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Harvard Forest Symposium Abstract 2008

  • Title: Forest Ecosystem Response to Experimental Hemlock Decline: Dead Wood and Nitrogen Dynamics
  • Primary Author: Heidi Lux (Harvard Forest)
  • Additional Authors: Audrey Barker Plotkin (Harvard Forest); David Orwig (Harvard Forest)
  • Abstract:

    Eastern hemlock (Tsuga canadensis) is declining regionally due to the spread of the invasive pest, hemlock woolly adelgid (Adelges tsugae). Hemlock is removed from the forest by insect-caused mortality and related hemlock logging activity influenced by the presence or perceived threat of the insect. To experimentally assess the ecosystem effects of hemlock removal on the forest, we designed an experiment that contrasts both the direct (death of standing hemlock) and indirect (moderate to heavy logging of hemlock forests) effects of the adelgid. In 2003, we established two replicates of the following four treatments in hemlock dominated stands at the Simes Tract of Harvard Forest: hemlock girdling (hemlocks of all sizes girdled), commercial logging (all merchantable hemlock and selected other trees cut and removed), untreated hemlock (control) and untreated hardwood forest (representing the predicted endpoint of the hemlock replacement process). Replicated large plots (0.8 ha) allow treatment effects to be detected at the stand scale, and accommodate a large suite measurements of ecosystem function. We are particularly interested in how sudden and more gradual hemlock mortality differentially influence nutrient cycling in this system. In this paper we discuss changes in coarse woody debris and soil nitrogen availability and cycling.

    Since tree death is the intended treatment in this study, examination of timing and distribution of dead wood inputs highlights one of the most obvious differences between adelgid-mediated death (here, simulated by girdling) and logging (Figure 1). In 2007, girdled plots contained the greatest amount of dead wood, more than 85% of which (by mass) came from standing snags and stumps. Girdled plots currently have nine times more standing dead eastern hemlocks than hemlock control plots, whereas in 2005 the masses differed by less than 1,000 kg/ha. Large snags (pieces standing ≥2m tall and measuring ≥25cm in base diameter) comprised most of the ≈126,000 kg/ha total dead wood mass in girdled plots. By comparison, less than 2,000 kg/ha out of ≈8,500 kg/ha standing dead wood in logged plots, ≈13,500 kg/ha out of 14,000 kg/ha in hemlock control plots, and 2,500 kg/ha out of 8,500 kg/ha in hardwood control plots were large snags.

    An immediate pulse of downed dead wood from logging slash leads to different inputs to the forest floor than the slower input of standing dead wood gradually transitioning to downed dead, and should lead to differences in processes such as soil nutrient cycling.

    We began measuring nitrogen dynamics at the Simes tract two years prior to the treatments, during the summer of 2003. Rates of nitrogen cycling are measured in situ, using pvc cores in four subplots from each treatment plot. Resin bags are used in the same plots to examine growing and winter season nitrogen availability in the soil. Nylon bags filled with mixed-bed ion exchange resins are installed in the soil, at the interface of the organic and mineral horizons, and incubated for approximately six months, from May to November and then November to May.

    Net N-mineralization rates were similar across all treatments, especially when comparing within years, from the onset of the experiment through the winter of 2005-2006, averaging 4.5 kg ha-1 yr-1 (Figure 2). The cut plots diverged in 2005, with significantly higher organic soil mineralization rates of 16.60 kg ha-1 yr-1 (SE= 6.95). Similarly, resin capture of both NH4+ and NO3- ions values were similar across plots within seasons, except for logged plots in the winter of 2004-2005, where NH4+ amounts captured on resin was 9 times greater than other treatments for that collection (Figure 3). The Simes logged plots were cut in February of 2005, and resins remained in place in the soil until May of that year, post-cutting. These differences in elevated ammonium capture were not seen in the growing season of 2005, or the following winter.

    Similar increases in soil NH4-N have been seen in Connecticut sites 1-2 years following logging (Kizlinski et al. 2002), although we did not see these increases at the urban counterpart to the Simes tract at the Arnold Arboretum (see Lux and Orwig, this volume). The much greater N availability and baseline rates of cycling at the arboretum, in addition to abundant vegetation in those logged plots may be obscuring changes in N cycling associated with cutting activity at this urban location.

    As Harvard Forest hemlock removal experiment continues, we will continue to investigate and tease out the impacts of changes in soil inputs as a response of different methods of hemlock removal at the Simes tract.

    Kizlinski, M.L., Orwig D. A., Cobb, R.C., & Foster, D.R. (2002) Direct and indirect ecosystem consequences of an invasive pest on forests dominated by eastern hemlock. Journal of Biogeography, 29,1489-1503.

  • Research Category: Invasive Plants, Pests & Pathogens, Large Experiments and Permanent Plot Studies

  • Figures:
  • ST Figure 1.pdf
    ST Figure 2.pdf
    ST Figure 3.pdf