INITIAL RESPONSE OF UNDERSTORY PLANT DIVERSITY AND OVERSTORY TREE DIAMETER GROWTH TO A GREEN TREE RETENTION HARVEST

The increasing use of harvest techniques other than clearcutting in fo rests west of the Cascade mountains has created an urgent need to unde rstand 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 mod erate-intensity fire or wind disturbance. We examined the response of understory plant diversity and basal area increment of retained overst ory trees after sixteen months and six years, respectively. in GTR cut s. Understory species cover was sampled on three adjacent treatment ar eas in the Seattle City Watershed, Washington: a clearcut, a dispersed green tree retention and the intact sixty-five year old forest. The c hange in basal area increment on dispersed green tree retention cuts w as 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 richne ss and evenness were significantly higher in the green tree retention cut than in the other two understory treatments. Although overall spec ies composition of the GTR was closer to the clearcut than the forest, the GTR retained more species and cover of shade-tolerant plants impo rtant for maintaining understory diversity as canopy closure reduces u nderstory light. While the basal area increment response of retained t rees varied between stands. the average response for all stands for a six year period following harvest was a 15% reduction in increment gro wth compared to the control stands. Additional study is needed to dete rmine the persistence of these effects and how basal area growth respo nse varies as a function of tree size and age.