What Causes Large Local-Scale Variation in Tree Diversity in a Neotropical Forest? An Experimental Approach

9615226哈贝尔出人意料地在热带森林的小空间尺度上发现了与地形和旱季土壤水分相关的树木多样性的巨大差异。这项研究是对一个简单假设的实验检验，即热带树木的多样性在很大程度上是由苗期和小苗期的干旱胁迫控制的。在较干燥的地方，与较潮湿的地方相比，较少的树种可能具有足够深的根系作为幼苗，可以在长时间的干旱中生存下来。这一假设将通过对移植到单一热带森林内干旱季节土壤水分和干旱胁迫较多或较少的地点的苗木的浇水实验来验证。本试验将论证旱季浇水是否比不浇水的对照地苗木提高成活率和生长量。实验还将显示，这种影响是否足够大，足以解释观察到的不同地点之间物种丰富度的巨大差异。世界范围内热带森林树种丰富度的模式显示出一种强烈的、无法解释的模式，即随着年平均降雨量的增加而增加丰富度。这项研究将有助于我们了解热带森林多样性地理变异较大的潜在原因，以及这种差异是否受苗期事件的控制。这项研究将改进对全球气候变化对热带森林多样性丧失影响的预测。全球气候模型预测，下个世纪全球年降雨量和季节降雨量的分布将发生重大变化，预计热带大部分地区将变得更加干燥。

9615226 Hubbell Unexpectedly large differences in tree diversity that correlate with topography and dry season soil moisture have been found on small spatial scales in tropical forests. This research is an experimental test of the simple hypothesis that tropical tree diversity is controlled largely by drought stress in the seedling and small sapling stages. In drier sites, a smaller fraction of tree species may have sufficiently deep root systems as seedlings to survive prolonged periods of drought than is the case in wetter sites. The hypothesis will be tested by a watering experiment on seedlings transplanted to sites with greater or lesser dry season soil moisture and drought stress within a single tropical forest. The experiment will demonstrate whether dry season watering increases seedling survival and growth compared with seedlings in non-watered control plots. The experiment will also show whether the effect is sufficiently large to explain the large observed differences in species richness between sites. World-wide patterns of tree species richness in tropical forests show a strong, unexplained pattern of increasing richness with mean annual rainfall. This research will help us to understand the potential causes of large geographical variation in tropical forest diversity and whether it is controlled by events in the seedling stages. The study will improve predictions on the impact of global climate change on loss of tropical forest diversity. Major changes are forecast in the next century by Global Climate Models in the worldwide distribution of annual and seasonal rainfall, and much of the tropics is expected to become drier.