LONG-TERM RESPONSES OF STEM GROWTH AND LEAF-AREA TO THINNING AND FERTILIZATION IN A DOUGLAS-FIR PLANTATION

Within a 53-yr-old Douglas-fir plantation, replicated thinning and fertilization plots were examined for responses in stem growth, leaf area and stem growth per unit leaf area. Although measurements occurred 20-30 yr after plot installation, substantial effects from the various treatment levels were still present. Thinning reduced leaf area of the stands but increased stem growth per unit of leaf area, resulting in little difference in stem growth per hectare over the 5-yr measurement period (1977-1981). Fertilization increased both stand leaf area and stem growth per unit leaf area, and more than doubled 5-yr stem growth per hectare. Consideration of the role of leaf area and stem growth per unit leaf area in determining stand treatment responses may account for much of the variation found among replicates of treatments or between studies on different sites.