Polylepis trees occur throughout the Andean mountain region and is the tree genus that grows at the highest elevation world-wide. In the humid Andes where moisture is rarely limiting, Polylepis trees must adapt to extreme environmental conditions, especially rapid fluctuations in temperature, ultraviolet radiation and vapor pressure deficit (VPD). However, Polylepis' water use patterns remain largely unknown, despite the importance of understanding their response to microclimate variation to determine their capacity to maintain resilience under future environmental change. We conducted a study in a Polylepis reticulata forest in the Ecuadorian Andes to evaluate its tree water use dynamics and identify the main environmental drivers of transpiration. Tree sap flow was monitored simultaneously with soil volumetric water content (VWC) and microclimate during two years for trees growing in forest edge and interior locations. We found that sap flow was primarily controlled by VPD and that VWC exerted a secondary role in driving sap flow dynamics. The highest values for sap flow rates were found when VPD > 0.15 kPa and VCW < 0.73 cm3 cm-3, but these threshold conditions only occurred during brief periods of time and were only found in 11% of our measurements. Moreover, these brief windows of more favorable conditions occurred more frequently in forest edge compared to forest interior locations, resulting in edge trees maintaining 46% higher sap flow compared to interior trees. Our results also suggest that P. reticulata has low stomatal control of transpiration, as sap flow did not decline with increasing VPD. This research provides valuable information about the potential impacts of projected future increases in VPD due to climate change on P. reticulata water use dynamics, which include higher sap flow rates leading to greater transpirational water loss due to this species' poor stomatal control.

中文翻译：

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更有利的大气条件的短暂窗口解释了高海拔安第斯森林中网状多鳞树的水利用模式。

水螅树遍及安第斯山脉地区，是世界上海拔最高的树属。在潮湿的安第斯山脉，水分很少受到限制，多鳞树必须适应极端的环境条件，特别是温度、紫外线辐射和蒸气压不足 (VPD) 的快速波动。然而，尽管了解它们对微气候变化的反应以确定它们在未来环境变化下保持恢复能力的能力非常重要，但它们的用水模式仍然很大程度上未知。我们在厄瓜多尔安第斯山脉的网状水螅森林中进行了一项研究，以评估其树木水分利用动态并确定蒸腾作用的主要环境驱动因素。两年期间，对生长在森林边缘和内部位置的树木的树液流量、土壤体积含水量（VWC）和微气候进行了同时监测。我们发现液流主要由 VPD 控制，VWC 在驱动液流动力学方面发挥次要作用。当 VPD > 0.15 kPa 且 VCW < 0.73 cm3 cm-3 时，液流流速达到最高值，但这些阈值条件仅发生在短暂的时间内，并且仅在我们的 11% 的测量中发现。此外，与森林内部位置相比，这些更有利条件的短暂窗口在森林边缘发生的频率更高，导致边缘树木的树液流量比内部树木高出 46%。我们的结果还表明，P. reticulata 对蒸腾作用的气孔控制较低，因为液流并没有随着 VPD 的增加而下降。这项研究提供了有关气候变化导致的 VPD 预计未来增加对 P. reticulata 用水动态的潜在影响的宝贵信息，其中包括由于该物种的气孔控制较差而导致较高的树液流速导致更大的蒸腾失水。