Collaborative Research: SG: Fire, climate, and vegetation change in an oak savanna ecosystem from annual to millennial time scales

火灾和干旱影响重要的生态系统功能，如生产力、碳储存和养分循环。如今，频繁的火灾对于维持占地球表面40%的草原和稀树草原非常重要。稀树草原是树木和草的动态混合物，受到火灾和干旱的双重控制。然而，树木和草之间的平衡究竟是如何在几个世纪或几千年里保持的，这仍然是一个谜。此外，目前尚不清楚热带稀树草原生态系统如何在长期内发生变化，以及变化如何影响碳储存和养分循环等重要生态过程。这项研究将结合1964年开始的火灾实验的知识，延续几十年的树木年轮记录，以及来自湖泊的沉积物核心，以了解干旱和火灾的变化如何在数年到数百年的时间尺度上影响橡树草原。这项研究将提供美国中西部橡树大草原的第一次火灾历史，以及该地区唯一的长期营养记录之一。结果分析还将为高草草原和橡树稀树草原火灾科学联盟的管理者提供信息，他们需要更多关于长期火灾制度和历史火灾模式的信息，以预测未来对环境变化的反应。许多研究成果将在开放获取期刊上提供。稀树草原与火灾的主题，将透过与热门的MinuteEarth频道制作影片，与大众分享。这项研究将把新的古生态学数据与美国明尼苏达州长期研究基地雪松溪生态系统科学保护区几十年的生态实验数据结合起来。这些在温带橡树稀树草原上进行的实验表明，火灾频率的增加减少了氮的数量，并减缓了氮的循环，这既是由于火灾引起的氮损失，也是由于植被从橡树为主的林地向草地的转变。在一个燃烧频率梯度上，草和树下面的土壤样本将被测量碳、氮浓度和同位素组成。沿着梯度的几个物种的树木年轮采样将用于开发新的年轮宽度年表和林分年龄结构，具有精确的年代测定和季节分辨率。这些年表将用于评估生长对氮循环变化、过去干旱事件和火灾频率的响应。对雪松沼泽湖全新世沉积物岩心进行分析，以评估火灾历史、过去的橡树：草动态和N循环指标。通过将树木年轮数据、湖泊沉积物测量数据和53年对照燃烧实验的信息结合起来，研究人员将解决三个研究问题：(1)与全新世早期的气候影响相比，人类和气候如何影响最近300年的火灾状况？(2)火与气候如何相互作用影响橡树在混生橡树草植被中的优势度？(3)在气候和火灾频率长期变化的背景下，橡树和草优势的变化如何改变氮循环？

Fire and drought influence important ecosystem functions like productivity, carbon storage and nutrient cycling. Today, frequent fires are important for maintaining grasslands and savannas, which cover 40% of the surface of the Earth. Savannas are dynamic mixtures of trees and grass that are controlled by both fire and drought. It remains a mystery, however, exactly how the balance between trees and grasses is maintained over centuries or millennia. Furthermore, it is unclear how savanna ecosystems are likely to change over the long term and how changes may affect important ecological processes like carbon storage and nutrient cycling. This research will combine knowledge from a fire experiment begun in 1964, tree-ring records extending decades into the past, and a sediment core from a lake to understand how changes in drought and fire affect the oak savanna over time scales ranging from years to centuries. This research will provide both the first fire history of the oak savanna in the upper Midwestern U.S. and one of the only long-term nutrient records for this region. The resulting analyses will also provide information to managers in the Tallgrass Prairie and Oak Savanna Fire Science Consortium who need more information regarding long-term fire regimes and historical fire patterns to forecast future responses to environmental change. Many research products will be provided in open-access journals. The topic of savannas and fire will be shared with the public through the development of a video with the popular MinuteEarth Channel.This research will integrate new paleoecological data with data from several decades of ecological experiments at Cedar Creek Ecosystem Science Reserve, a long-term research site in Minnesota, USA. These experiments, which are situated in a temperate oak savanna, suggest that increased fire frequency reduces the quantity of nitrogen and slows down its cycling both because of fire-induced nitrogen losses and because of a vegetation shift from an oak-dominated woodland to a grassland. Across a burn frequency gradient, soil samples below grass and trees will be measured for carbon, N concentration and isotopic composition. Tree-ring sampling of several species along the gradient will be used to develop new ring-width chronologies and stand age structures with precise dating and seasonal resolution. These chronologies will be used to assess growth response to N cycling changes, past drought episodes, and fire frequency. A Holocene-age sediment core from Cedar Bog Lake will be analyzed to assess fire history, past oak:grass dynamics, and N cycling metrics. With this novel combination of tree-ring data, lacustrine sediment measurements and information from a 53-year controlled burn experiment, researchers will address three research questions: (1) How have humans and climate influenced fire regime for the most recent 300 years compared with climate influences earlier in the Holocene? (2) How have fire and climate in turn interacted to influence oak dominance in mixed oak-grass vegetation? (3) How do changes in oak vs. grass dominance alter nitrogen cycling in the context of long-term changes in climate and fire frequency?