A global understanding of forest-savanna transitions

热带稀树草原在热带地区占主导地位，考虑到降雨和土壤，它经常出现在可能是森林的地区。长期以来，科学家们一直在关注热带稀树草原为什么会在这里，以及在气候变化下它们会如何变化，但一个普遍的理解尚未形成。遥感提供了一种解决这一问题的方法，特别是来自激光雷达和雷达传感器的新数据集，有可能描述和解释全球稀树草原向森林的转变。鉴于这些生态系统对当地居民、碳循环和生物多样性的重要性，这种理解对于应对气候和生物多样性危机至关重要。在这个博士学位课程中，你将使用这些新的遥感数据和机器学习来理解稀树草原如何以及为什么转变为森林，以及这种转变是如何变化的。你将绘制和分类世界上的稀树草原森林过渡，并测试是什么决定了他们的位置和时间稳定性。理论模型表明，稀树草原和森林之间的渐变和突变都可能发生，这取决于树木的特征。然而，在现实世界中，尚不清楚为什么在一些地方，渐变发生，而在另一些地方，则是稀疏的稀树草原和封闭的森林的马赛克。我们也不知道这些模式在气候变化、食草动物的变化（如大象的减少）和二氧化碳施肥的影响下会有多稳定。填补我们知识上的这些空白很重要，因为稀树草原被认为对气候变化特别敏感：它们被认为是全球重要的碳汇，并且随着二氧化碳的施肥使树木能够胜过草而迅速变化。因此，哪些因素控制着热带稀树草原的森林边界，以及这些因素将如何对气候变化和其他环境变化作出反应，将对热带稀树草原的未来至关重要。

Savannas dominate the tropics, and often occur in areas which could be forest, given the rainfall and soils. Why savannas are where they are, and how that might change under climate change has long preoccupied scientists, but a general understanding has yet to be developed. Remote sensing offers a way to address this problem, and new data sets from lidar and radar sensors in particular, have the potential to describe and explain savanna-forest transitions globally. Given the importance of these ecosystems for local people, carbon cycling and biodiversity, this understanding is vital for responding to the climate and biodiversity crises. In this PhD you will use these new remote sensing data and machine learning to understand how and why savannas transition into forests and how this is changing. You will map and classify the world's savanna-forest transitions and test what determines their location and temporal stability. Theoretical models suggest that both gradual and abrupt transitions between savannas and forest can occur, depending on the traits of trees. However, in the real world, it is not clear why in some places, gradual transitions occur, whilst in other, there is a mosaic of sparse savanna and closed forest. Nor do we understand how stable these patterns will be under climate change, changes to herbivory (such as the loss of elephants) and CO2 fertilisation. Filling these gaps in our knowledge is important because savannas are thought to be particularly sensitive to climate change: they are expected to both be a globally important carbon sink, and to change rapidly as CO2 fertilisation allows trees to outcompete grasses. Which factors control savanna[1]forest boundaries, and how these factors will respond to climate change and other environmental changes, will therefore be vital for the future of savannas.