Plant transpiration is a key component of the terrestrial water cycle, and it is important to understand whether rates are likely to increase or decrease in the future. Plant transpiration rates are affected by biophysical factors, such as air temperature, vapour pressure deficits and net radiation, and by plant factors, such as canopy leaf area and stomatal conductance. Under future climate change, global temperature increases, and associated increases in vapour pressure deficits, will act to increase canopy transpiration rates. Increasing atmospheric CO₂ concentrations, however, is likely to lead to some reduction in stomatal conductance, which will reduce canopy transpiration rates. The objective of the present paper was to quantitatively compare the importance of these opposing driving forces. First, we reviewed the existing literature and list a large range of observations of the extent of decreasing stomatal conductance with increasing CO₂ concentrations. We considered observations ranging from short-term laboratory-based experiments with plants grown under different CO₂ concentrations to studies of plants exposed to the naturally increasing atmospheric CO₂ concentrations. Using these empirical observations of plant responses, and a set of well-tested biophysical relationships, we then estimated the net effect of the opposing influences of warming and CO₂ concentration on transpiration rates. As specific cases studies, we explored expected changes in greater detail for six specific representative locations, covering the range from tropical to boreal forests. For most locations investigated, we calculated reductions in daily transpiration rates over the twenty-first century that became stronger under higher atmospheric CO₂ concentrations. It showed that the effect of CO₂-induced reduction of stomatal conductance would have a stronger transpiration-depressing effect than the stimulatory effect of future warming. For currently cold regions, global warming would, however, lengthen the growing seasons so that annual sums of transpiration could increase in those regions despite reductions in daily transpiration rates over the summer months.

中文翻译：

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变暖和升高的CO 2对蒸腾作用具有相反的影响。哪个更重要？

植物的蒸腾作用是地球水循环的关键组成部分，了解未来水的增加或减少的速率非常重要。植物的蒸腾速率受生物物理因素（例如气温，蒸气压不足和净辐射）的影响，还受植物因素（例如冠层叶面积和气孔导度）的影响。在未来的气候变化下，全球温度升高以及随之而来的蒸气压赤字增加，将使冠层蒸腾速率增加。大气CO ₂增加然而，浓度过高可能会导致气孔导度降低，从而降低冠层蒸腾速率。本文的目的是定量比较这些相反驱动力的重要性。首先，我们回顾了现有文献并列出了随着CO ₂浓度升高气孔导度降低程度的大量观察结果。我们考虑了以下观察结果：从在不同CO ₂浓度下生长的植物的短期实验室实验到暴露于自然增加的大气CO ₂的植物的研究，浓度。使用这些对植物反应的经验观察，以及一组经过良好测试的生物物理关系，我们然后估算了变暖和CO ₂浓度对蒸腾速率的相反影响的净影响。作为特定案例研究，我们更详细地探讨了六个特定代表性地点的预期变化，涵盖从热带森林到北方森林的范围。对于大多数调查地点，我们计算了二十一世纪日蒸腾速率的降低，在较高的大气CO ₂浓度下，蒸腾速率的降低更为明显。表明CO ₂的作用诱导的气孔导度下降比未来变暖的刺激作用具有更强的蒸腾抑制作用。然而，对于当前较冷的地区，全球变暖将延长生长期，从而使这些地区的年蒸腾量可能增加，尽管夏季月份的日蒸腾量有所减少。