Collaborative Research: Rainfall variability and the axes of tree-grass niche differentiation

稀树草原生物群系是一个世界性的分布区，其中一个不连续的乔木层与一个连续的草层共存。越来越多的研究表明，热带稀树草原中树木和草生物量的相对比例受到降雨模式的强烈影响，降雨量越大，木质生物量越大。尽管越来越多的证据表明降雨的影响，但是，目前还不清楚，如果更多的降雨导致更大的树木生物量在热带稀树草原，因为更深的水渗透，降雨强度的变化，或因为生长季节的长度与年降雨量增加。这个问题很重要，因为我们不能准确地预测未来的变化，热带稀树草原生物群落没有一个详细的了解树木和草物种如何应对降雨模式。该项目将以南非热带草原为模型系统，寻求这一问题和其他问题的答案。北美的热带稀树草原已被森林所取代，其原因尚不完全清楚。该项目的结果可用于为北美和其他地方的稀树草原的恢复和维护提供信息，并广泛预测稀树草原生物群落未来可能发生的变化。该项目还将通过多年实地课程向本科生介绍热带稀树草原的生态学，并教育当地社区了解降雨量变化对自然资源可用性的作用，该项目将联合收割机结合实验、观察和建模方法，以检验关于热带稀树草原降雨量与树木与草比例之间关系的各种假设。 该研究有三个具体目标：（1）确定有利于树木的降雨制度，（2）确定热带稀树草原树木和草的功能特征和权衡，（3）开发和测试树木和草动态作为降雨量的函数的机械模型。这项研究的核心内容将是一项实地实验，该实验将操纵降雨的时间、季节性和深度分布，并监测嵌入草基质中的已知树龄树木的多物种组合的反应。此外，该项目还将研究功能差异（例如，通过温室、生长室和田间试验，研究了6种乔木和6种禾本科植物的耐旱性、水分利用效率和功能根深等指标。最后，该项目将使用第二个组成部分的参数估计值，开发和测试一个基于整株性状的树木生长和树木与草比例的机械模型，并通过比较模拟和观测到的树木与草的比例变化与现场实验中的降雨量操纵，在现场条件下验证模型。该奖项反映了NSF的法定使命，并通过使用基金会的学术价值和更广泛的影响审查标准。

The savanna biome, in which a discontinuous tree layer coexists with a continuous grass layer, has a worldwide distribution. Increasingly, research suggests that the relative proportion of tree and grass biomass in savannas is strongly influenced by rainfall patterns, with higher rainfall leading to greater woody biomass. Despite mounting evidence for rainfall effects, however, it is unclear if more rainfall leads to greater tree biomass in savannas because of deeper water infiltration, changes in rainfall intensity, or because the length of the growing season increases with the amount of annual rainfall. This question is important because we cannot accurately predict future changes to the savanna biome without a detailed understanding of how tree and grass species respond to rainfall patterns. This project will seek answers to this and other questions using a South African savanna as a model system. North American savannas have been largely replaced by forests, and the reasons for this are not fully understood. Results from this project can be used to inform the restoration and maintenance of savannas in North America and elsewhere, as well as to predict with broad generality how the savanna biome might change in the future. The project will also introduce undergraduate students to the ecology of savannas through a multi-year field course and educate local communities about the role of rainfall variability on natural resource availability.The project will combine experimental, observational and modeling approaches to test alternative hypotheses about the relationship between rainfall and tree to grass ratios in savannas. The research has three specific aims: (1) identify the rainfall regimes that favor trees over grasses, (2) identify the functional traits and tradeoffs that differentiate savanna trees and grasses, and (3) develop and test a mechanistic model of tree and grass dynamics as a function of rainfall. The core element of the study will be a field experiment that will manipulate the timing, seasonality and depth distribution of rainfall, and monitor the response of a multi-species assemblage of known-age trees embedded in a grass matrix. In addition, the project will examine functional differences (e.g., drought tolerance, water use efficiency and functional rooting depth) between six tree and six grass species using greenhouse, growth chamber and field experiments. Finally, the project will use parameter estimates from this second component to develop and test a mechanistic model of tree growth and tree to grass ratios based on whole-plant traits, and validate the model under field conditions by comparing modeled and observed changes in tree to grass ratios to rainfall manipulations in the field experiment.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.