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

热带稀树草原生物群系是一个不连续的乔木层与连续的草层共存的群落，具有世界性的分布。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，木本生物量就越大。然而，尽管有越来越多的证据表明降雨会产生影响，但尚不清楚更多的降雨是否会导致热带稀树草原树木生物量增加，这是因为更深的水渗透、降雨强度的变化，还是因为生长季节的长度随着年降雨量的增加而增加。这个问题很重要，因为如果不详细了解树木和草类物种对降雨模式的反应，我们就无法准确预测热带稀树草原生物群落的未来变化。该项目将以南非稀树大草原为模型系统，寻求这个问题和其他问题的答案。北美的热带稀树草原大部分被森林所取代，其原因尚不完全清楚。该项目的结果可用于北美和其他地区稀树草原的恢复和维护，以及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的实地课程向本科生介绍热带草原的生态，并向当地社区介绍降雨变化对自然资源可用性的作用。该项目将结合实验、观察和建模方法来检验关于热带草原降雨和草木比之间关系的各种假设。该研究有三个具体目标：(1)确定有利于树木而不是草的降雨制度；(2)确定区分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的动态机制模型作为降雨的函数。该研究的核心要素将是一项野外实验，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年龄的多物种树木的反应。此外，该项目将通过温室、生长室和田间试验，研究6种树种和6种草的功能差异（如耐旱性、水分利用效率和功能生根深度）。最后，该项目将使用第二部分的参数估计来开发和测试基于全株性状的树木生长和树草比的机制模型，并通过比较模拟和观察到的树草比变化以及田间试验中的降雨操纵来验证该模型在田间条件下的有效性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.