Effects of hemlock logging on forest properties in hemlock-dominated stands free of hemlock woolly adelgid

Background/Question/Methods

Adelges tsugae (hemlock woolly adelgid; HWA) has well established populations in much of the Northeastern United States but has only recently been detected in Maine.  When present, this insect has caused widespread mortality of Tsuga canadensis (eastern hemlock).  Currently the Maine HWA infestation is limited to southern portions of the state providing a unique opportunity to collect baseline ecological data in and design proactive management strategies for uninfested forests in the northern regions of the hemlock range.  The objective of this study was to quantify the changes to forest community composition and structure resulting from hemlock removal in order to inform management approaches for addressing future HWA infestation.   At 50 sampling locations within 3 uninfested stands, we compared overstory, sapling, and understory structure and composition of intact hemlock dominated stands with stands from which hemlock had been removed by selective logging.  Community composition data were also collected for invertebrates by pitfall trapping of ants and carabid beetles and sampling earthworms using a liquid mustard extraction technique.  Microhabitat data collection focused on leaf litter composition and depth, fine and coarse woody debris abundances, soil moisture, and photosynthetically active radiation (PAR, μmol/m²). 

Results/Conclusions

Logged stands were harvested 1-2 years prior to data collection and had significantly lower overstory basal area (F_3,46 = 15.01, P < 0.0001) and stem density (F_3,46 = 5.11, P < 0.001). Sapling density was more than three times higher in the unlogged plots than the logged plots and Abies balsamea comprised 43% and 24% of the saplings in the unlogged and logged plots respectively. The unlogged stands were characterized, in order of abundance, by Pinus strobus, Acer saccharum, and T. canadensis.  The three most abundant species of woody seedlings in the logged stands were A. balsamea followed by A. saccharum, and T. canadensis.  A. rubrum was a minor component of seedling composition in both stand types.  Unlogged stands had significantly more deciduous leaf litter (F_1,43 = 20.6, p < 0.001) and greater leaf litter depth than logged stands (F_1,53 = 5.3, p = 0.02).  Earthworm density correlated positively with leaf litter depth (R² = 0.628).  Overall, T. canadensis regeneration appeared sufficient to maintain its dominance or co-dominance in these forests in the absence of HWA.  Advance regeneration of A. balsamea in the logged stands suggests the potential for this species to replace T. canadensis in HWA infested stands.