Eastern US forests are witnessing a devastating decline in one of their foundation tree species, Tsuga canadenis (Eastern Hemlock). The spread of the exotic pest Adelges tsugae (Hemlock Woolly Adelgid) has greatly altered ecosystems previously dominated by this important evergreen conifer. The implications of this invasion for ecosystem processes are far-reaching because coniferous Hemlock is most often replaced by deciduous tree species, such as Betula lenta (Black Birch), which have differing effects on forest floor microenvironments. We took advantage of an “accidental experiment” initiated by patch-level timber harvesting ~30 years ago at Smith College’s MacLeish Field Station (Whately, MA) to investigate how the removal of Hemlock, and its replacement by young deciduous trees, has affected soil organic layer mass, C:N content, soil respiration, leaf litter characteristics, and the microbial community. Forest floor samples were collected from a series of paired plots in Hemlock vs. young Birch stands, in which measures of soil respiration, moisture and bacterial counts were also made throughout the growing season. The inclusion of nearby plots in 80-100 year-old, mature Birch forest has allowed us to estimate longer-term forest successional trends in which Hemlock might be completely replaced in the landscape with dramatic impacts on ecosystem processes.
Three years of data collection reveal that all forest types exhibited similar trends in soil respiration over time, with the maximum seen during the peak of the growing season. The young Birch and Hemlock stands showed no significant differences in soil respiration rates. However, the mature Birch stands were significantly higher throughout the entire growing season. As forests transition from Hemlock to Mature Birch we see a decrease in the soil organic layer and the lowest C:N. The associated microbial communities differed, where Hemlock forest floors consistently showed tenfold higher bacterial counts than the Birch stands. Taken together, our results suggest that the carbon pool in the forest floor is likely to mobilize through greater decomposition and diminish as Hemlocks are replaced by Birch stands. The carbon sink decreases due to a reduction in the organic layer, altered microclimate, and a shift in the quality of litter inputs. Despite a consistent trend of Birch replacing Hemlock forests in the wake of hemlock woolly adelgid invasion, there has not previously been a demonstration of severely altered soil carbon cycling. Our results suggest that the ramifications of this change for carbon storage could be extensive, but may take decades to manifest.