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Harvard Forest Symposium Abstract 2019

  • Title: Oak forests prove remarkably resilient to expansive mortality from repeated insect defoliation
  • Primary Author: Brian Hall (Harvard Forest)
  • Additional Authors: David Foster (Harvard Forest); David Orwig (Harvard Forest)
  • Abstract:

    Three years of mild drought and insect defoliation (2006-2008) caused widespread death of oak canopies across the western portion of Martha’s Vineyard, Massachusetts. Since tree mortality is likely to increase in frequency and extent due to climate change and introduced pests in the near future, it is crucial that ecologists, conservationists, and natural source managers understand the short- and long-term effects of overstory mortality. Harvard Forest researchers set up 20 400 m2 vegetation-sampling plots across an oak mortality gradient within a 200-ha study area on Martha’s Vineyard in 2009. This large conservation area owned by The Nature Conservancy was kept intact and received no salvage harvesting or treatment outside of trail maintenance. Within each plot, overstory vegetation and sapling, shrub, and herbaceous layer composition were sampled. Growing season soil temperatures were sampled for two years with iButton data loggers and soil nitrate and ammonium availability were sampled for three years with resin bags. Vegetation was resampled in 2014/15 to determine short-term trends in vegetation structure and composition as forest regrowth began.

    The pre-infestation forests were dominated by white and black oak with a few plots having a high local beech component; these three species accounted for more than 95% of the total basal area of the study area. Plot overstory tree mortality ranged from 13 to 97% (mean of 60%) of total basal area at the beginning of the study following the three years of infestation. Oaks accounted for almost all the mortality, with black oak declining by 78% (12.7 to 2.8 m2/ha) and white oak declining by 47% (8.5 to 4.5 m2/ha). Total nitrogen availability (ammonium plus nitrate) over the three-years following the overstory mortality was significantly correlated with percent overstory mortality. Seasonal nitrogen availability was correlated with percent overstory mortality during the 2010 growing season (May-November) and the winter/spring seasons of 2010/11 and 2011/12, suggesting that the effects of overstory mortality may still be affecting nitrogen cycling during the nongrowing season . Mortality effects on increased nitrogen availability during the growing season were only detectable during the first summer after mortality.

    The second vegetation survey showed that the average live overstory basal area is 47% of what it was before the infestation (12.0 m2/ha vs. 25.3 m2/ha) and that plots with higher mortality had significantly greater increases in live basal area and greater increase in sapling and shrub densities than plots with lower overstory mortality. These increases in higher mortality plots may account for the somewhat rapid decline in available nitrogen during the growing season. Although the selective mortality of oak strongly favored the much less abundant beech, sassafras and red maple, oak regeneration through seedling establishment and sprouting was surprisingly abundant. In the absence of any salvage harvesting this forest ecosystem proved extremely resilient and is recovering rapidly while providing a time-transgressive and spatially varying array of habitats.

  • Research Category: Biodiversity Studies; Conservation and Management; Invasive Plants, Pests & Pathogens