• Energy Research

AbstractWhile Amazonian forests are extraordinarily diverse, the abundance of trees is skewed strongly towards relatively few ‘hyperdominant’ species. In addition to their diversity, Amazonian trees are a key component of the global carbon cycle, assimilating and storing more carbon than any other ecosystem on Earth. Here we ask, using a unique data set of 530 forest plots, if the functions of storing and producing woody carbon are concentrated in a small number of tree species, whether the most abundant species also dominate carbon cycling, and whether dominant species are characterized by specific functional traits. We find that dominance of forest function is even more concentrated in a few species than is dominance of tree abundance, with only ≈1% of Amazon tree species responsible for 50% of carbon storage and productivity. Although those species that contribute most to biomass and productivity are often abundant, species maximum size is also influential, while the identity and ranking of dominant species varies by function and by region.

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.

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.

AbstractThe Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin‐wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin‐wide ground‐based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground‐based observations of mortality and growth from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha−1, confidence interval (CI): −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This contrasted with a long‐term biomass sink during the baseline pre‐2010 drought period (1998 to pre‐2010) of 1.33 Mg ha−1 yr−1 (CI: 0.90, 1.74, p < 0.01). The resulting net impact of the 2010 drought (i.e., reversal of the baseline net sink) was −1.95 Mg ha−1 yr−1 (CI:−2.77, −1.18; p < 0.001). This net biomass impact was driven by an increase in biomass mortality (1.45 Mg ha−1 yr−1 CI: 0.66, 2.25, p < 0.001) and a decline in biomass productivity (−0.50 Mg ha−1 yr−1, CI:−0.78, −0.31; p < 0.001). Surprisingly, the magnitude of the losses through tree mortality was unrelated to estimated local precipitation anomalies and was independent of estimated local pre‐2010 drought history. Thus, there was no evidence that pre‐2010 droughts compounded the effects of the 2010 drought. We detected a systematic basin‐wide impact of the 2010 drought on tree growth rates across Amazonia, which was related to the strength of the moisture deficit. This impact differed from the drought event in 2005 which did not affect productivity. Based on these ground data, live biomass in trees and corresponding estimates of live biomass in lianas and roots, we estimate that intact forests in Amazonia were carbon neutral in 2010 (−0.07 Pg C yr−1 CI:−0.42, 0.23), consistent with results from an independent analysis of airborne estimates of land‐atmospheric fluxes during 2010. Relative to the long‐term mean, the 2010 drought resulted in a reduction in biomass carbon uptake of 1.1 Pg C, compared to 1.6 Pg C for the 2005 event.