Over the last decade, interest has grown in expanding the sources of energy produced in Minnesota, including burning wood alongside or in place of fossil fuels in commercial power plants. One possible source of this wood is through the harvest of residual material (hereafter "woody biomass harvest") left behind following traditional timber harvest. Although voluntary site-level harvest guidelines endorsed by the State initially called for the retention of residual material in the forest, the rules were modified in 2007 to allow for such harvest. It remained unclear, however, what, if any, ecological impacts woody biomass harvest could have in the region. I examined the immediate impacts of woody biomass harvest on small mammals and vegetation in 3 aspen (Populus spp) forests in northeast Minnesota. The study used of a "Before-After, Control-Impact" (BACI) design, with baseline surveys prior to treatment, follow up surveys after treatment, and a paired control that was not treated. I had two treatment categories: clearcut with slash left in situ (hereafter "slash-retention"), and clearcut with whole tree skidding and no replacement of residue (slash-removal). Each research site received a full complement of two treatments plus a control, giving me three replicates at each site.Prior to harvest, the stands contained an average coarse woody debris (CWD) density of 465 cm2/m2 (SE=49 cm2/m2). After harvest, the density of CWD increased in the slash-retention plots by an average of 422 cm2/m2, while slash-removal plots lost on average 29 cm2/m2. Prior to harvest there were no statistically significant differences in CWD density between treatments, but after logging CWD was higher in the retention than in the control plots, and higher in the control plots than in the removal plots.. In addition to gross changes in CWD area, the nature of the CWD in the harvested plots also shifted from initial conditions, with slash-retention plots gaining a disproportionate share of wood in less decayed conditions, while the slash-removal treatments shifted to a CWD base dominated by more decayed wood.Overall shrub stem counts increased similarly in both harvest treatments though more so in the slash-removal plots. Hazel (Corylus cornuta) stem counts increased by more than 3 stems/m2 in the slash-retention treatment, but stem counts were steady in the slash-removal treatment. Both treatments show a strong, similar regeneration of aspen stems after harvest (3-5 stems/m2).Both harvest types exhibited a decline in native forb species cover and an increase in bare ground, cover of non-native plants, and cover of graminoids compared to pre-existing conditions and associated control stands. Changes in native forb cover, non-native forb cover, and bare ground were greater in the slash-removal compared to the slash-retention treatment, but there was not a statistically significant difference between the plots for the increase in graminoid cover. Native species richness of survey plots 2 years post-harvest was not different from pre-harvest values for either treatment.In just over 29,000 trap nights I recorded 4,838 captures of 1,794 individual animals. I captured 15 mammal, 6 amphibian, and 1 snake species. Overall amphibian captures were low, but trended downward post-harvest in treatment plots. Population estimates of deer and white-footed mice (Peromyscus spp.) were little affected by treatment type or harvest status, while red-backed voles (Myodes gapperi) showed a positive response to harvest. Shrews (Sorex sinereus and Blarina brevicauda) and chipmunks (Tamias striatus) responded negatively to harvest of both types. The abundance of the communities overall (all individuals of all species lumped) were generally higher after treatment on the harvest sites than either before conditions or the adjacent controls, with slash-retention plots showing slightly greater gains than slash-removal plots.In conclusion, although many metrics responded similarly to either harvest type, in all cases where there were differences between the two treatments, the slash-removal treatment yielded a less favorable outcome for wildlife and a further shift from pre-treatment conditions than did the slash-retention treatment. This is especially true for the loss of native species cover, the increase in bare ground and non-native species cover, the loss of CWD, and the lower levels of C. cornuta. Although overall small mammal population sizes increased across both harvest types, increases were slightly greater for slash-retention treatments. Overall the results here do not warrant an avoidance of woody biomass harvest, but do argue for some caution, careful monitoring, and thoughtful siting. Future work should revisit these stands to assess longer term impacts.
University of Minnesota Ph.D. dissertation. Major: Conservation Biology. Advisor: Stephen Polasky. 1 computer file (PDF); xi, 194 pages.
Rentz, Michael S..
Impacts of woody biomass harvest on small mammals and plants in Northern Minnesota Aspen Forests.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.