Abstract :
Several pioneer tree species were compared in a trial in southern Sweden with the aim of evaluating non-coppice short rotation forestry systems on agricultural land. The trial comprised two clones of hybrid poplar (Populus trichocarpa×deltoides), hybrid aspen (Populus tremula×tremuloides), one Swedish provenance of Betula pendula, grey alder (Alnus incana), a mixture of A. incana and Salix fragilis, and a mixture of A. incana, B. pendula and Salix schwerinii. All plots were planted at a 1×2 m spacing and weeded annually until canopy closure. After four growing seasons some poplar plots were fertilised with 150 kg nitrogen ha−1 and/or thinned to 2500 trees ha−1. After 6 years, both poplar clones had produced 45 tonnes dry matter ha−1 (stem+branch), and neither thinning nor fertilisation had affected their yield. During the same period the hybrid aspen, grey alder and two species mixtures had produced 24–27 tonnes ha−1. The birch produced 14 tonnes ha−1 in 6 years. Competition did not lead to self-thinning in any of the species. The allometric relations between stem diameter and tree weight were highly age-specific. When allometric relations, derived from four-year-old trees, were used in measurements made the following year, biomass was underestimated by 3–15%. It was concluded that within a few years after planting on a given fertile arable soil, several tree species can attain about the same level of biomass production as agricultural crops.
