Effects of silvicultural treatments in young Scots pine-dominated stands on the potential for early biofuel harvests

The overall objective of the work underlying this thesis was to increase knowledge regarding growth of young Scots pine (Pinus sylvestris L.) - dominated mixed forests in northern Sweden and the potential for combining early biofuel harvest in such stands while leaving crop trees for future harvests. For this purpose, several studies were made. Biomass functions for the fractions stem (including bark), branches, foliage and whole trees were created for Scots pine, Norway spruce (Picea abies (L. Karst.), downy birch (Betula pubescens Ehrh.) and silver birch (Betula pendula Roth), based on measurements of the sampled trees in six young and dense stands (I). Growth and yield was studied for 8-11 years at four experimental sites (stand ages 17-20 years at start) in which density treatments (pre-commercial thinning, PCT, to 3 000 stems ha-1 and no-thinning; control, C) and fertilization treatments (N fertilization at 100 kg ha-1 every 6th year or annually: F1 and F2, respectively) had been applied alone and in two combinations (C+F1, C+F2 and PCT+F1). During the observation period total biomass yield was 58 - 79% higher (up to in total 100 ton ha-1 DW) in the dense, unthinned stands (>11 000 stems ha-1) than in the PCT stands. Fertilizing every year did not give significantly higher biomass production than the two fertilization applications. The 500 - 2 700 largest trees ha-1 showed significantly higher values of measured size parameters following treatment C+F2 compared to the unthinned control (C), but not to the PCT treatment, indicating that stand density only had minor effects on growth of the largest trees (II). When allocation patterns were analysed after six years, the only significant between-treatment differences found for Scots pine trees of various size classes were among the smallest trees (with a diameter at breast height, DBH, 9 000 stems ha-1. Further, the trees that died (and hence were most severely affected by competition) were the smallest trees (DBH<5 cm), and timing of PCT had only marginal effects on the risk of mortality (V). The main conclusion from the results is that substantial amounts of biofuel can be harvested from pine-dominated young stands at appropriate times, if conventional PCT is omitted, while still retaining appropriate numbers of crop trees for subsequent main harvests. Key words: Allocation patterns, Betula spp., biomass functions, branch characteristics, mortality, Picea abies (L. Karst), Pinus sylvestris (L.), production, young dense mixed stands

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