Initial response to understory plant diversity and overstory tree diameter growth to a green tree retention harvest

Abstract

The increasing use of harvest techniques other than clearcutting in forests west of the Cascade mountains has created an urgent need to understand the effects of these practices on ecosystem species composition and structure. One common alternative, "green tree retention" (GTR), leaves some live trees on a harvest site to more closely mimic a moderate-intensity fire or wind disturbance. We examined the response of understory plant diversity and basal area increment of retained overstory trees after sixteen months and six years, respectively, in GTR cuts. Understory species cover was sampled on three adjacent treatment areas in the Seattle City Watershed, Washington: a clearcut, a dispersed green tree retention and the intact sixty-five year old forest. The change in basal area increment on dispersed green tree retention cuts was sampled with increment cores collected in six stands and two uncut control stands in the Wind River and H.J. Andrews Experimental Forests in Washington and Oregon, respectively. Herb and shrub species richness and evenness were significantly higher in the green tree retention cut than in the other two understory treatments. Although overall species composition of the GTR was closer to the clearcut than the forest, the GTR retained more species and cover of shade-tolerant plants important for maintaining understory diversity as canopy closure reduces understory light. While the basal area increment response of retained trees varied between stands, the average response for all stands for a six year period following harvest was a 15% reduction in increment growth compared to the control stands. Additional study is needed to determine the persistence of these effects and how basal area growth response varies as a function of tree size and age.

Citation

North, Malcolm; Chen, Jiquan; Smith, Gordon; Krakowlak, Lucy; Franklin, Jerry. 1996. Initial response to understory plant diversity and overstory tree diameter growth to a green tree retention harvest. Northwest Science, Vol. 70(1): 24-35