Collaborative Research: The Role of Spatial Interactions in Determining the Distribution of Savanna and Forest

长期以来，生态学家一直强调气候是决定大尺度生态系统动态的主要因素，但气候和植被之间的关系并不总是确定性的。在植被-环境反馈显著的地方(例如在热带稀树草原)，预测对气候的反应可能特别困难。最近的研究表明，与仅由气候决定稀树草原的分布相比，火的反馈可能会使稀树草原更加常见；因此，稀树草原和森林对全球变化的反应可能是剧烈的、突然的和难以预测的。然而，现有的工作没有解释为什么稀树草原和稀树草原以及森林和森林在空间上聚集--这种模式表明空间过程也可能在决定生态系统对气候的反应方面发挥作用。研究人员将考虑稀树草原和森林之间的空间相互作用对它们的分布以及它们对过去和未来气候和土地利用变化的潜在响应的影响。研究人员确定了三个假说来解释稀树草原和森林的空间聚集：H1)稀树草原和森林是双稳的，初始条件下的空间结构(过去气候的结果)决定了它们的分布，H2)稀树草原和森林是双稳的，但稀树草原内的空间过程(例如，火蔓延)导致了空间结构的分布，和/或H3)稀树草原和森林之间的最近邻相互作用改变了它们在长时间尺度上的分布，影响了它们的长期稳定性。这些假设在经验文献中得到了不同程度的支持，现有的工作并没有试图理清这些过程。研究结果将使研究小组能够对全球稀树草原和森林过去和未来的分布做出明智的理论和经验预测。这项拟议的工作还将产生新的数学结果。理论、空间-随机模型的可能结果包括a)稀树草原和森林在景观中共存，b)稀树草原对森林的排斥，c)森林对稀树草原的排斥，或d)生物群落稀树草原/森林优势地位的替代稳定状态。最后的结果在齐次空间随机模型中是不太可能的，其中获胜的生物群由生物群边界(即前部)的移动方向决定，但最接近真实的生物群分布。空间随机模型的结果将通过理论和模拟模型与观测值进行协调。

Ecologists have long emphasized climate as primary factor determining ecosystem dynamics at large scales, but the relationship between climate and vegetation is not always deterministic. Where vegetation-environment feedbacks are significant (e.g., in savannas), predicting responses to climate can be especially difficult. Recent work suggests that feedbacks with fire may make savannas much more common than they would be were their distributions solely determined by climate; as a result, savanna and forest responses to global change may thus be drastic, sudden, and difficult to foresee. However, existing work does not explain why savanna is spatially aggregated with savanna and forest with forest -- a pattern that indicates that spatial processes may also play a role in determining ecosystem responses to climate. Here the investigators will consider the impacts of those spatial interactions between savanna and forest on their distributions and on their potential responses to climate and land-use change, both in the past and into the future.The investigators have identified three hypotheses to explain the spatial aggregation of savanna and forest: H1) that savanna and forest are bistable, and that spatial structure in initial conditions (as a result of past climate) determines their distributions, H2) that savanna and forest are bistable, but that spatial processes within savanna (e.g., fire spread) result in spatially structured distributions, and/or H3) that nearest neighbor interactions between savanna and forest change their distributions on long time scales, impacting their long-term stability. These hypotheses are variously supported in the empirical literature, and existing work has not attempted to disentangle these processes. Results will allow the research team to generate informed theoretical and empirical predictions about the past and future distribution of savanna and forest globally. The proposed work will also generate novel mathematical results. Possible outcomes of theoretical, spatial-stochastic models include a) savanna and forest coexistence in landscapes, b) forest exclusion by savanna, c) savanna exclusion by forest, or d) alternative stable states in biome savanna/forest dominance. The last outcome would be unlikely in a homogeneous spatial stochastic model, where the winning biome is decided by the direction of movement of the biome boundary (i.e., the front), but most closely resembles real biome distributions. Spatial stochastic model results will be reconciled with observations using theoretical and simulation modeling.