• Energy Research

Climate change effects on growth rates of tropical trees may lead to alterations in carbon cycling of carbon-rich tropical forests. However, climate sensitivity of broad-leaved lowland tropical trees is poorly understood. Dendrochronology (tree-ring analysis) provides a powerful tool to study the relationship between tropical tree growth and annual climate variability. We aimed to establish climate-growth relationships for five annual-ring forming tree species, using ring-width data from 459 canopy and understory trees from a seasonal tropical forest in western Thailand. Based on 183/459 trees, chronologies with total lengths between 29 and 62 years were produced for four out of five species. Bootstrapped correlation analysis revealed that climate-growth responses were similar among these four species. Growth was significantly negatively correlated with current-year maximum and minimum temperatures, and positively correlated with dry-season precipitation levels. Negative correlations between growth and temperature may be attributed to a positive relationship between temperature and autotrophic respiration rates. The positive relationship between growth and dry-season precipitation levels likely reflects the strong water demand during leaf flush. Mixed-effect models yielded results that were consistent across species: a negative effect of current wet-season maximum temperatures on growth, but also additive positive effects of, for example, prior dry-season maximum temperatures. Our analyses showed that annual growth variability in tropical trees is determined by a combination of both temperature and precipitation variability. With rising temperature, the predominantly negative relationship between temperature and growth may imply decreasing growth rates of tropical trees as a result of global warming.

Summary The relationship between species richness and ecosystem function, as measured by productivity or biomass, is of long‐standing theoretical and practical interest in ecology. This is especially true for forests, which represent a majority of global biomass, productivity and biodiversity. Here, we conduct an analysis of relationships between tree species richness, biomass and productivity in 25 forest plots of area 8–50 ha from across the world. The data were collected using standardized protocols, obviating the need to correct for methodological differences that plague many studies on this topic. We found that at very small spatial grains (0.04 ha) species richness was generally positively related to productivity and biomass within plots, with a doubling of species richness corresponding to an average 48% increase in productivity and 53% increase in biomass. At larger spatial grains (0.25 ha, 1 ha), results were mixed, with negative relationships becoming more common. The results were qualitatively similar but much weaker when we controlled for stem density: at the 0.04 ha spatial grain, a doubling of species richness corresponded to a 5% increase in productivity and 7% increase in biomass. Productivity and biomass were themselves almost always positively related at all spatial grains. Synthesis. This is the first cross‐site study of the effect of tree species richness on forest biomass and productivity that systematically varies spatial grain within a controlled methodology. The scale‐dependent results are consistent with theoretical models in which sampling effects and niche complementarity dominate at small scales, while environmental gradients drive patterns at large scales. Our study shows that the relationship of tree species richness with biomass and productivity changes qualitatively when moving from scales typical of forest surveys (0.04 ha) to slightly larger scales (0.25 and 1 ha). This needs to be recognized in forest conservation policy and management.

The responses of tropical forests to global anthropogenic disturbances remain poorly understood. Above-ground woody biomass in some tropical forest plots has increased over the past several decades, potentially reflecting a widespread response to increased resource availability, for example, due to elevated atmospheric CO2and/or nutrient deposition. However, previous studies of biomass dynamics have not accounted for natural patterns of disturbance and gap phase regeneration, making it difficult to quantify the importance of environmental changes. Using spatially explicit census data from large (50 ha) inventory plots, we investigated the influence of gap phase processes on the biomass dynamics of four ‘old-growth’ tropical forests (Barro Colorado Island (BCI), Panama; Pasoh and Lambir, Malaysia; and Huai Kha Khaeng (HKK), Thailand). We show that biomass increases were gradual and concentrated in earlier-phase forest patches, while biomass losses were generally of greater magnitude but concentrated in rarer later-phase patches. We then estimate the rate of biomass change at each site independent of gap phase dynamics using reduced major axis regressions and ANCOVA tests. Above-ground woody biomass increased significantly at Pasoh (+0.72% yr−1) and decreased at HKK (−0.56% yr−1) independent of changes in gap phase but remained stable at both BCI and Lambir. We conclude that gap phase processes play an important role in the biomass dynamics of tropical forests, and that quantifying the role of gap phase processes will help improve our understanding of the factors driving changes in forest biomass as well as their place in the global carbon budget.

AbstractThe 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.

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.

