Global atmospheric CO₂ concentrations are increasing at historically unprecedented but ecologically gradual rates.  The implications of this perturbation for carbon sequestration and feedback on global climate change are difficult to predict due in part to its gradual and largely uniform nature.  We used long-term (>40 years) spatial gradients in atmospheric CO₂ concentration, produced by spatially heterogeneous fossil fuel combustion along a rural to urban transect, to test the hypotheses that 1) rural to urban CO₂ spatial gradients are useful analogs for gradual climate change and 2) higher atmospheric CO₂ concentration promotes tree growth and C sequestration.  Fossil fuel derived CO₂ imparts a distinctive ¹⁴C isotopic signature on atmospheric CO₂; as this CO₂ is fixed into annual tree rings, a proxy for fossil fuel derived CO₂ is preserved.  Ten four-year tree ring segments were analyzed for α-cellulose ¹⁴C content by AMS from trees within 10 closed canopy forested sites in the Baltimore Maryland metropolitan area.  Tree growth parameters were assessed by measuring the annual ring width change of 224 trees across the 10 sites.  A hierarchical Bayesian model was constructed to determine the influence of CO₂ concentration and other site and environmental factors on tree growth.

Results/Conclusions

Our proxy for historical CO₂ concentrations indicates a detectable but diminishing spatial CO₂ gradient across the rural to urban transect that ranged from a 5.6% gradient during the 1970s to a 1.4% gradient in recent years (2000-2008).  This observation is consistent with urban deindustrialization and concurrent expansion of suburban development.  As an analog for future atmospheric conditions, this spatial gradient is equivalent to a temporal gradient of ca. 15, 7.2, 9.8, 2.6 years of atmospheric CO₂ rise during the past four decades.  The CO₂ spatial gradient had an overall positive effect on tree size adjusted ring width growth.  Modeled air surface temperature differences among sites indicate that warmer sites also promote tree growth.  In- growth root cores, soil N mineralization and nitrification assays, and soil C and N contents all suggest that N is unlikely to be limiting current tree productivity on most sites across our rural to urban transect.  Furthermore, soil lead content varied little across these forest sites, suggesting that heavy metal contamination is not likely a significant control on forest productivity in our study.  These results lend support for the overarching hypothesis that terrestrial ecosystems will sequester more C under greater atmospheric CO₂ concentrations and warmer air temperatures.