Rewilding our understanding of how climate extremes affect grasslands

多年的极端干旱在过去对大平原的草原产生了很大影响，而且可能会变得更加普遍。在大平原上的最后一次大干旱--“沙尘暴”--之前，人类基本上已经通过狩猎移走了野牛和其他大型动物。因此，这些曾经广泛分布的动物在极端干旱期间的影响是未知的。然而科学家们知道，在正常时期，像野牛这样的大型食草动物会极大地改变草原的土壤和植物的生命。例如，一项长期研究发现，重新引入野牛增加了适应干旱和放牧的植物物种的丰富度。自沙尘暴以来，野牛被重新引入到整个大平原相对较小的草原地区。为了应对未来的气候变化，科学家们需要进行实验，测量野牛和其他大型食草动物如何塑造草原对极端干旱的反应。例如，经过几年的干旱，放牧的高草草原能否提供优质饲料？支持传粉者的植物生物多样性是有弹性的还是会下降？这些问题仍然没有答案，因为用于模拟干旱的实验设计--将雨水从下面的植物上转移开的大型避难所--在重新引入野牛的地方还没有经过测试。特别是，野牛可能会避开、聚集在或破坏这样的“雨停”庇护所，在这种情况下，依赖于它们的实验将无法测试它们应该测试的东西。本研究将评估多种类型的防雨避难所的有效性，以及a）野牛是否以与非防雨避难所相似的速度使用防雨避难所下的区域，B）野牛是否破坏或损坏防雨避难所，以及c）防雨避难所是否阻挡了足够的降雨来模拟极端干旱。结果将为未来的实验设计提供信息，并帮助牧场管理者确定在日益干旱的条件下放牧的最佳做法。干旱和食草动物都强烈影响草地系统，大量的工作已经量化了草地对这些驱动因素的响应。但干旱和放牧可能相互作用，影响草地的形式和功能。例如，放牧植物群落的物种组成往往与未放牧群落不同，这些物种对干旱的敏感性可能更高（或更低）。放牧植物对极端干旱的额外压力的反应能力也会降低。我们对这种相互作用的影响的理解是有限的，因为在大型食草动物存在的情况下，与操纵干旱相关的后勤挑战。该项目将使用各种实验设计，在有大型食草动物和没有大型食草动物的情况下实验性地控制干旱。目的是测试独立操纵干旱和放牧对草原植物群落的可行性。具体而言，研究人员将开发方法，以建立实验干旱操纵在大平原，野牛，在Konza草原生物站和长期生态研究网站的一个关键的大型食草动物的存在。研究人员将建立防雨庇护所，并量化庇护所下的野牛使用和活动水平。他们将对庇护所设计进行迭代方法，首先测试一种设计，然后根据需要进行修改，以引起预期的反应（即，在庇护所下和远离庇护所的地方使用类似水平的野牛，以及庇护所下土壤湿度减少50%）。研究结果将为设计一个更大规模的干旱和放牧对植物群落的相互作用实验提供必要的试验数据。长期目标是了解这两个基本驱动因素如何影响草原种群、社区和生态系统。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Multi-year extreme droughts have greatly impacted grasslands of the Great Plains in the past and will likely become more common. Before the last big drought in the Great Plains -- the “Dust Bowl” -- humans had essentially removed bison and other large animals by hunting. Therefore, the influence of these once widespread animals during extreme drought is unknown. Yet scientists know that in normal times, large grazers such as bison drastically alter grasslands’ soil and plant life. For example, one long-term study found that reintroduction of bison increases the abundance of plant species adapted to drought and grazing. Since the Dust Bowl, bison have been reintroduced to relatively small areas of grassland throughout the Great Plains. To prepare for future changes in climate, scientists need experiments that measure how bison and other large grazers shape grasslands’ response to extreme droughts. For instance, can grazed tallgrass prairie provide quality forage after several years of drought? Will the plant biodiversity that supports pollinators be resilient or decline? Such questions remain unanswered because the experimental design used to simulate droughts -- large shelters that divert rain away from plants underneath -- is untested in places where bison have been reintroduced. In particular, bison may avoid, congregate under, or destroy such “rainout” shelters, in which case experiments that rely on them would be unable to test what they were supposed to test. This study will assess the effectiveness of multiple types of rainout shelters and whether a) bison use areas under rainout shelters at similar rates as areas not under rainout shelters, b) bison destroy or damage rainout shelters, and c) whether rainout shelters block enough rainfall to simulate extreme drought. Results will inform future experimental designs and help rangeland managers determine best practices for grazing under increasingly arid conditions.Both drought and grazers strongly influence grassland systems, and a large body of work has quantified the response of grasslands to each of these drivers separately. But drought and grazing may interact to influence grassland form and function. For example, grazed plant communities often have different species composition than ungrazed communities, and those species could be more (or less) sensitive to drought. Grazed plants could also have a reduced ability to respond to the additional stressor of extreme drought. Our understanding of such interacting effects is limited because of the logistical challenges associated with manipulating drought in the presence of large grazers. This project will experimentally manipulate drought in the presence and absence of large grazers, using a variety of experimental designs. The goal is to test the feasibility of independently manipulating drought and grazing on communities of prairie plants. Specifically, researchers will develop methodologies to establish experimental drought manipulations in the presence of a key large grazer in the Great Plains, bison, at Konza Prairie Biological Station and Long-Term Ecological Research Site. Researchers will establish rainout shelters and quantify bison use and activity levels under the shelters. They will conduct an iterative method of shelter designs, testing one design first and then modifying as necessary to elicit the intended responses (namely, similar levels of bison use under and away from the shelters and a 50% reduction in soil moisture under the shelters). Results will provide pilot data essential for designing a much larger experiment on the interaction of drought and grazing on plant communities. The long-term objective is to understand how these two fundamental drivers influence grassland populations, communities, and ecosystems.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.