Interpreting foliar analyses from Douglas-fir as weight per unit of leaf area

To evaluate whether changes in leaf weight might affect the interpretation of foliar analyses, seasonal samples of currentand older Douglas.-fir (Pseudotsuga menziesii (Mirb.) Franco) needles were compared from contrasting environments in theCoast Range and Cascade Mountains of western Oregon, U.S.A. Foliage from trees grown in the climatically mild and moistCoast Range maintained nearly constant weight and nitrogen content per unit of leaf area throughout the year, regardless ofwhether stands were thinned or left unthinned. In contrast, foliage from trees growing in the harsher environment of the CascadeMountains averaged only half the weight per unit of leaf area of samples from the Coast Range during the dormant season.During the summer, foliage weight and nitrogen content per unit of leaf area increased by more than 50% in the Cascades.These values were 20-40% less, respectively, than those observed in the foliage of Coast Range trees. In the Cascades, thinningsignificantly increased both foliage weight and nitrogen content. Implications of these findings are discussed in regard tointerpreting foliar analyses.