Establishment and early management of Populus species in southern Sweden

Sammanfattning: Populus species are among the most productive tree species in Sweden. Interest in growing them has increased during the 21st century due to political goals to increase the share of renewable energy and to increase the proportion of hardwood species in forests. Populus species have been shown to be potentially profitable, but currently they are mostly planted on abandoned agricultural land. There is a lack of knowledge about the establishment of Populus species on forest sites. There is also a lack of knowledge of how second generations can be established by root and stump sprouts, and about management of young stands. The main objective of this thesis was to focus on these issues. Site properties are important for successful establishment of Populus species and factors such as soil pH and soil moisture should be considered. Site preparation is important to control weed competition and to modify the microclimate, including soil moisture. In this thesis, mounding in relation to patch scarification, soil inversion and no intervention, were found to provide the highest survival and growth. Hybrid aspen was in most cases found to be more robust than other Populus species, but poplar plants or long unrooted cuttings showed similar survival and faster growth at the site where soil pH was high. In general, short unrooted poplar cuttings showed high mortality on forest sites. In water saturated soils, the growth of poplar cuttings was inhibited and roots were located closer to the soil surface. This was mainly caused by the absence of callus roots originating at the base of the cutting. Poplar stump sprouts can be used successfully for regeneration if the clones can produce living straight sprouts. Variability in this trait was found in one of the studies in this thesis. A second generation of naturally-regenerated hybrid aspen can produce over 100,000 root suckers per hectare. If no early management is conducted, biomass will be lost through self-thinning. It was shown in this study that a substantial amount of biomass can be harvested schematically through corridors or cross-corridors, resulting in stimulated stem diameter development. An additional early thinning further increased the diameter on crop trees. In this thesis, site-adapted management of Populus species is suggested to be the most important measure to achieve successful regeneration. In particular, this includes choosing species and clones adapted to specific site properties. A second generation can be established either by stump sprouts (poplars) or root suckers (hybrid aspen). If larger-dimensioned trees are desired, early thinnings are important in the second generation.