Comparative Ecosystem Ecology: Acclimation of ecosystem function to warmer temperatures and altered moisture

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Photosynthesis and plant productivity are essential for supporting the operation of our biosphere. A current consensus suggests that future climate warming will result in reduced photosynthesis and net loss of carbon dioxide from ecosystems, but there is large uncertainty about the mechanisms involved, the magnitude of the expected positive feedback, and the consequences for future ecosystem function. Despite this consensus, several recent measurements, made at the global-scale, indicate that photosynthesis and net ecosystem CO2 uptake have been increasing for the last several decades. Two important mechanisms proposed for higher global-scale photosynthesis are: (i) shifts in the timing and duration of leaf area and photosynthetic activity, and (ii) increases in water-use efficiency (WUE). Further studies are required to test if these mechanisms, which have contributed to the ecosystem service of enhanced global photosynthesis, could be altered or restricted by further changes in environmental conditions. I propose to investigate the mechanisms that control how plants and ecosystems respond and acclimate to warmer temperatures and altered precipitation. A comparative approach, involving a network of four sites instrumented with eddy covariance flux towers, will be used to study ecosystems dominated by two different plant functional types (grassland, deciduous forest) in two locations in southwestern Alberta. The first objective is to test the effect of warmer temperatures on shifts in the timing and duration of photosynthesis and the associated consequences for growing season net carbon gain. In addition, we will determine the role of water stress and photoperiod in limiting potential warming-induced extensions to the growing season and ecosystem carbon gain. The second objective will test for temperature acclimation of photosynthesis and respiration at the ecosystem-scale and determine how this potential acclimation interacts with other environmental changes. Temperature acclimation of either or both photosynthesis and ecosystem respiration can lead to an altered temperature response of net carbon dioxide exchange, although it is not yet clear which acclimation process would have the largest effect on growing season net carbon uptake. The third objective will test the sensitivity of plant- and ecosystem-scale WUE to changes in environmental conditions and determine whether potential future increases in WUE will be constrained by projected warmer and drier conditions. 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 responses of plants and ecosystems to the environmental changes. Finally, the new information generated from these studies will be used to improve models that evaluate the consequences of global environmental changes for photosynthesis and ecosystem productivity.