Environmental Dynamics Department, Brazil's National Institute for Amazon Research, Manaus, Brazil.
Department of Forest Sciences, Federal University of Amazonas, Manaus, Brazil.
Glob Chang Biol. 2018 Sep;24(9):4266-4279. doi: 10.1111/gcb.14293. Epub 2018 May 30.
Sustained drought and concomitant high temperature may reduce photosynthesis and cause tree mortality. Possible causes of reduced photosynthesis include stomatal closure and biochemical inhibition, but their relative roles are unknown in Amazon trees during strong drought events. We assessed the effects of the recent (2015) strong El Niño drought on leaf-level photosynthesis of Central Amazon trees via these two mechanisms. Through four seasons of 2015, we measured leaf gas exchange, chlorophyll a fluorescence parameters, chlorophyll concentration, and nutrient content in leaves of 57 upper canopy and understory trees of a lowland terra firme forest on well-drained infertile oxisol. Photosynthesis decreased 28% in the upper canopy and 17% in understory trees during the extreme dry season of 2015, relative to other 2015 seasons and was also lower than the climatically normal dry season of the following non-El Niño year. Photosynthesis reduction under extreme drought and high temperature in the 2015 dry season was related only to stomatal closure in both upper canopy and understory trees, and not to chlorophyll a fluorescence parameters, chlorophyll, or leaf nutrient concentration. The distinction is important because stomatal closure is a transient regulatory response that can reverse when water becomes available, whereas the other responses reflect more permanent changes or damage to the photosynthetic apparatus. Photosynthesis decrease due to stomatal closure during the 2015 extreme dry season was followed 2 months later by an increase in photosynthesis as rains returned, indicating a margin of resilience to one-off extreme climatic events in Amazonian forests.
持续的干旱和随之而来的高温可能会降低光合作用并导致树木死亡。降低光合作用的可能原因包括气孔关闭和生化抑制，但在亚马逊强干旱事件中，它们在树木中的相对作用尚不清楚。我们通过这两种机制评估了最近（2015 年）强厄尔尼诺干旱对中央亚马逊树木叶片水平光合作用的影响。在 2015 年的四个季节中，我们在排水良好的贫瘠氧化土上的低地硬木森林中，测量了 57 棵上层树冠和下层树木的叶片气体交换、叶绿素 a 荧光参数、叶绿素浓度和叶片养分含量。与 2015 年的其他季节以及下一个非厄尔尼诺年的气候正常干旱季节相比，2015 年极端干旱季节上层树冠和下层树木的光合作用分别下降了 28%和 17%。在 2015 年干旱季节极端干旱和高温下，光合作用的降低仅与上层树冠和下层树木的气孔关闭有关，而与叶绿素 a 荧光参数、叶绿素或叶片养分浓度无关。这种区别很重要，因为气孔关闭是一种可以在水分可用时逆转的短暂调节反应，而其他反应则反映了对光合作用装置的更永久性变化或损害。在 2015 年极端干旱季节由于气孔关闭导致的光合作用下降，在两个月后随着降雨的恢复而增加，这表明在亚马逊森林中，一次性极端气候事件具有一定的恢复力。
