Forty years of forest measurements support the continuation of the Northeastern carbon sink

Background/Question/Methods

Using atmospheric and forest inventory measurements, previous studies have demonstrated that northeastern forests act as a carbon sink, but few have examined relationships between forest stand dynamics and carbon uptake. Using 42 years of forest census data from a 2.9 hectare permanent plot at the Harvard Forest (Petersham, MA), changes in species composition and total forest aboveground biomass were examined to determine if stand dynamics impact the forest’s carbon uptake. The diameter at breast height, canopy class, and condition were recorded for all living and dead individuals in 1969, 1975, 1991, 2001, and 2011, with additional measurements taken on dead individuals, creating record of over 6000 living and dead individuals.

Results/Conclusions

From 1969 to 2011, Red oak (Quercus rubra) increased its dominance in the stand’s total basal area from 52% to 60%. However, red maple (Acer rubrum) has become relatively less abundant, decreasing from 30% to 23%. While red oak and red maple continue to account for the majority of the basal area in the stand, the secondary species experienced a dramatic increase in relative abundance of individuals in the stand; yellow birch (Betula alleghaniensis), black birch (Betula lenta), American chestnut (Castanea dentata), American beech (Fagus grandifolia), witch hazel (Hamamelis virginiana), eastern white pine (Pinus strobus), and eastern hemlock (Tsuga canadensis) have increased from comprising 25% of the individuals in the stand in 1969 to 52% in 2011. The total biomass of living individuals is increasing linearly (R²=0.99, p=0.0002), which implies that the stand has not yet experienced an age-induced decrease in biomass accumulation. These results suggest that New England forests may be increasing the region’s carbon sink at a constant rate, largely due to the continued growth of dominant individuals. However, while red oak comprises the largest percentage of the total forest biomass and basal area, it is not prominent in the understory, suggesting the forest composition may change significantly in the future.