Deciphering environmental controls over the hysteresis of biome switches at savanna-forest boundaries

就面积、生物多样性、总碳储量和人类利用而言，稀树草原和常绿森林是最重要的热带植被类型，因此这些生物群分布的变化对保护、全球变化和人类福祉具有重大意义。然而，预测这些转变是具有挑战性的。一些建模研究预测森林将广泛扩展到热带稀树草原，而另一些研究则预测热带森林将大规模崩溃。预测和管理稀树草原-森林过渡的困难很大程度上源于复杂的火-植被反馈，这些反馈受到人类、气候、以前的火灾历史和地点植物物种特征的影响。提高对这些反馈的理解对于开发下一代地球系统模型和为季节性干旱的热带地区的管理提供信息是必要的。后者在巴西尤其如此，在巴西，由于未能认识到火的自然作用，导致在土著稀树草原地区采取灭火政策，导致森林侵占自然稀树草原的许多地区，并使依赖这种栖息地的物种丧失。除了为管理这些系统提供有价值的数据外，该项目还将促进人们对火的自然作用的接受，为一本科普杂志编写一篇文章，并向巴西稀树草原保护区的管理者和环境部长分发一份白皮书。这些文件将审查火灾的自然作用和灭火后果的证据。随着该项目产生结果，记者将被邀请访问研究地点，向公众宣传巴西大草原火灾的自然作用。四名美国本科生将获得国际研究经验，这些经验将直接有助于该提案的研究目标，同时激发美国学生对热带生态学的兴趣和专业知识。了解火灾和火灾历史对植被动态的影响对于美国的火灾管理很重要。森林-稀树草原交错带物种变化的驱动因素的理解受到了强烈的正反馈和其他非线性过程的阻碍，这些过程导致了复杂的动力学和滞后(即系统不仅依赖于当前环境，而且依赖于过去的环境)。这些遗留效应扰乱了我们观察植被对环境变化反应的能力。这项拟议的研究将结合现场数据和建模来测试和量化稀树草原森林动态中的滞后来源。研究的目的是(1)量化热带稀树草原和森林状态之间转换的关键过程，(2)利用这些信息改进和参数化CLM(ED-Spitfire)模型，以模拟热带稀树草原-森林动态，以及(3)进行模拟，以了解热带稀树草原和森林分布的环境控制，重点是滞后的原因。该模型代表了基于人口统计学的生态系统模型(ED)、火灾发生的机制模型(Spitfire)和陆地表面过程模型(CLM)的耦合，后者可以选择性地耦合到大气环流模型(CESM)。改进、参数化和验证该模型所需的大部分数据都来自以前由NSF资助的研究。这一信息的差距将通过在巴西天然稀树草原-森林边界进行火灾试验来解决。在这项实验中，可燃性试验将被用来了解调节植被-火灾反馈的阈值。对生物量生产和周转的测量将量化初级生产力和平均停留时间在管理生物群转移中的相对作用。对树木动态的监测将使人们更全面地了解稀树草原和森林树种在这些转变中的不同作用。

Savannas and evergreen forests are the most important tropical vegetation types in terms of area, biodiversity, total carbon stocks, and use by humans, so shifts in the distribution of these biomes have large implications for conservation, global change, and human welfare. Predicting these shifts, however, is challenging. Some modeling studies forecast widespread expansion of forest into tropical savanna, while others forecast massive collapse of tropical forest. The difficulty of predicting and managing savanna-forest transitions arises largely from complex fire-vegetation feedbacks that are influenced by humans, climate, previous fire history, and the characteristics of plant species at a site. An improved understanding of these feedbacks is necessary for developing the next generation of earth system models and for informing management of the seasonally dry tropics. The latter is particularly true in Brazil, where a failure to recognize the natural role of fire has led to policies of fire suppression in areas of native savanna, resulting in forest encroachment into many areas of natural savanna and loss of species that depend upon this habitat. In addition to providing valuable data for managing these systems, this project will promote the acceptance of the natural role of fire with an article to be prepared for a popular science magazine and a white paper to be distributed to managers of Brazilian savanna reserves and state ministers of environment. These documents will review the evidence for the natural role of fire and the consequences of fire suppression. As the project generates results, journalists will be invited to visit the study sites to inform the public of the natural role of fire in Brazilian savannas. Four US undergraduate students will be given international research experiences that will contribute directly to the research objectives of this proposal while stimulate interest and expertise in tropical ecology among US students. Understanding the impacts of fire and fire history on vegetation dynamics is important for fire management in the US.Understanding of the drivers of species change at the forest-savanna ecotone has been hindered by strong positive feedbacks and other non-linear processes that cause complex dynamics and hysteresis (i.e. the dependency of system not only on its current environment but also on its past environment). These legacy effects confound our ability to observe vegetation responses to environmental change. The proposed research will combine field data and modeling to test for and quantify sources of hysteresis in savanna-forest dynamics. The aims of the research are (1) quantify key processes that underlie switches between savanna and forest states, (2) use this information to refine and parameterize the CLM(ED-SPITFIRE) model for simulating savanna-forest dynamics, and (3) perform simulations to understand environmental controls on the distribution of tropical savanna and forest, with emphasis on causes of hysteresis. This model represents the coupling of a demography-based ecosystem model (ED), a mechanistic model of fire occurrence (SPITFIRE), and a model of land surface processes (CLM) that can optionally be coupled to a general circulation model (CESM). Much of the data needed to refine, parameterize, and validate the model are available from previous NSF-funded research. Gaps in this information will be resolved with a fire experiment at a natural savanna-forest boundary in Brazil. In the experiment, flammability trials will be used to understand thresholds that mediate vegetation-fire feedbacks. Measurements of biomass production and turnover will quantify the relative roles of primary productivity and mean residence time in governing biome shifts. Monitoring of tree dynamics will provide a more complete understanding of the distinct roles of savanna and forest tree species in these shifts.