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Harvard Forest Symposium Abstract 2012
- Title: Dead Wood Dynamics in Response to Experimental Hemlock Decline
- Primary Author: Audrey Barker Plotkin (Harvard Forest)
- Abstract:
Eastern hemlock (Tsuga canadensis) is declining regionally, with the spread of the invasive pest, hemlock woolly adelgid (Adelges tsugae). Hemlock is killed directly by insect-caused mortality, and indirectly by increased hemlock logging activity as human management decisions are influenced by the perceived threat of the insect. To experimentally assess the ecosystem effects of hemlock removal on forest structure and function, we designed an experiment that contrasts death of standing hemlock with moderate to heavy logging of hemlock forests. In 2003, we established two replicates of the following four treatments at the Simes Tract of Harvard Forest: hemlock girdling (hemlocks of all sizes girdled; simulates many of the effects of insect mortality), commercial logging (all merchantable hemlock and selected other trees cut and removed), untreated hemlock (control) and untreated hardwood forest. Large plots (0.8 ha) allow treatment effects to be detected at the stand scale, and accommodate a large suite of responses. In Winter/Spring 2005, the treatments were implemented, so we now have seven seasons of post-treatment data.
Examining the timing and distribution of dead wood inputs highlights one of the most visible differences between adelgid-mediated death and logging. Dead wood plays a key role in forest carbon dynamics and provides habitat to numerous vertebrate, invertebrate and microbial species. We established permanent transects in all plots in Summer 2005 just after the girdling and logging were completed, so logging debris was captured but girdled trees were still living. We surveyed coarse downed wood (CWD), fine woody debris, and standing snags and stumps in 2005, 2007, 2009 and 2011.
CWD volume in the logged plots was significantly higher (2-3 times more volume; mixed-effects ANCOVA, treatment F3,23=58.2, p<0.0001) than the hemlock control (Figure 1, top panel). This effect was immediate and persisted through the seven years, although CWD mass has declined from 2005-2011 as the wood becomes more decayed. The other treatments did not differ from the hemlock control. In contrast, standing dead wood volume in the girdled plots rose by an order of magnitude between 2005 and 2009 (Figure 1, bottom panel). The girdled plot volumes began at similar levels to the other plots, then rose significantly (mixed-effects ANCOVA, time x treatment F3,23 = 5.4, p = 0.006) as the girdled trees died. Standing dead wood volume declined somewhat between 2009 and 2011. This is probably because standing dead wood has begun to fall to the ground; however, a strong uptick in CWD volumes in 2011 was not observed in the girdled plots.
We will continue the bi-annual survey of dead wood in this experiment, in order to better understand the long-term consequences of differing modes of hemlock death on this aspect of forest structure and carbon budget. Future analyses will examine more closely differing rates of decay by species and size, and consequences for forest carbon dynamics following foundation species removal.
- Research Category: Invasive Plants, Pests & Pathogens
Large Experiments and Permanent Plot Studies - Figures: abp-symp12-SimesFig1.pdf
