Climatic sensitivity and growth of southern temperate trees in the eastern United States: Implications for the carbon cycle

Climatic warming over the past 200 years is expected to accelerate over the next century. Such a change will have significant impacts on forest composition, species ranges and the terrestrial carbon pool. Forecasts of the impact of future climate on temperate, eastern North America forests, however, vary widely. The range of forecasts is primarily related to the paucity of data on the climate sensitivity of most tree species. Tree-ring analysis gives the opportunity to explore the long-term relationship between climate and tree growth as well as long-term growth trends. Three tree-ring based networks comprised of seven southern-temperate tree species covering over more than 400,000 km2 in the eastern U.S. were created to study the response of radial growth to climate and growth rates of these species. Winter temperatures were found to be a significant factor limiting growth of most species, with Atlantic white-cedar (Chameacyparis thyoides) being the most limited. Individual species' temperature response differed the most during the dormant seasons, which could be biologically important. Geography plays an important role in the climatic response of species. Growth rates of the northern red oak (Quercus rubra) in the forest surrounding the Harvard Forest eddy-flux tower are among the lowest of 20 populations studied across the northeastern U.S., which suggests other northern red oak-dominated forests may be sequestering significantly more atmospheric CO2 than previously thought. Tree age does not appear to limit individual-tree growth rates of white oak (Q. alba), chestnut oak ( Q. prinus), yellow-poplar (Liriodendron tulipifera) and northern red oak. In fact, growth rates for these species and Atlantic white-cedar have accelerated over the last 50--150 years. Although accelerated tree growth rates are not proportional to increased productivity at the stand or ecosystem level, these results suggest that old trees and forests can be an important element of the carbon cycle. Results here on a small number of the tree species in the eastern U.S. suggest that there is much to learn on the impact of climate on tree growth and climate response and that there are still large uncertainties on how forested ecosystems will respond to future climate change.