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

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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.