The decline of a northeastern foundation species (Tsuga canadensis) and its implications for forest carbon storage capacity

Background/Question/Methods

Introduced into the eastern US in the 1950’s, the hemlock woolly adelgid (HWA) has quickly spread to 18 states and is anticipated to continue its distribution throughout the native range of eastern hemlocks (Tsuga canadensis). The aphid-like insect feeds on the parenchyma cells of T. canadensis, bringing extensive mortality to this foundation species and, therefore, altering the functional characteristics of these ecosystems. In southern New England, this event has led to a rapid increase in the population of black birch (Betula lenta), a shade-intolerant hardwood species. Amidst the rising of atmospheric CO₂ concentrations, it becomes increasingly important to examine the role of forest ecosystems as natural mechanisms for storing carbon (C).  By quantifying changes in the forest biomass, we can begin to understand how the HWA changes the potential of eastern forests to store C. Sites representing the transition from undisturbed hemlock forest, through recently dead hemlock stands, to secondary black birch forests were used for a chronosequence approach to perceive the forest biomass response to the HWA infestation. Carbon content in above- and belowground biomass, woody debris, and organic and mineral soils was measured in twenty 30x30 m forest plots in Massachusetts and Connecticut.

Results/Conclusions

Biomass increased slightly in the years after the infestation and, once reaching maturity, the black birch forest had a C storage capacity approximately 1.5x higher than that of a secondary hemlock forest (52.95kgC/m² + 7.04, and 32.6kgC/m² + 1.66, respectively), suggesting that northeastern forests can actually increase their storage capacity as a consequence of the HWA infestation rather than decrease, as hypothesized. Furthermore, though primary growth forests stored approximately 3.5x more C in the organic horizon than the secondary hemlock forest (15.28 kgC/m² + 4.4, and 9.03 kgC/m² +  1.10, respectively), and 6x more than the black birch forest (2.56 kgC/m² + 0.17), both secondary growth forests were found to have a much more rock-free belowground volume (16.60% + 3.9 and 6.32% + 0.7 in the hemlock and black birch forests, respectively) than that of the primary growth forest (27.27% + 1.4), indicating an agricultural legacy in New England’s forest C storage capacity.