1. Scott Collins, Cliff Dahm, Marcy Litvak, Will Pockman, Kristin Vanderbilt, Esteban Muldavin, Don Natvig, Bob Sinsabaugh and Blair Wolf SEVILLETA LTER: Hierarchical pulse dynamics in an aridland ecosystem ABSTRACT: The Sevilleta LTER is located at the intersection of several aridland ecosystem types which makes it an ideal location for the study of pulse dynamics and boundary transitions. Rainfall pulses drive aridland ecosystem processes, and climate change is predicted to alter the frequency, intensity and size of precipitation pulses. The traditional pulse-dynamics framework focused on individual rainfall events within a growing season. The Sevilleta LTER has developed a hierarchical, pulse dynamics framework in which rainfall pulses affect (1) event-scale plant-soil and plant-microbe resource exchanges, (2) Plant-plant and plant-consumer interactions at seasonal to interannual time scales, (3) landscape transitions, such as shrub encroachment, that occur over decadal time scales, and (4) transfers of organic matter and nutrients among landscape elements. To address this framework SEV research includes (1) long-term mechanistic experimental and empirical studies of exchanges and interactions, (2) monitoring and measurement of causes and consequences of boundary transitions, and (3) large scale monitoring of transfers among landscape elements, Overall, our research program integrates studies on multiple global change drivers and pulse precipitation dynamics across scales to determine how nutrient exchanges and species interactions lead to temporal dynamics, compositional transitions and landscape scale interactions. This poster presents our conceptual framework (Fig 1) and representative long-term experiments. Together, our observational and experimental research yields a comprehensive understanding of how key abiotic drivers affect pattern and process in aridland ecosystems over multiple spatial and temporal scales. Figure 1. A crude rendition of the Sevilleta LTER hierarchical pulse dynamics framwork. Warming, El Nino, N-deposition Experiment: Long- term data show that nighttime temperatures and N deposition are increasing in our region. In addition, global change models predict increasing frequency of El Nino events in response to climate warming. The purpose of this experiment is to determine the individual and interactive effects of nighttime warming, nitrogen deposition, and increased frequency of El Nino events on species interactions and plant- microbe exchanges. The study was established in June 2005. Mega-Monsoon Experiment: Mega-ME was established in 2012 to assess the effects of above average monsoon precipitation on seed germination and seedling survival of creosote bush in the grass- shrub transition zone and in creosote dominated shrubland. Empirical evidence suggests that creosote seedlings emerge only in years where precipitation is well above average. Treatments include clipping of grasses in the transition zone plots to determine if grazing enhances creosote reproduction, and addition of black grama seeds in creosote dominated shrubland to determine if dispersal limitation prevents grasses from increasing in areas dominated by creosote. Soil moisture at depth – Ecotone Experiment: This experiment addresses water and N competition between grasses and shrubs during shrub encroachment. The functional response of co- occurring plants to a rain event varies widely, depending on multiple factors. SMaD-Ecotone will determine how patterns of available water drive plant responses and why shrubs invest in roots in soil that is rarely wet during the growing season. Specifically, SMaD-Ecotone will determine how patterns of soil moisture interact with plant traits to influence plant C and water relations during the process of shrub encroachment. Piñon-Juniper Rainfall Experiment: This experiment is designed to determine the causes of tree mortality under chronic drought and the potential for pine to outperform juniper under conditions of increased annual precipitation. Treatments include chronic drought (-45% of annual precipitation) or increased (+50% of the long-term average) rainfall. Measurements include soil moisture, temperature, Rs, and piñon and juniper sap flow. Our goal is to distinguish among hypothesized mechanisms for the observed differential mortality during drought and responses during periods of above-average rainfall that may pre-dispose the system to catastrophic responses under future climate scenarios. Sevilleta Ecological Observatory Network (SEON): Terrestrial and aquatic components of arid landscapes are periodically coupled through trigger- transfer events. The goal SEON is to understand how pulse events (fires, floods) move materials from terrestrial to aquatic components of the landscape affecting water quantity and quality in the Middle Rio Grande. Activities include a sonde network that measures temperature, pH, conductivity, turbidity, and dissolved oxygen at 27 locations and 11 flux towers measuring CO2, H2O and energy balance throughout the Middle Rio Grande. Sprinkler systems increase monsoon precipitation to stimulate seed germination of creosote bush Warming shelters deploy reflective blankets to elevate nighttime temperatures