Intermittent rain events drive dynamic pulses of carbon and water exchange in many arid and semiarid ecosystems. Although soil moisture is known to control these pulses, the effect of atmospheric dryness on pulses is not well documented. Here we hypothesized that vapor pressure deficit (VPD) modulates net ecosystem production (NEP) and ecosystem‐scale water use efficiency (WUE) during pulse events due to its effects on canopy stomatal conductance and evapotranspiration. We quantified relationships between VPD and carbon and water exchange during growing season rain events and tested their generality across four semiarid flux sites with varied vegetation in the southwest United States. Across grassland, shrubland, and savanna sites, we found that high VPD during pulses suppressed ecosystem photosynthesis and surface conductance to a greater degree than respiration or evapotranspiration, particularly when soil moisture was high. Thus, periods of high VPD were associated with a 13–64% reduction in NEP and an 11–25% decrease in WUE, relative to moderate VPD conditions. Sites dominated by shrubs with the C3 photosynthetic pathway were more sensitive to VPD than sites dominated by C4 grasses. We found that a 1 kPa increase in VPD reduced the average NEP of pulse events by 13–56%, which illustrates the potential for projected increases in atmospheric demand to reduce the net productivity of semiarid ecosystems.

中文翻译：

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高蒸气压赤字会降低半干旱生态系统中降雨诱发脉冲的生产力和水分利用效率

间歇性降雨事件推动了许多干旱和半干旱生态系统中碳和水交换的动态脉冲。尽管已知土壤湿度可控制这些脉冲，但大气干燥对脉冲的影响尚未得到充分记录。在这里，我们假设在脉搏事件期间，由于蒸汽压亏缺（VPD）对冠层气孔导度和蒸散量的影响，因此会调节净生态系统产量（NEP）和生态系统规模的水分利用效率（WUE）。我们量化了VPD之间的关系生长季节降雨期间的碳和水交换，并测试了美国西南部四个植被不同的半干旱通量站点的通量。在草原，灌木丛和热带稀树草原地区，我们发现脉冲期间的高VPD比呼吸作用或蒸散作用更大程度地抑制了生态系统的光合作用和表面电导，特别是当土壤湿度较高时。因此，相对于中等VPD条件，高VPD时期可使NEP降低13–64％，WUE降低11–25％。具有C3光合途径的灌木为主的部位对VPD更为敏感比以C4草为主的地方 我们发现，VPD升高1 kPa可使脉冲事件的平均NEP降低13–56％，这说明预计的大气需求增加可能会降低半干旱生态系统的净生产力。