AbstractThe important role of tropical forests in the global carbon cycle makes it imperative to assess changes in their carbon dynamics for accurate projections of future climate–vegetation feedbacks. Forest monitoring studies conducted over the past decades have found evidence for both increasing and decreasing growth rates of tropical forest trees. The limited duration of these studies restrained analyses to decadal scales, and it is still unclear whether growth changes occurred over longer time scales, as would be expected if CO2‐fertilization stimulated tree growth. Furthermore, studies have so far dealt with changes in biomass gain at forest‐stand level, but insights into species‐specific growth changes – that ultimately determine community‐level responses – are lacking. Here, we analyse species‐specific growth changes on a centennial scale, using growth data from tree‐ring analysis for 13 tree species (~1300 trees), from three sites distributed across the tropics. We used an established (regional curve standardization) and a new (size‐class isolation) growth‐trend detection method and explicitly assessed the influence of biases on the trend detection. In addition, we assessed whether aggregated trends were present within and across study sites. We found evidence for decreasing growth rates over time for 8–10 species, whereas increases were noted for two species and one showed no trend. Additionally, we found evidence for weak aggregated growth decreases at the site in Thailand and when analysing all sites simultaneously. The observed growth reductions suggest deteriorating growth conditions, perhaps due to warming. However, other causes cannot be excluded, such as recovery from large‐scale disturbances or changing forest dynamics. Our findings contrast growth patterns that would be expected if elevated CO2 would stimulate tree growth. These results suggest that commonly assumed growth increases of tropical forests may not occur, which could lead to erroneous predictions of carbon dynamics of tropical forest under climate change.

