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

森林砍伐和全球环境变化导致的热带生态系统退化是一个重大的全球性问题，危害生物多样性和人民的生计。退化可界定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有所不同，因为它们提供的许多服务都依赖于生长在分散的树木之间的草地植被。因此，在这些生态系统中，树木种植或自然树木入侵都可能导致退化，热带稀树草原树木密度的增加会导致退化，因为树木遮蔽了草地。在非洲标志性的热带草原上，卫星数据显示，在整个非洲大陆，热带草原正在被树木自然入侵，排除了草，形成茂密的荆棘丛。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，消耗饮用水供应，密集的灌木丛对在开放的草地植被中茁壮成长的热带草原野生动物不利。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因素，特别是大气中不断上升的二氧化碳如何与水资源的可用性相互作用。我们将进行一个大规模的野外实验，调查全球变化如何导致树木侵入稀树草原，特别是评估两个观察结果。首先，卫星数据和实地调查表明，树木在潮湿的稀树草原上的侵蚀正在加速，但在干燥的稀树草原上的侵蚀速度很慢。我们将通过观察不断上升的二氧化碳如何与降雨相互作用来测试对这一观察结果的可能解释，并测试在干旱条件下，不断上升的二氧化碳对草比树更有利的想法，导致草比树竞争。第二，实地调查表明，只有特定的树种才是造成稀树草原侵蚀的原因。我们将测试这样一种想法，即二氧化碳的上升增强了这些树木的生长特性，使它们能够逃脱火灾的燃烧，在被食草动物吃掉后恢复，并有效地与草竞争。相反，我们不希望看到不侵入热带稀树草原的树木也能达到同样的增强水平。我们将在南非一个独特的设施中进行实验，该设施使用3米长的开顶室，在不久的将来将大气中的二氧化碳浓度提高到预期的水平。我们的实验将比较热带稀树草原树木和草的生长在当前和不久的将来的CO2浓度下，在竞争下，扎根在大盆充满了热带稀树草原土壤。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，我们将在有和没有草竞争的情况下种植树木，并在土壤水分可用性高或低的情况下种植树木。在干旱条件下，二氧化碳上升对草比树木更有利的想法是新的，我们还没有意识到它的广泛意义。因此，在进行实验的同时，我们将建立一个实地监测方案，以了解稀树草原的草和树何时何地受到水的限制。我们将利用这些信息来解释我们的实验结果。水资源限制有多普遍和频繁，水资源限制和二氧化碳上升之间的相互作用在何处/何时最有可能变得重要？由于二氧化碳已经从历史水平上升，对草的任何影响也应该在长期的热带稀树草原管理实验中很明显，在这些实验中，相同的火灾制度已经应用了几十年。因此，我们将分析这些实验的数据，以寻找草产量的变化。总的来说，我们的工作将带来一个重要的进步，了解全球变化如何导致稀树草原树木入侵，以及所涉及的生态机制。

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.