Nutrient content in stems of hybrid aspen as affected by tree age and tree size, and nutrient removal with harvest

The content of macro-nutrients in stems of hybrid aspen (N, P, K, Ca, Mg, S) was analysed, and nutrient removal at concurrent thinning operations was calculated. Five stands on fertile soils in southern Sweden were used in the study, and stand age spanned from 4 to 15 yr. The average nutrient concentration of the stems decreased with increasing age (1.52 kg N m(-3) at age 4 yr to 0.71 kg N m(-3) at age 15 yr). However, differences in nutrient concentration between trees of varying size within a stand were never observed for N, P. and K, and were only occasionally seen for Ca, Mg, and S. As expected, nutrient concentration always increased with stem height, except for Ca where this pattern was only seen in two of the stands. This means that a biofuel assortment of tops and branches will have higher concentrations of nutrients than merchantable wood, and thereby put the focus even more on growth sustainability and reapplication of nitrogen and ash. Since the share of merchantable wood increases with stem size, the absolute removal of biomass and nutrients in logging residues will decrease with age. In this study, a total of 23-93 kg N ha(-1) was removed in the thinnings. (C) 2002 Elsevier Science Ltd. All rights reserved.