How do global change and functional traits influence savanna woody plant encroachment?

森林砍伐和全球环境变化造成的热带生态系统退化是损害生物多样性和人民生计的一个重大全球问题。退化可以被定义为生态系统功能和服务的侵蚀。在热带森林中，它主要是由于砍伐森林造成的。然而，热带稀树草原的情况不同，因为他们提供的许多服务依赖于生长在零星树木之间的青草植被。因此，在这些生态系统中，植树或自然入侵可能会导致退化。稀树草原上树木密度的增加会导致退化，因为它们遮蔽了草地。在非洲标志性的大草原上，卫星数据显示，整个非洲大陆的大草原都在受到树木的自然入侵，不包括草，形成了茂密的带刺灌木丛。这是一个退化的过程，因为人们需要草来放牧他们的动物，树木比草消耗更多的水，耗尽饮用水供应，茂密的灌木丛对在开阔的草丛中茁壮成长的稀树大草原野生动物不利。我们的项目将研究全球环境变化如何导致树木入侵非洲大草原。它侧重于标志性生态系统的这种大陆转型的根本原因，旨在解决有关全球变化驱动因素的根本未知，特别是大气二氧化碳上升如何与水供应相互作用。我们将进行大规模的野外实验，调查全球变化如何导致树木入侵稀树草原，特别是评估两个观测结果。首先，卫星数据和实地调查表明，在潮湿的热带大草原上，树木的侵蚀正在加速，但在干燥的热带大草原上，这种侵蚀是缓慢的。我们将通过观察二氧化碳上升如何与降雨相互作用，并测试在干旱条件下二氧化碳上升对草比树木更有利的观点，来测试对这一观察结果的可能解释，从而导致草比树木更具竞争力。其次，实地调查表明，只有特定树种才是入侵稀树草原的罪魁祸首。我们将测试这样的想法，即二氧化碳增加增强了这些树的生长特征，使它们能够躲避火灾，在被草食动物吃掉后恢复，并有效地与草竞争。相反，我们预计不会在不侵蚀稀树草原的树木中看到同样水平的增强。我们将在南非一个独特的设施中进行实验，该设施使用3米高的开顶式气室来浓缩大气中的二氧化碳，预计在不久的将来会达到一个水平。我们的实验将比较稀树草原树和草在当前和不久的将来二氧化碳浓度下的生长，在竞争下，当生根于充满稀树大草原土壤的大盆中时。为了测试我们关于树木、草、二氧化碳和水之间相互作用的想法，我们将在有草竞争和没有草竞争的情况下种植树木，并在土壤水分有效性高或低的情况下种植树木。在干旱条件下，增加二氧化碳对草的好处比对树木的好处更大这一想法是新的，我们还没有意识到它的广泛意义。因此，在实验的同时，我们将建立一个实地监测计划，以查看稀树草原草和树木何时何地受到水分限制。我们将使用这些信息来解释我们的实验结果。限水的范围和频率有多大，以及限水和二氧化碳上升之间的相互作用在哪里/什么时候最有可能变得重要？由于二氧化碳已经从历史水平上升，对牧草的任何影响在长期的稀树草原管理实验中也应该是明显的，在这些实验中，几十年来一直应用相同的火灾制度。因此，我们将分析这些实验的数据，以寻找牧草产量的变化。总体而言，我们的工作将在理解全球变化如何导致稀树草原树入侵以及涉及的生态机制方面取得重要进展。

The degradation of tropical ecosystems by deforestation and global environmental change is a major global issue that harms biodiversity and people's livelihoods. Degradation can be defined as the erosion of ecosystem functions and services. In tropical forests, it arises largely from deforestation. However, the situation is different in tropical savannas because many of the services they provide depend on the grassy vegetation growing between scattered trees. In these ecosystems, degradation can therefore be caused by tree planting or natural tree invasion.Increasing tree densities in savannas lead to degradation because they shade out the grasses. In the iconic savannas of Africa, satellite data shows that, all across the continent, savannas are being naturally invaded by trees, excluding grasses to make dense thorny thickets. This is a process of degradation because people need grasses to graze their animals, trees use more water than grasses and deplete drinking water supplies, and dense thickets are bad for the savanna wildlife that thrives in open grassy vegetation.Our project will study how global environmental change is causing the invasion of trees into African savannas. It focuses on the root cause of this continental transformation of iconic ecosystems, aiming to resolve fundamental unknowns about global change drivers, especially how rising atmospheric CO2 interacts with water availability. We will carry out a large-scale field experiment, investigating how global change causes tree encroachment into savannas, and particularly evaluating two observations. First, satellite data and field surveys have shown that tree encroachment is accelerating in wet savannas but is slow in dry savannas. We will test a possible explanation for this observation, by looking at how rising CO2 interacts with rainfall, and testing the idea that rising CO2 benefits grasses more than trees in dry conditions, causing grasses to out-compete trees. Secondly, field surveys have shown that only particular tree species are responsible for encroachment into savannas. We will test the idea that rising CO2 enhances the growth characteristics of these trees that enable them to escape burning by fires, recover after being eaten by herbivores, and compete effectively with grasses. Conversely, we don't expect to see the same level of enhancement in trees that don't encroach into savannas.We will run the experiment in a unique South African facility that uses 3-metre open-top chambers to enrich the atmosphere in CO2 to a level expected in the near future. Our experiment will compare savanna tree and grass growth under current and near-future CO2 concentrations, under competition when rooted in large pots filled with a savanna soil. To test our ideas about the interactions between trees, grasses, CO2 and water, we will grow the trees with and without grass competition, and with a high or low soil water availability.The idea that rising CO2 benefits grasses more than trees in dry conditions is new and we do not yet appreciate its broad significance. Therefore, in parallel with the experiment, we will set up a field monitoring programme to look at when and where savanna grasses and trees are limited by water. We will use this information to interpret our experimental results. How widespread and frequent is water-limitation, and where / when is the interaction between water-limitation and rising CO2 most likely to become important? Because CO2 has already risen from historical levels, any effects on grasses should also be evident in long-term savanna management experiments where the same fire regime has been applied for decades. We will therefore analyse data from these experiments to look for changes in grass production.Overall, our work will bring an important advance in understanding of how global change causes savanna tree encroachment, and the ecological mechanisms involved.