Rewilding our understanding of how climate extremes affect grasslands

过去，多年的极端干旱对大平原的草原造成了极大的影响，而且可能会变得更加常见。在大平原上一次大干旱之前--“沙尘暴”--人类基本上是通过捕猎来消灭野牛和其他大型动物。因此，这些曾经分布广泛的动物在极端干旱期间的影响尚不得而知。然而，科学家们知道，在正常情况下，野牛等大型食草动物会极大地改变草原的土壤和植物生命。例如，一项长期研究发现，重新引入野牛会增加适应干旱和放牧的植物物种的数量。自沙尘暴以来，野牛被重新引入大平原相对较小的草原地区。为了为未来的气候变化做准备，科学家们需要测量野牛和其他大型食草动物如何影响草原对极端干旱的反应的实验。例如，经过几年干旱后，放牧的马尾草草原能否提供优质牧草？支持传粉者的植物生物多样性是有弹性的还是下降的？这些问题仍然没有答案，因为用于模拟干旱的实验设计--大型避难所将雨水从植物下面分流出去--在重新引入野牛的地方没有经过测试。具体地说，野牛可能会避开、聚集在或摧毁这样的“防雨”掩体，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。这项研究将评估多种类型的避雨场所的有效性，以及a)野牛使用避雨场所下的区域的比率是否与非避雨场所相似，b)野牛破坏或损坏避雨场所，以及c)避雨场所是否阻挡了足够的降雨，以模拟极端干旱。研究结果将为未来的实验设计提供参考，并帮助牧场管理者确定在日益干旱的条件下放牧的最佳实践。干旱和放牧对草原系统的影响很大，大量工作已经分别量化了草原对这些驱动因素的反应。但干旱和放牧可能会相互作用，影响草地的形态和功能。例如，放牧植物群落的物种组成往往与非放牧群落不同，这些物种对干旱可能更敏感(或更不敏感)。被放牧的植物对极端干旱的额外压力的反应能力也可能会降低。我们对这种相互作用的影响的了解是有限的，因为在有大型食草动物的情况下操纵干旱会带来后勤挑战。这个项目将利用各种实验设计，在有和没有大型食草动物的情况下实验性地操纵干旱。目标是测试在草原植物群落中独立操纵干旱和放牧的可行性。具体地说，研究人员将开发方法，在大平原的关键大型放牧动物野牛存在的情况下，在Konza Prairie生物站和长期生态研究地点建立实验性干旱操纵。研究人员将建立避雨庇护所，并量化庇护所下野牛的使用和活动水平。他们将进行一种重复的庇护所设计方法，首先测试一种设计，然后根据需要进行修改，以获得预期的反应(即，庇护所内外的野牛使用量相似，庇护所下面的土壤湿度减少50%)。研究结果将为设计一项关于干旱和放牧对植物群落的相互作用的更大规模的实验提供必要的试点数据。长期目标是了解这两个基本驱动力如何影响草原人口、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。

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.