Abstract. Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Neotropics. This data was used to train two machine learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2 = 0.67, NRMSE = 9.5 %) and stem growth (R2 = 0.51, NRMSE = 11.2 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1°). Modelled time series allowed to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. Both droughts were estimated to have caused widespread declines in stem growth (−0.6σ ~ −1.8σ), coinciding with enhanced leaf fall (+0.7σ ~ +0.9σ). Regions in the Amazon basin that flushed leaves at the onset of both droughts (+1.1σ ~ +1.9σ), showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be explained by enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline of stem growth and a simultaneous but weaker increase in leaf litterfall in the Amazon basin since 1982 that is not observed in long-term inventory plots. These trends are associated with increased warming and drying of the Amazonian climate.

中文翻译：

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干旱对新热带森林的落叶，叶片潮红和茎生长的影响；调和遥感数据和实地观察

摘要。每年，大量的碳流过热带生态系统，其中一部分通过树木生长被隔离在生物质中。然而，持续变暖和干燥对热带森林树木生长和碳固存的影响仍然高度不确定。实地观察稀少，仅限于几个地点，而遥感分析显示，对过去干旱的生长反应各不相同，无法自信地解释。为了调和来自野外观测和遥感的数据，我们从新热带地区的清单中整理了茎生长和凋落物的原位测量结果。该数据用于训练两个机器学习模型，并评估模型在繁殖茎生长和凋落物率方面的性能。该模型利用多个气候变量和其他地理空间数据集作为解释变量。输出包括在1982年至2019年之间以较高的空间分辨率（0.1°）估算的新热带地区的叶子凋落物（R2 = 0.67，NRMSE = 9.5％）和茎生长（R2 = 0.51，NRMSE = 11.2％）的月度估算值。通过建模的时间序列，可以评估亚马逊流域2005年和2015年干旱对区域规模的影响。据估计，这两种干旱都导致茎生长普遍下降（-0.6σ〜-1.8σ），同时叶片下落也有所增加（+0.7σ〜+0.9σ）。亚马逊干旱盆地在两次干旱（+1.1σ〜+1.9σ）发生时都冲刷了叶子，在遥感增强的植被指数上显示出正异常，而太阳诱导的荧光和植被光学深度则降低了。因此，以前通过许多遥感分析检测到的亚马逊流域干旱期间冠层绿色抬高的反直觉反应可以通过干旱开始时的叶子潮红来解释。自1982年以来，亚马逊流域的叶子凋落物和茎生长的长期估计表明茎生长的下降和叶子凋落物的同时但较弱的增加，这在长期存货区中并未观察到。这些趋势与亚马逊气候的变暖和干燥有关。自1982年以来，亚马逊流域的叶子凋落物和茎生长的长期估计表明茎生长的下降和叶子凋落物的同时但较弱的增加，这在长期存货区中并未观察到。这些趋势与亚马逊气候的变暖和干燥有关。自1982年以来，亚马逊流域的叶子凋落物和茎生长的长期估计表明茎生长的下降和叶子凋落物的同时但较弱的增加，这在长期存货区中并未观察到。这些趋势与亚马逊气候的变暖和干燥有关。