and predict changes in the forest ...carbon cycle--particularly net primary productivity and carbon storage--increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands (2,3). Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree (4-7), in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level (8-10) and stand-level (10) productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation (11) and plant senescence (12).
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.
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.
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