Biogeography models and dynamic ecosystem simulation models used in the assessment of climatic change routinely predict species displacement and population migration following mortality. Although such predictions are based on reasonable hypotheses regarding plant response to warming and associated inter-specific competition, predicted mortality rates driving current models and mechanisms remain largely untested. We propose a manipulative field study to provide data on the impact of acute drought on mechanisms responsible for growth and mortality of deciduous forest canopy trees representative of common plant functional types (Liriodendron and Quercus). We propose to deploy replicated understory canopies for the removal of 100 percent throughfall and stem flow around mature trees to allow the artificial introduction of a spring drought. Such droughts overlap the period of optimum stem growth and have a greater potential to disrupt plant function. Key measurements include weekly dendrometer band observations, periodic evaluation of plant nonstructural carbohydrates status, automated measurements of sapflow, periodic observations of foliar photosynthesis and conductance, and observations of soil moisture status by depth and horizontal extent of the tree root zone. A primary goal of this research will be to translate such data into mechanistic expressions of the threshold levels of moisture stress responsible for limiting plant function and growth and the inclusion of mechanistic expressions in biogeochemical and biogeography models to enhance their usefulness for assessments of climatic change. While the proposed field experiments and observations will be limited to acute precipitation manipulations, interactions between acute drought, future warming, elevated CO2 and increasing tropospheric ozone will be addressed with stand-level ecosystem models to evaluate the potential for mitigating (CO2) or exacerbating (temperature, ozone) impacts.
1) Demonstrate that a late spring/early summer drought will have a direct impact on current year growth due to reductions in development of functional sapwood.
2) Identify the level and extent of drought needed to induce reduce stored carbohydrate and nutrient reserves, accelerate canopy senescence, and lead to reduced canopy production in subsequent years for two common plant functional types.
3) Translate these observations into appropriate algorithms for inclusion in biogeochemical and biogeography models being used for assessments of climatic change impacts.
The TARP Experiment is supported by the U.S. Department of Energy's Office of Science (BER) through the Program for Ecosystem Research (PER) in the DOE Office of Science, Office of Biological and Environmental Research (BER).
Experimental System Instrumentation (to be added)
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