Xu Xiangtao, Medvigy David, Rodriguez-Iturbe Ignacio
Department of Geosciences, Princeton University, Princeton, NJ 08544;
Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544
Proc Natl Acad Sci U S A. 2015 Oct 20;112(42):12992-6. doi: 10.1073/pnas.1517382112. Epub 2015 Oct 5.
Tree abundance in tropical savannas exhibits large and unexplained spatial variability. Here, we propose that differentiated tree and grass water use strategies can explain the observed negative relation between maximum tree abundance and rainfall intensity (defined as the characteristic rainfall depth on rainy days), and we present a biophysical tree-grass competition model to test this idea. The model is founded on a premise that has been well established in empirical studies, namely, that the relative growth rate of grasses is much higher compared with trees in wet conditions but that grasses are more susceptible to water stress and lose biomass more quickly in dry conditions. The model is coupled with a stochastic rainfall generator and then calibrated and tested using field observations from several African savanna sites. We show that the observed negative relation between maximum tree abundance and rainfall intensity can be explained only when differentiated water use strategies are accounted for. Numerical experiments reveal that this effect is more significant than the effect of root niche separation. Our results emphasize the importance of vegetation physiology in determining the responses of tree abundance to climate variations in tropical savannas and suggest that projected increases in rainfall intensity may lead to an increase in grass in this biome.
热带稀树草原的树木丰度呈现出巨大且无法解释的空间变异性。在此，我们提出，树木和草本植物不同的水分利用策略可以解释观测到的最大树木丰度与降雨强度（定义为降雨日的特征降雨深度）之间的负相关关系，并且我们提出了一个生物物理树木 - 草本植物竞争模型来验证这一观点。该模型基于一个在实证研究中已得到充分证实的前提，即，在湿润条件下，草本植物的相对生长速率比树木高得多，但草本植物更容易受到水分胁迫，在干旱条件下生物量损失更快。该模型与一个随机降雨发生器相结合，然后使用来自几个非洲稀树草原地点的实地观测数据进行校准和测试。我们表明，只有考虑到不同的水分利用策略时，才能解释观测到的最大树木丰度与降雨强度之间的负相关关系。数值实验表明，这种效应比根系生态位分离的效应更为显著。我们的结果强调了植被生理学在确定热带稀树草原树木丰度对气候变化的响应中的重要性，并表明预计降雨强度的增加可能导致该生物群落中草本植物数量增加。
