Tree-Ring Evidence for [CO2] Fertilization of Forests and its Influence on the "Divergence Problem"

HR: 0800h
AN: PP51C-0662 [Abstracts]
TI: Tree-Ring Evidence for [CO2] Fertilization of Forests and its Influence on the "Divergence Problem"
AU: * Gedalof, Z
EM: zgedalof@uoguelph.ca
AF: Department of Geography University of Guelph, Guelph, ON N1G 2W1, Canada
AU: Berg, A
EM: aberg@uoguelph.ca
AF: Department of Geography University of Guelph, Guelph, ON N1G 2W1, Canada
AB: The "divergence problem" refers to the observation that increasing temperatures are not matched by increasing radial growth at many northern forest sites. A related, and potentially offsetting, effect may be caused by CO₂ fertilization of trees. Increased atmospheric CO₂ could increase photosynthetic rates and cause trees to use water more efficiently, thereby increasing overall growth rates relative to climatic limiting factors. Accelerated tree growth in response to elevated atmospheric CO₂ has been seen across a range of forest types, however these results have been inconsistent, and are generally based on short-term studies. Long-term studies based on tree-rings have generally been restricted to a few sites, and have produced conflicting results. Here we analyze the global record of tree-ring width for evidence of increasing growth relative to drought, and for changing sensitivity of radial growth to drought. This analysis shows that a small but highly significant proportion of trees exhibit increasing growth relative to drought over the past 130 years. These growth increases cannot be attributed to increasing water use efficiency or elevation effects, and no differences between species were detectable. These results suggest that while CO₂ fertilization is occurring at some locations and will influence future forest dynamics, it cannot be expected to offset the effects of increasing temperatures, or to substantially slow the rate of carbon accumulation in the atmosphere. It also suggests that the "divergence problem" may be more widespread than is currently recognized, but its effects are obscured by the offsetting effect of CO₂ fertilization.
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1812 Drought
DE: 1851 Plant ecology (0476)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4920 Dendrochronology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting