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Nonlinear responses of tree seedlings to gradients of superambient air temperature and atmospheric CO2.
Wolfe-Bellin, Kelly*,1, He, Jin-Sheng1, Bazzaz, F.1, 1 Harvard University, Cambridge, MA
ABSTRACT- Increasing global air temperature and atmospheric CO2 concentration are expected to have considerable impact on the function of forest ecosystems, yet there is great uncertainty in the future predictions of each. Given the sizeable range in these predictions, it is important to understand how forest ecosystems may respond to a range of air temperatures and CO2 concentrations. We conducted a study to investigate how gradients of both elevated air temperature and atmospheric CO2 affect seedling recruitment of three species that co-occur as dominant canopy trees in northeastern hardwood forests. Seedlings of gray birch, red oak, and red maple were exposed to gradients of either elevated air temperature or a combination of elevated air temperature and atmospheric CO2 for one growing season. The environmental gradients were created in greenhouse tunnels. All seedlings were exposed to an air temperature gradient that ranged from ambient to approximately 3°C above ambient during daylight hours. Additionally, half the seedlings were exposed to a CO2 gradient that ranged from 370 to 810 ppm. Nonlinear responses to the gradients were evident at the assemblage and species-specific levels. Total biomass at the end of the first growing season exhibited little response to the temperature gradient alone, but exhibited a strong curvilinear (parabolic, concave-side down) response to the combination of elevated temperature and CO2. The biomass of individual species exhibited a range of responses to the temperature gradient alone, while each species exhibited the same curvilinear response to the temperature and CO2 gradients evident in the total biomass response. These results demonstrate that tree seedling recruitment may exhibit important nonlinear responses to gradients of both environmental factors, and that the combination of elevated air temperature and atmospheric CO2 to which forests may be exposed in the future should be considered when conducting experiments designed to predict ecosystem responses to global change.
Key words: carbon dioxide, air temperature, tree seedlings, environmental gradients