Collaborative Research: Are Amazonian and Andean ecosystems close to a tipping point?

亚马孙热带雨林以其支持的巨大生物和文化多样性而闻名。这些森林也是全球淡水循环的重要组成部分，它们的丧失对数百万人的粮食安全以及全球气候动态产生了影响。气候变化正在威胁着这些系统，人们担心，随着气温上升和降雨变得不那么可靠，这些森林可能会被低多样性的草原所取代。然而，尚不清楚气候变化的程度可能会引发森林向草原的转变。古生态学可以提供有关过去生物多样性的信息，以及在过去变暖事件期间生物多样性模式是如何变化的。这项研究项目将调查两个气候变暖时期，一个持续大约8000年到5000年前，另一个更温暖的时期持续在12.8万年到12万年前。随着时间的推移，古代湖泊积累了沉积物，通过钻探沉积物，可以在湖泥中捕捉到该地区的历史。研究人员将分析亚马逊和安第斯山脉七个地点的湖泊沉积物岩心的化石和化学成分，以确定过去气候变化了多少，以及这些变化是否足以引发从森林到草原的转变。通过观察一系列湿度不同的地区，从非常潮湿到非常干燥，以及变暖的程度，研究人员将估计现代热带雨林距离森林到草原的转变有多近。此外，合作研究小组之间的新互动将包括共同教授的古生态学/古气候学课程，当地电影节的赞助类别，以及至少10名本科生、2名博士生和1名博士后研究人员的培训。研究将解决与气候-植被反应相关的一些假设，如闪烁、遥相关、生态系统缓冲和临界点。通过使用有机地球化学数据来估计古温度，用树叶蜡同位素来估计降水，用沉积物化学来描述干旱，用化石花粉来重建植被，用木炭来重建火灾频率，研究人员将在比现在高出2摄氏度(4华氏度)的条件下调查生态系统的变化。最后一次间冰期将提供气候比现代高1-2摄氏度时特征、火灾发生率和群落组成变化的规模指数。在直接确定日期的全新世，这些数据将评估生态系统指标的亚十年变化，并允许调查变化率。目前，还不存在亚马逊地区温度的高分辨率记录，因此将气候变化与生态指标区分开来的新方法将对定量的新热带古生态学产生变革。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Amazonian rainforests are famous for the immense biological and cultural diversity they support. These forests are also a vital part of global recycling of freshwater, and their loss has implications for food security for millions of people as well as global climate dynamics. Climate change is threatening these systems and there are concerns that as temperatures rise and rainfall becomes less reliable, these forests might be replaced by low-diversity grasslands. However, it is unclear what level of climate change might trigger a forest-to-grassland transition. Paleoecology can provide information about biological diversity in the past, and how biodiversity patterns changed during past warming events. This research project will investigate two periods of climate warming, one that lasted from about 8000 to 5000 years ago, and another even warmer time that lasted from 128,000 to 120,000 years ago. Ancient lakes build up sediments through time, and by taking a drill-core down through the sediment the history of the region can be captured in the lake mud. The researchers will analyze the fossil and chemical content of lake sediment cores from seven locations in the Amazon and Andes to establish how much the climate changed in the past, and whether those changes were enough to induce a shift from forest to grassland. By looking at a range of locations that vary in moisture, from very wet to quite dry, and degree of warming, the researchers will estimate how close the modern rainforest is to a forest-to-grassland transition. Additionally, new interactions between the collaborating research groups will include a co-taught paleoecology/paleoclimatology course, a sponsored category in a local film festival, and training of at least 10 undergraduates, two doctoral students, and one postdoctoral researcher.The research will address a number of hypotheses relating to climate-vegetation responses such as flickering, teleconnections, ecosystem buffering, and tipping points. By using organic geochemical data to estimate paleotemperature, leaf wax isotopes to estimate precipitation, sediment chemistry to characterize droughts, fossil pollen to reconstruct vegetation, and charcoal to reconstruct fire frequency, the researchers will investigate ecosystem change under conditions as much as 2 degrees C (4 degrees F) warmer than the present. The Last Interglacial will provide an index of scale of change in traits, fire prevalence and community composition in a climate 1-2 degrees C warmer than modern. In the directly-dated Holocene, the data will assess sub-decadal variance in ecosystem indicators, and allow rates of change to be investigated. Currently, no high-resolution records of Amazonian temperature exist, so the novel approach of separating climatic change from that of ecological proxies will be transformational for quantitative neotropical paleoecology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.