• Energy Research

Significance Understanding how changes in climate will affect terrestrial ecosystems is particularly important in tropical forest regions, which store large amounts of carbon and exert important feedbacks onto regional and global climates. By combining multiple types of observations with a state-of-the-art terrestrial ecosystem model, we demonstrate that the sensitivity of tropical forests to changes in climate is dependent on the length of the dry season and soil type, but also, importantly, on the dynamics of individual-level competition within plant canopies. These interactions result in ecosystems that are more sensitive to changes in climate than has been predicted by traditional models but that transition from one ecosystem type to another in a continuous, non–tipping-point manner.

AbstractUnderstanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest data set assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity (GPP) has a simple relationship with seasonal water deficit, but that (ii) site‐to‐site variations in GPP have little power in explaining site‐to‐site spatial variations in net primary productivity (NPP) or growth because of concomitant changes in carbon use efficiency (CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Moreover, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling.

Understanding climatic controls on tropical forest productivity is key to developing more reliable models for predicting how tropical biomes may respond to climate change. Currently there is no consensus on which factors control seasonal changes in tropical forest tree growth. This study reports the first comprehensive plot-level description of the seasonality of growth in a Peruvian tropical forest. We test whether seasonal and interannual variations in climate are correlated with changes in biomass increment, and whether such relationships differ among trees with different functional traits. We found that biomass increments, measured every 3 months on the two plots, were reduced by between 40 and 55% in the peak dry season (July-September) relative to peak wet season (January-March). The seasonal patterns of biomass accumulation are significantly (p < 0.01) associated with seasonal patterns of rainfall and soil water content; however, this may reflect a synchrony of seasonal cycles rather than direct physiological controls on tree growth rates. The strength of the growth seasonality response among trees is significantly correlated to functional traits: consistent with a hypothesised trade-off between maximum potential growth rate and hydraulic safety, tall and fast-growing trees with broad stems had the most strongly seasonal biomass accumulation, suggesting that they are more productive in the wet season, but more vulnerable to water limitation in the dry season.

Several independent lines of evidence suggest that Amazon forests have provided a significant carbon sink service, and also that the Amazon carbon sink in intact, mature forests may now be threatened as a result of different processes. There has however been no work done to quantify non-land-use-change forest carbon fluxes on a national basis within Amazonia, or to place these national fluxes and their possible changes in the context of the major anthropogenic carbon fluxes in the region. Here we present a first attempt to interpret results from ground-based monitoring of mature forest carbon fluxes in a biogeographically, politically, and temporally differentiated way. Specifically, using results from a large long-term network of forest plots, we estimate the Amazon biomass carbon balance over the last three decades for the different regions and nine nations of Amazonia, and evaluate the magnitude and trajectory of these differentiated balances in relation to major national anthropogenic carbon emissions.The sink of carbon into mature forests has been remarkably geographically ubiquitous across Amazonia, being substantial and persistent in each of the five biogeographic regions within Amazonia. Between 1980 and 2010, it has more than mitigated the fossil fuel emissions of every single national economy, except that of Venezuela. For most nations (Bolivia, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname) the sink has probably additionally mitigated all anthropogenic carbon emissions due to Amazon deforestation and other land use change. While the sink has weakened in some regions since 2000, our analysis suggests that Amazon nations which are able to conserve large areas of natural and semi-natural landscape still contribute globally-significant carbon sequestration.Mature forests across all of Amazonia have contributed significantly to mitigating climate change for decades. Yet Amazon nations have not directly benefited from providing this global scale ecosystem service. We suggest that better monitoring and reporting of the carbon fluxes within mature forests, and understanding the drivers of changes in their balance, must become national, as well as international, priorities.

Along with total radiation received, the proportion of diffuse to direct solar radiation can influence forest photosynthesis and carbon cycling. However, tropical diffuse radiation regimes are poorly described, and to date there are few or no site-based or regional diffuse radiation datasets. The relationship between cloud fraction and diffuse solar radiation was investigated using data from two sites in western and eastern Amazonia. Radiation regimes for diffuse and total radiation were characterised for each site, and the variation in clear sky diffuse radiation fraction between wet and dry season demonstrated and quantified, as well as the dependence of diffuse radiation on cloud amount. Using high frequency measurements of diffuse and total solar radiation data from the two sites, and estimated top of the canopy clear-sky radiation, a number of alternative models to predict diffuse radiation fraction from cloud fraction were formulated and tested. Results showed that cloud fraction can be approximated using the relationship between observed and calculated top of canopy radiation, after which diffuse radiation can then be predicted from cloud fraction. We also demonstrate that satellite cloud data (from the International Satellite Cloud Climatology Project) can be used as inputs to the diffuse radiation model to provide estimates of annual and monthly diffuse radiation proportion.

