9321626 Oberbauer Global warming will substantially increase the length of the active season for high latitude ecosystems, but many plant species in these areas have phenological patterns adapted to short growing seasons. Some, but not all, of the factors that affect phenological stages will change with global warming. Those species constrained by cues that will be unchanged with global warming, such as photoperiod, will be unable to respond to extended season length. As a result, species composition, productivity, and carbon fluxes from these ecosystems will change substantially. Using Alaskan tundra as a model system, this project addresses the questions: 1) What are the constraints on phenological patterns for the dominant species? 2) What physiological adjustments will species make in response to an extended period of resource availability? 3) How will growth of the dominant species, ecosystem productivity, and ecosystem carbon fluxes change under field-simulated extended seasons? These questions will be addressed using a combination of controlled-environment studies and field manipulations of season length. Growth chamber studies will examine the effects of photoperiod, soil and air temperature, and leaf age on phenological patterns and carbon gain of selected species of the dominant growth forms. Field studies will examine the effects of extended active period on phenology, growth, productivity, and ecosystem carbon balance. Results from this study will be used to develop a model of phenological patterns of tundra plants that can be used to evaluate species' responses to extended growing season. ***
