Abstract
AbstractEastern U.S. forests are witnessing an ecologically disruptive decline in one of the region's distinctive foundation tree species, the eastern hemlock (Tsuga canadenis). The exotic insect pests hemlock woolly adelgid (Adelges tsugae, HWA) and elongate hemlock scale (Fiorinia externa) have greatly altered many forest ecosystems previously dominated by this important evergreen conifer. The consequences for ecosystem processes are far reaching because hemlock is often replaced by deciduous tree species, such as black birch (Betula lenta), which have strongly divergent effects on forest floor microenvironments and nutrient cycling. We took advantage of an accidental experiment initiated by patch‐level timber harvesting ~30 yr ago to investigate how the removal of hemlock, and its replacement by deciduous trees, has affected leaf litter characteristics, soil organic layer mass, C:N content, and soil respiration rates. We also contrasted these areas to nearby forest plots where deciduous B. lenta has been dominant for almost a century. The inclusion of healthy, intact hemlock stands in the design, and the close proximity of plots, allowed for a powerful space‐for‐time approach to detect ecosystem changes that are likely to occur across the broader landscape with HWA‐induced hemlock loss in coming years. Three years of data collection from a series of plots in hemlock, young birch, and mature birch stands revealed dramatic differences in soil carbon pools and cycling. Between forests dominated by hemlock vs. mature birch, we saw a significant decrease in soil organic layer mass and in the C:N of the remaining organic material. Although hemlock and young birch stands showed no significant differences in soil respiration rates, mature birch stands had significantly higher soil respiration rates throughout the entire growing season, regardless of wet or dry years. Our results suggest that the carbon pool in the forest floor is likely to mobilize through greater decomposition with a 6.8× decline in soil organic layer C storage as hemlocks are replaced by deciduous trees, leading to a potential net release of ~4.5 tons C per hectare. We conclude that the ramifications of this change for carbon storage could be extensive, but may take decades to manifest.
Highlights
Biotic disturbances are having major effects on forest ecosystems across the United States and Canada (Ellison et al 2005, Lovett et al 2006, Hicke et al 2012)
Mature birch plots were heavily dominated by black birch (78.7% of basal area) and other deciduous tree species (21.2%), with only a single stem of hemlock sampled (0.2%)
The young birch plots were dominated by sapling-stage black birch (83.4%) and other deciduous tree species (16.6%), with no hemlock present into the canopy, subcanopy, or ground layer
Summary
Biotic disturbances are having major effects on forest ecosystems across the United States and Canada (Ellison et al 2005, Lovett et al 2006, Hicke et al 2012). Favors certain forest floor organisms such as bryophytes and soil mesofauna, like mites and collembolans (Snyder et al 2002, Ellison et al 2005, Zukswert et al 2014, Siddig et al 2016). In addition to their influence on ecological communities and other species, foundation tree species like hemlock affect ecosystem processes such as decomposition, nutrient cycling, and carbon sequestration (Ellison et al 2005). Hemlock stands dramatically influence forest floor structure and edaphic conditions, and typically develop deep, acidic organic layers with slow rates of decomposition and nutrient cycling (Lovett et al 2006)
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