Historical CO2 growth enhancement declines with age in Quercus and Pinus

Despite experimental evidence showing that elevated CO2 levels increase growth in most plants, the isolation of a signal consistent with anthropogenically caused increases in atmospheric CO2 from the dendrochronological record has shown mixed results. Our extensive sets of tree ring data from the Ozark Mountains in Missouri showed that since 1850, Quercus velutina Lam., Quercus coccinea Muench., and Pinus echinata Mill. trees increased in stem growth coincidently with increases in atmospheric CO2. Those long-term increases in radial growth appear unrelated to historical disturbance levels for the region, to long-term changes in relevant climatic variables, or to productivity of sites sampled for the purpose of creating a time sequence of tree ring growth. It is still unclear what the potential role of nitrogen deposition might have been for tree growth. We cross-dated a large number of increment cores and aligned the ring width data by pith date for accurate age constant assessments of growth over the past 150 years. Thus, we circumvented changes in growth trend associated with differences in physiological functioning during development, as well as the need for statistical detrending that removes an unknown degree of long-term environmental signal, the so called segment length curse that applies to standard dendrochronological investigations. When the positive relationship between CO2 and ring width was examined at different ages, an ontogenetic decline in the rate of growth stimulation was found. Specifically, both the pooled Quercus spp. and P. echinata were characterized by a negative exponential pattern of response over a developmental sequence through age 50. Further knowledge of an intrinsic decline in CO2 sensitivity with tree age or size such as this may be important for increased accuracy in estimating terrestrial carbon stocks across successional landscapes.