Acclimation of plants and ecosystems to warmer temperatures and altered precipitation

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Human activities are increasing atmospheric carbon dioxide with consequences such as warmer air temperatures and altered precipitation patterns. Some models suggest that warmer and drier conditions will result in an increased release of carbon dioxide from ecosystems, contributing a positive feedback to the atmosphere and further climate change. However, the magnitude of this potential positive feedback and the mechanisms controlling it represent major unsolved problems in global change science. A long-term objective of my research program is to better understand the physiological mechanisms that control how plants and ecosystems respond and acclimate to warmer temperatures and altered precipitation. In particular, I study photosynthesis and respiration in plants and ecosystems, and these processes determine whether ecosystems are net sources or sinks of carbon dioxide. I propose to initiate a new series of experiments to investigate the response and acclimation of plants and ecosystems to experimental warming and precipitation manipulation, alone and in combination. These experiments and analyses will be conducted in a native, mixed grass prairie ecosystem. Grassland ecosystems offer special opportunities to study plant and ecosystem physiology because of the wide range of environmental conditions and plant responses that can occur within growing seasons and among study years in these systems. The proposal defines four short-term objectives that provide a framework for student research projects. The first objective is to test the hypothesis that warmer temperatures increase carbon exudation by roots, which subsequently stimulates microbial respiration and the breakdown of old soil organic matter, releasing carbon dioxide to the atmosphere. A second objective will examine the hypothesis that higher temperature, particularly in association with exposure to high solar radiation, stimulates plant respiration because of a shift in electron partitioning to the alternative pathway respiration under stressful conditions. Over the last 50 years, nighttime temperatures have increased 1.4 times faster than daytime temperatures. This asymmetric, day-night warming is an under-appreciated climate characteristic affecting ecosystems. A third objective is to determine the consequences of the potential opposing effects of daytime and nighttime warming on ecosystem productivity. Changes in the intensity and timing of rainfall events, even without changes in the annual precipitation total, can have significant effects on plant and soil physiological processes. The fourth objective will test the hypothesis that warmer temperatures interact with larger rainfall events to stimulate higher plant productivity and soil microbial activity, despite longer dry periods between rainfall events. Data from these studies will be compared to the predictions of quantitative models to gain further understanding of the mechanistic controls and the reasons for the observed physiological responses of plants and ecosystems to the environmental changes. Finally, the new physiological information generated from these studies will be used to improve models that predict, and help to evaluate, the consequences of global environmental changes for plant and ecosystem physiology.