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

Abstract

To evaluate whether changes in leaf weight might affect the interpretation of foliar analyses, seasonal samples of current and older Douglas-fir (P. menziesii (Mirb.) Franco) needles were compared from contrasting environments in the Coast Range and Cascade Mountains of western Oregon, USA. Foliage from trees grown in the climatically mild and moist Coast Range maintained nearly constant weight and N content per unit of leaf area throughout the year, regardless of whether stands were thinned or left unthinned. In contrast, foliage from trees growing in the harsher environment of the Cascade Mountains 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 N 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, thinning significantly increased both foliage weight and N content. Implications of these findings are discussed in regard to interpreting foliar analyses.