| 英文摘要: | A simple conceptual model helps to answer the question of which forests are more likely to die following droughts. The Rolling Stones famously sang “you can't always get what you want”. However, it is not clear whether forests will be able to always get what they need under conditions of future climate change. This nicely sums up a current theme of research on how forests are expected to cope with the increased frequency and intensity of regional drought events and heat waves predicted under global climate change. Evidence of large-scale defoliation and mortality has been reported for forests around the world1, but our ability to predict these events remains weak2. When and where will droughts occur and what consequences will they have for the world's forests? Writing in Nature Climate Change, McDowell and Allen3 discuss a conceptual tool that points out some of the vegetation characteristics that predispose trees to drought-induced mortality. Global air temperatures are rising. While the capacity of the atmosphere to hold humidity increases exponentially with temperature, climate models and observations show that absolute atmospheric humidity does not keep pace with warming4, 5, 6, especially over land areas7. The amount by which humidity increases with warming is a very important quantity as humidity strongly affects the terrestrial biosphere. Plant physiologists employ the concept of leaf-to-air vapour pressure deficit (VPD), the difference in vapour pressure between the humid air spaces inside plant leaves and the surrounding air. Leaf-to-air VPD is predicted to increase strongly with warming4, 5, 6, 7. Plants routinely lose very large amounts of water from small pores in their leaves called stomata and this flux to the atmosphere is referred to as transpiration. The most direct response of vegetation to increased atmospheric dryness is to moderate water losses via transpiration. In the short term, plants regulate these losses by partial or complete stomatal closure. The downside of this is that photosynthesis is reduced because CO2 uptake is restricted. Therefore, plants are challenged by increased atmospheric dryness and also by the risk of starvation from declining carbohydrates8. The short-term regulation of transpiration by stomatal closure remains one of the least understood aspects of plant physiology. Yet it has important implications for global-scale modelling of carbon and water cycles and of energy balance. This regulation is likely to involve responses to VPD9, which is therefore a key variable in the interactions between forests and the atmosphere. Increased global temperatures and VPD may therefore require plants to adjust their morphological and physiological traits. Which plant traits are effective at enabling regulation of transpiration and avoidance of drought-induced mortality? Several groups have presented modelling and empirical analyses to distil this set of plant traits (for example, ref. 10). McDowell and Allen3 employ a corollary of Darcy's law, a widely used principle of environmental biology linking water supply to demand within plants. As in traditional economic analysis, a break-even point must exist between how much water a tree can scavenge from the soil and how much it can spend via transpiration to maintain rates of photosynthesis. Surprisingly, this principle also provides information on the characteristics acting when these physiological feedbacks break down under drought. In other words, the same variables that maintain balance (homeostasis) in response to background dryness also quantify the limits of homeostatic control and eventual mortality under extreme drought conditions. McDowell and Allen3 list a number of empirical observations where this breakdown applies, at scales from individual trees to forests. Stomatal responses to atmospheric dryness help us to understand how plants cope with increased water demand in the short term. But what happens in the long term? The principle that leaf transpiration is regulated to moderate water losses also works to describe the long-term response of forests to atmospheric dryness. Darcy's law makes quantitative predictions about the reduction in the area of leaves necessary to maintain homeostasis within a forest.
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