Does restoring oak woodlands from closed-canopy forests reduce carbon sequestration?

Background/Question/Methods

Forests and woodlands in much of the eastern US were greatly altered by fire suppression efforts beginning in the 1900s, with negative consequences for biodiversity. One potential concern regarding attempts to reverse the effects of past fire suppression (via canopy reduction and prescribed burning) is the effect restoration treatments have on carbon sequestration (e.g., the photosynthetic assimilation of atmospheric CO₂into structural or stored carbon such as wood). Although the death of trees associated with restoration treatments can have the immediate effect of reducing sequestered carbon, reducing stand density and canopy cover could also increase productivity of the forest, which could compensate for the losses. I present the results of a ten-year study of the effects of ecological restoration of upland oak woodlands from fire-suppressed closed canopy forests on carbon sequestration in trees. I measured diameters of all trees in adjacent 0.5 ha treated and control plots at each of two sites before and after treatments (deadening using girdling/herbicide and biennial spring burning) and heights after treatments and estimated changes in stand volume and carbon. I then used a growth model to contrast the net rate of carbon accumulation over a 100-year period in treated and control plots.

Results/Conclusions

The direct effect of the restoration treatments was a significant reduction in stand-level carbon at both sites. Most of the initial reduction was caused by the thinning treatments (5% reduction in carbon), whereas repeated biennial fires caused some mortality of adults (~1% over 10 years). Recruitment was greater in the control (3.6% over 10 years). Growth of trees not killed by fire, however, was significantly greater in the treated plots than in the control plots (0.9% v. 0.47% per year) and was predicted to overcompensate for losses directly attributable to the treatments in less than 30 years, assuming no differences in recruitment and no density dependence. These results suggest that the initial reduction in carbon sequestration in trees following thinning treatments is at least partially compensated for by increased growth. Carbon sequestration can be increased if treatments can be implemented in a way that promotes recruitment. Slower growth following crown expansion and filling of gaps should eventually reduce the net rate of tree carbon sequestration in treated stands relative to that in control stands.