Dans les forêts tropicales amazoniennes, des études récentes ont rapporté des augmentations de la biomasse aérienne et de la productivité primaire, ainsi que des changements dans la composition des espèces végétales favorisant les espèces à croissance rapide par rapport aux espèces à croissance lente. Cette altération omniprésente des forêts tropicales matures a été attribuée aux changements environnementaux mondiaux, tels qu'une augmentation de la concentration atmosphérique de CO2, des dépôts de nutriments, de la température, de la fréquence des sécheresses et/ou de l'irradiance. Nous avons utilisé des mesures standardisées et répétées de plus de 2 millions d'arbres dans dix grandes parcelles forestières (16 à 52 ha chacune) sur trois continents pour évaluer la généralité de ces résultats dans les forêts tropicales. La biomasse aérienne a augmenté dans sept de nos dix parcelles, de manière significative dans quatre parcelles, et a montré une forte diminution dans une seule parcelle. L'accumulation de carbone regroupée entre les sites était significative (+0,24 MgC ha(-1) y(-1), intervalles de confiance à 95 % [0,07, 0,39] MgC ha(-1) y(-1)), mais inférieure à celle rapportée précédemment pour l'Amazonie. Sur trois sites pour lesquels nous avions des données pour plusieurs intervalles de recensement, nous n'avons trouvé aucune augmentation concertée du gain de biomasse, en contradiction avec l'hypothèse de productivité accrue. Sur l'ensemble des dix parcelles, le quartile d'espèces à la croissance la plus rapide a gagné de la biomasse (+0,33 [0,09, 0,55] % y(-1)) par rapport à la communauté arboricole dans son ensemble (+0,15 % y(-1)) ; cependant, cette tendance significative était due à une seule parcelle. La biomasse des espèces à croissance lente a augmenté de manière significative lorsqu'elle a été calculée sur toutes les placettes (+0,21 [0,02, 0,37] % y(-1)), et dans la moitié de nos placettes lorsqu'elle a été calculée individuellement. Nos résultats ne corroborent pas l'hypothèse selon laquelle les espèces à croissance rapide sont de plus en plus dominantes dans les communautés d'arbres tropicaux. Au lieu de cela, ils suggèrent que nos parcelles peuvent se remettre simultanément des perturbations passées et être affectées par les changements dans la disponibilité des ressources. Des études à plus long terme sont nécessaires pour clarifier la contribution du changement global au fonctionnement des forêts tropicales. En los bosques tropicales amazónicos, estudios recientes han reportado aumentos en la biomasa aérea y en la productividad primaria, así como cambios en la composición de las especies de plantas que favorecen a las especies de rápido crecimiento sobre las de crecimiento lento. Esta alteración generalizada de los bosques tropicales maduros se atribuyó al cambio ambiental global, como un aumento en la concentración atmosférica de CO2, la deposición de nutrientes, la temperatura, la frecuencia de la sequía y/o la irradiancia. Utilizamos mediciones estandarizadas y repetidas de más de 2 millones de árboles en diez grandes parcelas forestales (16-52 ha cada una) en tres continentes para evaluar la generalidad de estos hallazgos en los bosques tropicales. La biomasa superficial aumentó en siete de nuestras diez parcelas, de manera significativa en cuatro parcelas, y mostró una gran disminución en una sola parcela. La acumulación de carbono agrupada en todos los sitios fue significativa (+0,24 MgC ha(-1) y(-1), intervalos de confianza del 95% [0,07, 0,39] MgC ha(-1) y(-1)), pero inferior a la informada anteriormente para la Amazonia. En tres sitios para los que teníamos datos para múltiples intervalos de censo, no encontramos un aumento concertado en la ganancia de biomasa, en conflicto con la hipótesis del aumento de la productividad. En las diez parcelas, el cuartil de especies de más rápido crecimiento ganó biomasa (+0,33 [0,09, 0,55] % y(-1)) en comparación con la comunidad arbórea en su conjunto (+0,15 % y(-1)); sin embargo, esta tendencia significativa se debió a una sola parcela. La biomasa de especies de crecimiento lento aumentó significativamente cuando se calculó en todas las parcelas (+0,21 [0,02, 0,37] % y(-1)), y en la mitad de nuestras parcelas cuando se calculó individualmente. Nuestros resultados no respaldan la hipótesis de que las especies de rápido crecimiento estén aumentando constantemente en dominio en las comunidades de árboles tropicales. En cambio, sugieren que nuestras parcelas pueden estar recuperándose simultáneamente de perturbaciones pasadas y verse afectadas por cambios en la disponibilidad de recursos. Se necesitan más estudios a largo plazo para aclarar la contribución del cambio global al funcionamiento de los bosques tropicales. In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16-52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha(-1) y(-1), 95% confidence intervals [0.07, 0.39] MgC ha(-1) y(-1)), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y(-1)) compared with the tree community as a whole (+0.15 % y(-1)); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y(-1)), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests. في غابات الأمازون الاستوائية، أبلغت الدراسات الحديثة عن زيادات في الكتلة الحيوية فوق الأرض وفي الإنتاجية الأولية، فضلاً عن التحولات في تكوين الأنواع النباتية التي تفضل الأنواع سريعة النمو على الأنواع بطيئة النمو. ويعزى هذا التغيير المنتشر في الغابات الاستوائية الناضجة إلى التغير البيئي العالمي، مثل الزيادة في تركيز ثاني أكسيد الكربون في الغلاف الجوي، وترسب المغذيات، ودرجة الحرارة، وتواتر الجفاف، و/أو الإشعاع. استخدمنا قياسات موحدة ومتكررة لأكثر من مليوني شجرة في عشر قطع غابات كبيرة (16-52 هكتار لكل منها) في ثلاث قارات لتقييم عمومية هذه النتائج عبر الغابات الاستوائية. زادت الكتلة الحيوية فوق الأرض في سبع من قطعنا العشر، بشكل ملحوظ في أربع قطع، وأظهرت انخفاضًا كبيرًا في قطعة أرض واحدة. كان تراكم الكربون المجمع عبر المواقع كبيرًا (+0.24 MgC ha(-1) y(-1)، فواصل ثقة 95 ٪ [0.07، 0.39] MgC ha(-1) y(-1))، ولكنه أقل مما تم الإبلاغ عنه سابقًا في الأمازون. في ثلاثة مواقع كان لدينا بيانات عنها لفترات تعداد متعددة، لم نجد أي زيادة منسقة في كسب الكتلة الحيوية، بما يتعارض مع فرضية زيادة الإنتاجية. على جميع القطع العشر، اكتسب الربع الأسرع نموًا من الأنواع الكتلة الحيوية (+0.33 [0.09، 0.55 ]٪ y(-1)) مقارنة بمجتمع الأشجار ككل (+0.15 ٪ y(-1 ))؛ ومع ذلك، كان هذا الاتجاه الكبير بسبب قطعة أرض واحدة. زادت الكتلة الحيوية للأنواع بطيئة النمو بشكل كبير عند حسابها على جميع القطع (+0.21 [0.02، 0.37 ]٪ y(-1))، وفي نصف قطعنا عند حسابها بشكل فردي. لا تدعم نتائجنا الفرضية القائلة بأن الأنواع سريعة النمو تتزايد باستمرار في الهيمنة في مجتمعات الأشجار الاستوائية. وبدلاً من ذلك، يشيرون إلى أن قطع أراضينا قد تتعافى في وقت واحد من الاضطرابات السابقة وتتأثر بالتغيرات في توافر الموارد. من الضروري إجراء المزيد من الدراسات طويلة الأجل لتوضيح مساهمة التغير العالمي في عمل الغابات المدارية.