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，频繁的火灾对维持草原和稀树草原至关重要，草原和稀树草原覆盖了地球表面的40%。热带稀树草原是树木和草的动态混合物，受火灾和干旱的控制。然而，树木和草之间的平衡如何维持数百年或数千年仍然是一个谜。此外，目前还不清楚热带稀树草原生态系统可能如何长期变化，以及这些变化如何影响重要的生态过程，如碳储存和营养循环。这项研究将结合联合收割机的知识，从火灾实验开始于1964年，树木年轮记录延伸到过去几十年，和沉积物核心从一个湖泊，以了解如何变化的干旱和火灾影响橡树稀树草原随着时间的尺度从几年到几个世纪。这项研究将提供美国中西部上游橡树稀树草原的第一次火灾历史和该地区唯一的长期营养记录之一。由此产生的分析还将提供信息的管理人员在高草草原和橡树草原火灾科学联盟谁需要更多的信息，长期火灾制度和历史火灾模式，以预测未来的环境变化的反应。许多研究产品将在开放获取期刊上提供。热带稀树草原和火的话题将通过与流行的MinuteEarth频道制作视频与公众分享。这项研究将把新的古生态数据与美国明尼苏达州长期研究地点雪松溪生态系统科学保护区几十年来的生态实验数据相结合。这些实验，这是位于温带橡树稀树草原，表明火灾频率的增加，减少了氮的数量，并减缓其循环，因为火灾引起的氮损失，因为植被从橡树为主的林地到草地的转变。在燃烧频率梯度上，将测量草和树下的土壤样本的碳、氮浓度和同位素组成。将利用沿着梯度对几个树种进行年轮取样，以制定新的年轮宽度年表和具有精确测年和季节分辨率的林分年龄结构。这些年表将被用来评估生长响应N循环的变化，过去的干旱事件，火灾频率。一个全新世时代的沉积物芯雪松沼泽湖将进行分析，以评估火灾的历史，过去的橡树：草的动态，和N循环指标。通过这种新颖的树木年轮数据，湖泊沉积物测量和来自53年受控燃烧实验的信息的组合，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)火和气候又是如何相互作用影响橡树在混合橡树草植被中的优势地位的？(3)在气候和火灾频率长期变化的背景下，橡树与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?