LTREB: Long-term ecosystem responses to directional changes in precipitation amount and variability in an arid grassland

在美国西南部，降雨模式正在发生变化，并可能在未来几十年继续变化。大的干旱和洪水将更频繁地发生，并将更加严重。这些变化可能对草原及其提供的生态系统服务产生重大影响。科学家们研究了草原生态系统和生活在其中多年的植物物种。然而，随着环境的变化，预测草原的未来仍然是一个挑战，因为仅仅通过观察很难理解模式的原因。受控实验可以揭示是什么导致了草原的变化，并有足够的信心预测它们的未来，因为它们开始经历前所未有的条件。仅通过短期实验也很难预测长期变化。该项目将扩大长达十年的实地实验，其中降雨量和变化被操纵，以了解草原生态系统可能如何变化。这些实验的结果将产生新的认识，为美国西南部和世界各地的旱地管理人员提供指导。该项目还将培训新一代的科学家，其中许多人是代表性不足的少数群体的成员，该项目以两种关于内源和外源现象的假设为指导，这两种现象相互作用，使旱地对降水量和降水变异性的变化作出反应。(1)内源性的生物地球化学和人口现象会引起人口、群落和生态系统进程的突然或持续变化。(2)诸如厄尔尼诺现象等发生频率较低的外源现象与内源机制相互作用，影响系统的复原力，导致人口动态和生态系统进程的变化。该项目将捕捉罕见的外源现象的影响和内源因素对沙漠物种、群落和生态系统的累积影响。该项目将建立在位于新墨西哥州的沙漠草原上的两个现存实验的基础上：（a）降雨量的操纵（降水量减少80%，环境，增加80%）与氮可用性的变化交叉，（B）降雨量的操纵变异性（控制，增强变异性50%和80%）。降水量的长期操纵是通过使用防雨棚和灌溉系统来完成的。通过施肥可获得不同水平的氮可用性。 通过在同一地块中每年翻转干旱和灌溉处理来进行降水变化的操纵。响应变量包括地上和地下净初级生产力，物种组成，净氮矿化和叶片氮同位素比值。这项研究有可能改变对降水模式变化的长期生态系统响应的理解，这对于评估生态系统服务在未来几十年可能如何变化至关重要。该奖项反映了NSF的法定使命，并被认为值得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来支持。

In the American Southwest, rainfall patterns are changing, and are likely to continue to change in the coming decades. Major droughts and floods will occur more often and will be more intense. These changes are likely to have a large impact on grasslands and the ecosystem services they provide. Scientists have studied grassland ecosystems and the plant species that live in them for many years. Predicting the future of grasslands as the environment changes remains a challenge, however, because it is very difficult to understand causes of patterns using observations alone. Controlled experiments can uncover what causes changes in grasslands with enough confidence to predict their future as they begin to experience conditions never seen before. It is also difficult to predict long-term changes using only short-term experiments. This project will expand on decade-long field experiments in which rainfall amount and variability were manipulated to learn how grassland ecosystems are likely to change. Results from these experiments will yield new understanding that will provide guidance to managers of drylands in the Southwest United States, and around the world. This project will also train a new generation of scientists, many of whom are members of underrepresented minority groups.This project is guided by two hypotheses about endogenous and exogenous phenomena that interact with each other to generate dryland responses to changes in amount and variability of precipitation. (1) Endogenous biogeochemical and demographic phenomena mediate abrupt or continuous changes in populations, communities and ecosystem processes. (2) Exogenous phenomena, such as El Niño events that occur with low frequency, interact with endogenous mechanisms to affect system resilience, leading to changes in population dynamics and ecosystem processes. This project will capture the impacts of infrequent exogenous phenomena and the cumulative effects of endogenous factors on populations of desert species, communities and ecosystems. The project will build upon two extant experiments located in a Desert Grassland in New Mexico: (a) manipulation of rainfall amount (80% reduced precipitation, ambient, 80% increased) crossed with changes in nitrogen availability, and (b) manipulation of rainfall variability (control, enhanced variability by 50% and 80%). Long-term manipulations of precipitation amount are accomplished using rainout shelters coupled with irrigation systems. Different levels of nitrogen availability are achieved via fertilization. Manipulations of precipitation variability are conducted by flipping drought and irrigation treatments from year to year in the same plots. Response variables include above and belowground net primary production, species composition, net nitrogen mineralization and leaf nitrogen isotopic ratios. This research has the potential to transform understanding of long-term ecosystem responses to changes in patterns of precipitation, which is critical to assessment of how ecosystem services are likely to change over the coming decades.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.