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Alternative stable states between subtropical forest and grassland: A palaeoecological case study in southern Brazil

亚热带森林和草原之间的替代稳定状态：巴西南部的古生态案例研究

基本信息

批准号：
299080296
负责人：
Professor Dr. Thomas Giesecke
金额：
--
依托单位：
Senckenberg Biodiversität und Klima Forschungszentrum (BiK-F)
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2016
资助国家：
德国
起止时间：
2015-12-31 至 2020-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/299080296?language=en
关键词：
Alternative stable states between subtropical

项目摘要

Forest and grassland are vastly contrasting terrestrial ecosystems that provide different ecosystem services. While frequent fires maintain grasslands, fires are suppressed by forests. Thus both systems are self-stabilizing and may occur under the same climate conditions. Large regions of the tropics and sub-tropics are climatically suitable for both ecosystems, and shifts between states may have consequences for regional and possibly global climate. Understanding the determinants of this bi-stability has recently received much attention, but has been mainly based on modern observations and model simulations. Palaeoecological techniques provide the opportunity to study the dynamics of state changes over longer time periods and thus contribute to the ongoing discussion. The natural vegetation in southern Brazil represents a mosaic of humid forests and grasslands, with forests expanding into the grasslands over the last 4000 years. This situation provides an ideal setting for a case study uncovering tipping points, determining factors of system resilience, as well as the dynamics of such a state shift. A number of pollen diagrams exist that describe this change, but are from small peat bogs with low sedimentation rate during the early and mid-Holocene, precluding detailed analysis of the process. The proposal aims to carry out a high resolution multi-proxy investigation on fire and vegetation dynamics from a 10 m Holocene sediment core of a small lake as a base for comparison with simulations from dynamic vegetation models. The timing of the forest expansion at different localities within the region provides additional information to understand the drivers and triggers of this vegetation shift, and the project aims to improve the dating of this change for already investigated sites and to provide additional local investigations. Grassland and forest species differ in pollen production and dispersal, and to reduce this bias and help in interpretation, an investigation will address the pollen-vegetation relationship. This will allow quantified estimates of the extent of grassland and its replacement by forest. The combined information will be used to test three competing hypotheses: a) Holocene precipitation increased gradually with a strengthened South American Monsoon, while forest expansion was limited by frequent fires and only occurred once a threshold was passed; b) a stepwise change in the precipitation regime triggered the rapid forest expansion; humans assisted in the spread of the Araucaria forest as the seeds have high nutritious value. A mechanistic vegetation model will be used for hypotheses testing, by forcing it with different climate scenarios and comparing the simulations against the quantitative vegetation reconstructions. Further data model experiments will be designed to test the current theory on alternative stable states between grasslands and forests.

森林和草原是截然不同的陆地生态系统，提供不同的生态系统服务。频繁发生的火灾可以保护草原，而森林则可以抑制火灾。因此，这两个系统都是自稳定的，并且可以在相同的气候条件下发生。热带和亚热带的大片地区在气候上都适合这两种生态系统，状态之间的变化可能会对区域甚至全球气候产生影响。了解这种双稳定性的决定因素最近受到了广泛关注，但主要基于现代观察和模型模拟。古生态技术提供了研究较长时期内状态变化动态的机会，从而有助于正在进行的讨论。巴西南部的自然植被是潮湿森林和草原的镶嵌体，在过去 4000 年里森林不断向草原扩展。这种情况为揭示临界点、系统弹性的决定因素以及这种状态转变的动态的案例研究提供了理想的环境。存在许多描述这种变化的花粉图，但它们来自全新世早期和中期沉降速率较低的小型泥炭沼泽，无法对该过程进行详细分析。该提案旨在以小湖 10 m 全新世沉积岩芯为基础，对火灾和植被动态进行高分辨率多代理调查，作为与动态植被模型模拟进行比较的基础。该地区不同地点森林扩张的时间提供了额外的信息，以了解这种植被变化的驱动因素和触发因素，该项目旨在改进已调查地点的这种变化的年代测定，并提供额外的当地调查。草原和森林物种在花粉产生和传播方面存在差异，为了减少这种偏差并帮助解释，一项调查将解决花粉与植被的关系。这将有助于对草原范围及其被森林的替代情况进行量化估计。综合信息将用于检验三个相互竞争的假设：a）随着南美季风的加强，全新世降水量逐渐增加，而森林扩张受到频繁火灾的限制，只有在超过阈值后才会发生； b) 降水状况的逐步变化引发了森林的快速扩张；由于种子具有很高的营养价值，人类帮助了南洋杉森林的传播。机械植被模型将用于假设检验，通过不同的气候情景来强制其模拟，并将模拟与定量植被重建进行比较。将设计进一步的数据模型实验来测试当前关于草原和森林之间替代稳定状态的理论。