Long-Term Impacts of the Hemlock Woolly Adelgid on Forest Carbon at Harvard Forest 2008-2011

The abundance of eastern hemlock (Tsuga canadensis) in eastern US forests has declined since the 1950s owing to the introduction of the non-native insect, hemlock woolly adelgid (HWA, Adelges tsugae). In southern New England, eastern hemlock is being replaced by the deciduous tree species, black birch (Betula lenta). To date there is little understanding of whether hemlock loss will fundamentally alter ecosystem C balance and component fluxes. In this study, we use a comparative approach to study potential changes in C fluxes and storage and N cycling associated with HWA-induced hemlock decline and replacement. The stands include primary- and secondary-growth hemlock forests (230 and 132 years old, respectively), recently disturbed stands (5 and 18 years old) that now have rapidly growing black birch saplings, and a mature black birch stand of age similar to the second-growth hemlock stand. We addressed the question of whether the quantity and distribution of C pools in black birch forests are the same as those found in the hemlock stands they replace and, if so, over what time scales. We found that if HWA did not infest hemlock stands in central MA, C stocks in these secondary stands could still increase by at least 30% over a period of 100 years. Girdling, intended to simulate HWA infestation, resulted in a large transfer of C from live biomass to coarse woody debris five years after treatment, but had little effect on total ecosystem C content. A former hemlock stand killed by HWA and now dominated by black birch saplings also had a large pool of highly decayed CWD and a rapidly aggrading C pool in live biomass. C pools in biomass in a mature, secondary growth black birch stand 135 years since pastureland abandonment were as large as those in a primary-growth hemlock stand 235 years of age. We also found that aboveground net primary production was higher in the aggrading black birch stand and significantly so at 18-years post-HWA compared to the secondary-hemlock stand it would likely replace. Rapid forest regrowth was accompanied by significantly higher rates of N uptake from the soil but also higher N-use efficiency because most of the N taken up from the soil was allocated to the production of wood with a high C-to-N ratio. In contrast to patterns of aboveground production, the rate of soil respiration was lowest in the young stands and not significantly different from the second-growth hemlock stand, suggesting little net effect of stand replacement on soil C efflux. The leaf litter decomposition study showed that black birch litter decomposed more rapidly than hemlock litter but that there was no effect of stand type on the rate of decomposition. Analyses of extracellular microbial exoenzyme activity painted a more nuanced pattern of variation among stands, with fine root biomass the only weakly explanatory variable. Based on this analysis, it appears that ecosystem C storage is resilient to the loss of hemlock because of vigorous regrowth by black birch. These results suggest that forests affected by HWA in southern New England will remain a sink for atmospheric CO2 despite reorganization of stand structure and species composition.