The effects of stem girdling on biogeochemical cycles within a mixed deciduous forest in eastern Tennessee

Summary

Studies on the effects of stem girdling of a tulip poplar (Liriodendron tulipifera L.) dominated mixed deciduous forest revealed that trees continued to grow above the point of girdling for more than two years after girdling. Soil core samples showed that root biomass was not significantly reduced until two years after trees were girdled. Litterfall during the fall of 1977 from trees girdled in Semptember 1975 and allowed to develop stump sprouts (plot no. 1) was 72% of the control plot. Litterfall during the same time from trees girdled in May 1976 but with stump sprouts removed (plot no. 2) was 82% of the control. Diameter growth during the growing season of 1977 was 53% of control in plot no. 1 and 68% of control in plot no. 2. No significant differences (P>0.05) in forest floor CO₂ efflux rates were observed between plots in the field. However, respiration rates were found to be higher in the girdled plot soil than in the control soil when roots and litter were removed by sieving and CO₂ efflux from just the mineral soil and associated detritus was measured.

Increased leaching rates of nutrients revealed the effects of girdling on biogeochemical cycles of the forest. The most pronounced effect was increased concentrations of NO₃ ions in the soil water of the girdled plots, resulting in losses of NO₃ ions below the root zone (>60 cm) during the second year. These losses amounted to 25.4 kg ha^-1 in plot no. 2 and 9.0 kg ha^-1 in plot no. 1 as compared to 0.15 kg/ha^-1 from the control plot. Calcium losses below the root zone during the second year were ≃10 kg ha^-1 greater in plot no. 2 than the control plot, while losses from plot no. 1 were about equal to the control. Calcium and nitrogen uptake by the sprouts in plot no. 1 offset the slightly reduced uptake by the girdled trees and in fact uptake in plot no. 1 may have exceeded the control during the first year. Differences in uptake could account for only a portion of the differences in NO₃ ion concentrations based on litterfall and diameter growth in girdled plots.