Los registros de dióxido de carbono atmosférico indican que la superficie terrestre ha actuado como un fuerte sumidero de carbono global en las últimas décadas, y una fracción sustancial de este sumidero probablemente se encuentra en los trópicos, particularmente en la Amazonía. Sin embargo, no está claro cómo evolucionará el sumidero de carbono terrestre a medida que el clima y la composición atmosférica continúen cambiando. Aquí analizamos la evolución histórica de la dinámica de la biomasa de la selva amazónica a lo largo de tres décadas utilizando una red distribuida de 321 parcelas. Si bien este análisis confirma que los bosques amazónicos han actuado como un sumidero neto de biomasa a largo plazo, encontramos una tendencia decreciente a largo plazo de la acumulación de carbono. Las tasas de aumento neto de la biomasa superficial disminuyeron en un tercio durante la última década en comparación con la década de 1990. Esto es consecuencia de que los aumentos de la tasa de crecimiento se estabilizaron recientemente, mientras que la mortalidad por biomasa aumentó persistentemente en todo momento, lo que llevó a un acortamiento de los tiempos de residencia del carbono. Los posibles impulsores del aumento de la mortalidad incluyen una mayor variabilidad climática y retroalimentaciones de un crecimiento más rápido de la mortalidad, lo que resulta en una menor longevidad de los árboles. La disminución observada del sumidero amazónico difiere notablemente del reciente aumento de la absorción de carbono terrestre a escala global, y es contraria a las expectativas basadas en modelos. Les enregistrements atmosphériques de dioxyde de carbone indiquent que la surface terrestre a agi comme un puissant puits de carbone mondial au cours des dernières décennies, une fraction substantielle de ce puits étant probablement située sous les tropiques, en particulier en Amazonie. Néanmoins, on ne sait pas comment le puits de carbone terrestre évoluera à mesure que le climat et la composition atmosphérique continueront de changer. Nous analysons ici l'évolution historique de la dynamique de la biomasse de la forêt amazonienne sur trois décennies à l'aide d'un réseau distribué de 321 parcelles. Bien que cette analyse confirme que les forêts amazoniennes ont agi comme un puits de biomasse net à long terme, nous constatons une tendance à la baisse à long terme de l'accumulation de carbone. Les taux d'augmentation nette de la biomasse aérienne ont diminué d'un tiers au cours de la dernière décennie par rapport aux années 1990. C'est une conséquence de la stabilisation récente de l'augmentation du taux de croissance, tandis que la mortalité liée à la biomasse a constamment augmenté, ce qui a entraîné une réduction des temps de résidence du carbone. Les facteurs potentiels de l'augmentation de la mortalité comprennent une plus grande variabilité du climat et des rétroactions d'une croissance plus rapide sur la mortalité, entraînant une réduction de la longévité des arbres. Le déclin observé du puits amazonien s'écarte nettement de la récente augmentation de l'absorption terrestre de carbone à l'échelle mondiale, et est contraire aux attentes basées sur des modèles. Atmospheric carbon dioxide records indicate that the land surface has acted as a strong global carbon sink over recent decades, with a substantial fraction of this sink probably located in the tropics, particularly in the Amazon. Nevertheless, it is unclear how the terrestrial carbon sink will evolve as climate and atmospheric composition continue to change. Here we analyse the historical evolution of the biomass dynamics of the Amazon rainforest over three decades using a distributed network of 321 plots. While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This is a consequence of growth rate increases levelling off recently, while biomass mortality persistently increased throughout, leading to a shortening of carbon residence times. Potential drivers for the mortality increase include greater climate variability, and feedbacks of faster growth on mortality, resulting in shortened tree longevity. The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale, and is contrary to expectations based on models. تشير سجلات ثاني أكسيد الكربون في الغلاف الجوي إلى أن سطح الأرض كان بمثابة حوض كربون عالمي قوي على مدى العقود الأخيرة، وربما يقع جزء كبير من هذا الحوض في المناطق المدارية، لا سيما في الأمازون. ومع ذلك، من غير الواضح كيف ستتطور بالوعة الكربون الأرضية مع استمرار تغير المناخ وتكوين الغلاف الجوي. نحلل هنا التطور التاريخي لديناميكيات الكتلة الحيوية لغابات الأمازون المطيرة على مدى ثلاثة عقود باستخدام شبكة موزعة من 321 قطعة أرض. في حين يؤكد هذا التحليل أن غابات الأمازون كانت بمثابة بالوعة صافية طويلة الأجل للكتلة الحيوية، فإننا نجد اتجاهًا تنازليًا طويل الأجل لتراكم الكربون. انخفضت معدلات الزيادة الصافية في الكتلة الحيوية فوق الأرض بمقدار الثلث خلال العقد الماضي مقارنة بالتسعينيات. وهذا نتيجة لاستقرار زيادات معدل النمو في الآونة الأخيرة، في حين زادت وفيات الكتلة الحيوية باستمرار طوال الوقت، مما أدى إلى تقصير أوقات بقاء الكربون. وتشمل الدوافع المحتملة لزيادة الوفيات زيادة تقلب المناخ، وردود الفعل على النمو الأسرع للوفيات، مما يؤدي إلى تقصير عمر الأشجار. يختلف الانخفاض الملحوظ في حوض الأمازون بشكل ملحوظ عن الزيادة الأخيرة في امتصاص الكربون الأرضي على النطاق العالمي، ويتعارض مع التوقعات القائمة على النماذج.