Production and ecological aspects of short rotation poplars in Sweden

Poplars (Populus sp.) are widely used in short rotation forestry for production of biomass for bioenergy, fibre and environmental services. Swedish short rotation forestry is based on Salix sp., and little is known about the production potential of poplar plantations and their effects on the environment. This thesis focuses on four aspects of intensive short rotation forestry with poplars: 1) Biomass production and partitioning at several initial densities and a range of latitudes and growing conditions in Sweden, 2) the effects of poplar plantation on floristic diversity in the Swedish agricultural landscape, 3) the pattern of wind damage and its effects on production in poplar plantations in southern Sweden, and 4) ecological characterisation of poplar varieties in short-term experiments with pot-grown plants. Annual biomass production in poplar plots and plantations over a rotation period of 9-14 years ranges between 3.3 and 9.2 Mg ha-1 yr-1. These high production figures are achieved on relatively fertile, non-fertilised and non-irrigated agricultural land. The production assessments for commercial poplar plantations established at lower initial densities (1000 trees ha-1) in southern Sweden indicate a similar production potential as in closely spaced cultures (5000 trees ha-1 yr-1), though at 3-5 years longer rotations. Lower initial densities enable higher pulpwood yields along with the production of biomass for bioenergy. A comparison among 21 poplar plots, 0.1-13 ha large and adjacent arable fields, indicates that small poplar plantations may increase floristic diversity on a landscape scale, mainly by providing a different type of habitat that may favour shade-tolerant and draught-sensitive species. This is reflected by a relatively low number of species shared by both types of habitat. Wind damage in two poplar plantations, 15 and 33 ha large, was assessed using wind damage classes based on leaning angle of individual trees on plots established before wind damage occurred. The loss of increment on the strongest damaged plots during the two-year period after the storm was 30%, whereas there was no difference in growth between damaged and undamaged plots in the third year after the storm. A short-term experiment using pot-grown plants revealed differences in clonal growth response in terms of physiological and morphological variables that determined relative growth rate and nutrient productivity, despite that most of clones were of the same species and geographic origin. The importance of different response variables in determining growth also shifted as an effect of irrigation and fertilisation treatment. Provided that suitable plant material is selected and widely available, commercial SRF with poplars represent a valuable alternative crop for surplus agricultural land with a potential to produce multiple benefits to society through the high production of biomass and fiber and positive effects on the environment