Enhanced carbon storage through management for old growth characteristics in northern hardwood conifer forests
Forest management practices emphasizing stand structural complexity are of interest across the northern forest region of the United States because of their potential to enhance carbon storage. Our research is part of a long term study evaluating silvicultural treatments that promote late successional forest characteristics in northern hardwood conifer forests. We are testing the hypothesis that aboveground biomass development (carbon storage) is greater in structural complexity enhancement (SCE) treatments when compared to conventional uneven aged treatments. Structural complexity enhancement treatments were compared against selection systems (single tree and group) modified to retain elevated structure. Manipulations and controls were replicated across 2 ha treatment units at two study areas in Vermont, United States. Data on aboveground biomass pools (live trees, standing dead, and downed wood) were collected pre and post treatment, then again a decade later. Species group specific allometric equations were used to estimate live and standing dead biomass, and downed log biomass was estimated volumetrically. We used the Forest Vegetation Simulator to project "no treatment" baselines specific to treatment units, allowing measured carbon responses to be normalized against differences in site characteristics affecting tree growth and pre treatment stand structure. Results indicate that biomass development and carbon storage 10 yr post treatment were greatest in SCE treatments compared to conventional treatments, with the greatest increases in coarse woody material (CWM) pools. Structural complexity enhancement treatments contained 12.67 Mg/ha carbon in CWM compared to 6.62 Mg/ha in conventional treatments and 8.84 Mg/ha in areas with no treatment. Percentage differences between post treatment carbon and simulated/projected baseline values indicate that carbon pool values in SCE treatments returned closest to pre harvest or untreated levels over conventional treatments. Total carbon storage in SCE aboveground pools was 15.90% less than that projected for no treatment compared to 44.94% less in conventionally treated areas. Results from classification and regression tree models indicated treatment as the strongest predictor of aboveground C storage followed by site specific variables, suggesting a strong influence of both on carbon pools. Structural enhancement treatments have the potential to increase carbon storage in managed northern hardwoods. They offer an alternative for sustainable management integrating carbon, associated climate change mitigation benefits, and late successional forest structure and habitat.