Long-term studies of community and population dynamics indicate that abrupt disturbances often catalyse changes in vegetation and carbon stocks. These disturbances include the opening of clearings, rainfall seasonality, and drought, as well as fire and direct human disturbance. Such events may be super-imposed on longer-term trends in disturbance, such as those associated with climate change (heating, drying), as well as resources. Intact neotropical forests have recently experienced increased drought frequency and fire occurrence, on top of pervasive increases in atmospheric CO2 concentrations, but we lack long-term records of responses to such changes especially in the critical transitional areas at the interface of forest and savanna biomes. Here, we present results from 20 years monitoring a valley forest (moist tropical forest outlier) in central Brazil. The forest has experienced multiple drought events and includes plots which have and which have not experienced fire. We focus on how forest structure (stem density and aboveground biomass carbon) and dynamics (stem and biomass mortality and recruitment) have responded to these disturbance regimes. Overall, the biomass carbon stock increased due to the growth of the trees already present in the forest, without any increase in the overall number of tree stems. Over time, both recruitment and especially mortality of trees tended to increase, and periods of prolonged drought in particular resulted in increased mortality rates of larger trees. This increased mortality was in turn responsible for a decline in aboveground carbon toward the end of the monitoring period. Prolonged droughts influence the mortality of large trees, leading to a decline in aboveground carbon stocks. Here, and in other neotropical forests, recent droughts are capable of shutting down and reversing biomass carbon sinks. These new results add to evidence that anthropogenic climate changes are already adversely impacting tropical forests.

中文翻译：

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长期干旱对热带森林地上生物量的影响：监测20年。

对社区和人口动态的长期研究表明，突然的干扰通常会促进植被和碳储量的变化。这些干扰包括开放的空地，降雨季节和干旱，以及火灾和直接的人为干扰。此类事件可能会叠加在干扰的长期趋势上，例如与气候变化（加热，干燥）以及资源相关的干扰。完整的新热带森林最近经历了干旱频率和火灾发生的增加，大气中二氧化碳浓度的普遍增加，但是我们缺乏对这种变化的反应的长期记录，尤其是在森林和稀树草原生物群落交界的关键过渡区域。这里，我们提供了20年监测巴西中部山谷森林（潮湿的热带森林离群值）的结果。森林经历了多次干旱事件，包括有火和没有火的土地。我们关注于森林结构（茎密度和地上生物量碳）和动力学（茎和生物量死亡率与补充）如何对这些扰动机制作出反应。总体而言，由于森林中已有树木的生长，生物量碳储量增加了，而树茎的总数却没有增加。随着时间的流逝，树木的采伐和尤其是死亡率均趋于增加，而长期干旱尤其导致较大树木的死亡率增加。死亡率的增加反过来又导致监测期末地上碳的减少。长期干旱会影响大树的死亡率，导致地上碳储量下降。在这里以及其他新热带森林中，最近的干旱能够关闭和逆转生物质碳汇。这些新结果进一步证明，人为气候变化已经对热带森林产生了不利影响。