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Harvard Forest Symposium Abstract 2019

  • Title: Soil respiration in the Prospect Hill old hemlock forest in 2018 compared to prior years: Effect of interannual climate variation versus hemlock woolly adelgid
  • Primary Author: Julian Hadley (Harvard Forest)
  • Abstract:

    This study was designed to detect an effect of hemlock woolly adelgid [(Adelges tsugae), abbreviated HWA] infestation on soil respiration in an old eastern hemlock (Tsuga canadensis) forest in the Prospect Hill tract of the Harvard forest. HWA was widely observed in the hemlock forest by 2012 and had caused widespread crown vigor reduction and the death of about 20% of canopy trees by 2018. I measured soil respiration on sixteen dates between May 2 and December 19, 2018, with an average of about two weeks between measurements. The data clearly showed the effect of an unusually warm and wet summer, which raised soil respiration above values measured in any previous year in the hemlock forest. The average measured soil respiration rate reached 6.9 µmol m-2 s-1 in late July and was at or above 5.2 µmol m-2 s-1 until October 12. In contrast, the highest average rate measured at the same locations in the years 2000, 2001, and 2004 through 2007 was 5.1 µmol m-2 s-1 (Figure 1). The primary cause of the high respiration rates appeared to be warm, wet weather. At the Harvard Forest meteorological station July, August and September in 2018 were as warm as in any of the previous years, and the average air temperature for the three months combined was at least 1.5 degrees higher in 2018 than all but one of the previous years, and 0.5 degrees higher than the remaining one, 2005. Rainfall was also higher than in previous years, especially in August and September, each of which had about 270 mm 10.6 inches) of rain. Total rainfall for July through August 2018 was nearly 1 foot (11.8 inches) higher than any of the previous years. The conditions resulted in moderate to high moisture content in the organic layer of soil in the hemlock stand from late July through the rest of the year. Response curves of average soil respiration versus either soil temperature (Figure 2) or versus a combined measure of soil temperature and soil moisture did not show a different response pattern in 2018 than in earlier years. However, two of the twelve locations where soil respiration was measured showed possible effects of hemlock decline and mortality, in terms of a lower increase in soil respiration in response to increasing soil temperature and moisture in 2018 than previously. Moreover, high soil temperatures which contributed to high soil respiration in 2018 may have been partially due increasing sunlight reaching the forest floor, which was in turn a result of canopy defoliation and tree mortality caused by hemlock woolly adelgid.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • Hadley HF symp Figure 1.docx
    Hadley HF symp Figure 2.docx