Amazonian ecosystems are major biodiversity hotspots and carbon sinks that may lose species to extinction and become carbon sources due to extreme dry or warm conditions. We investigated the seasonal patterns of high-resolution solar-induced chlorophyll fluorescence (SIF) measured by the satellite Orbiting Carbon Observatory-2 (OCO-2) across the Amazonian ecoregions to assess the area´s phenology and extreme drought vulnerability. SIF is an indicator of the photosynthetic activity of chlorophyll molecules and is assumed to be directly related to gross primary production (GPP). We analyzed SIF variability in the Amazon basin during the period between September 2014 and December 2018. In particular, we focused on the SIF drought response under the extreme drought period during the strong El Nino in 2015–2016, as well as the 6-month drought peak period. During the drought´s peak months, the SIF decreased and increased with different intensities across the ecoregions of the Amazonian moist broadleaf forest (MBF) biome. Under a high temperature, a high vapor pressure deficit, and extreme drought conditions, the SIF presented differences from -31.1% to +17.6%. Such chlorophyll activity variations have been observed in plant-level measurements of active fluorescence in plants undergoing physiological responses to water or heat stress. Thus, it is plausible that the SIF variations in the ecoregions’ ecosystems occurred as a result of water and heat stress, and arguably because of drought-driven vegetation mortality and collateral effects in their species composition and community structures. The SIF responses to drought at the ecoregional scale indicate that there are different levels of resilience to drought across MBF ecosystems that the currently used climate- and biome-region scales do not capture. Finally, we identified monthly SIF values of 32 ecoregions, including non-MBF biomes, which may give the first insights into the photosynthetic activity dynamics of Amazonian ecoregions.

中文翻译：

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OCO-2太阳辐射引起的亚马逊河流域生态区的叶绿素荧光变异和厄尔尼诺现象的极端干旱影响（2015–2016）

亚马孙生态系统是主要的生物多样性热点和碳汇，由于极端干燥或温暖的条件，它们可能使物种灭绝并成为碳源。我们调查了由亚马逊生态区上的卫星轨道碳观测站2（OCO-2）测得的高分辨率太阳诱发的叶绿素荧光（SIF）的季节性模式，以评估该地区的物候和极端干旱脆弱性。SIF是叶绿素分子光合活性的指标，并被认为与初级总产值（GPP）直接相关。我们分析了2014年9月至2018年12月期间亚马逊河流域的SIF变异性。特别是，我们重点研究了2015-2016年厄尔尼诺现象期间，极端干旱时期的SIF干旱响应，以及六个月的干旱高峰期。在干旱的高峰月份，在亚马逊湿地阔叶林（MBF）生物群落的生态区中，SIF随强度的增加而降低和升高。在高温，高蒸气压赤字和极端干旱条件下，SIF的差异为-31.1％至+ 17.6％。在对水或热胁迫进行生理反应的植物中，在活性荧光的植物水平测量中已经观察到这种叶绿素活性变化。因此，有理由认为，生态区的生态系统因子变化是由于水分和热胁迫而发生的，并且可以说是干旱引起的植被死亡及其物种组成和群落结构的附带影响。SIF在生态区域范围内对干旱的反应表明，MBF生态系统在干旱上的抵御能力水平不同，目前使用的气候和生物群落区域范围无法捕获。最后，我们确定了32个生态区（包括非MBF生物群落）的每月SIF值，这可能使我们首次了解了亚马逊生态区的光合作用动态。