This review summarises the knowledge about the ecology, breeding and management of hybrid aspen (Populus)wettsteinii Hämet-Ahti 0P. tremula L. )P. tremuloides Michx.). The review is restricted mainly to Northern Europe, where hybrid aspen has been most intensively studied and cultivated and where it has proved to be one of the fastest-growing hardwoods, suitable for the production of pulp-and energy-wood using the principles of short-rotation forestry. During recent decades over 4500 ha have been cultivated with hybrid aspen in the region. A number of research articles and domestic project reports involving hybrid aspen have been published, providing the basis for this review. Breeding has resulted in clones with high productivity and improved resistance to pests and diseases. Thus, hybrid aspen has fulfilled the preconditions for becoming an economically valuable hardwood in Northern Europe. Hybrid aspen plantations can be established on abandoned agricultural land, on forest land, and for the reclamation of exhausted surface mines. However, fast growth rate occurs only in fertile sites with good nutritional and hydrophysical properties. An increased area of Populus plantations on forest or agricultural land can have both positive and negative impacts on biodiversity, depending on landscape context, management activities and considered organisms. Further studies are needed concerning silviculture, site-growth relations, stability of clones, environmental and biodiversity impacts in large-scale plantations at various sites and adaptation of hybrid aspen to climate change.

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Highlights • Annual growth is 287 million m 3 in the forests of the Nordic and Baltic countries. • Growth can be increased by new tree species, tree breeding, high-productive management systems, fertilization and afforestation of abandoned agricultural land. • We predict a forest growth increment of 50-100% is possible at the stand scale. • 65% of annual growth is harvested today.

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The aim of the present study was to calculate fine-root turnover rates in stands of basket willow (Salix viminalis L.). Fine-root number was recorded in minirhizotrons in two adjacent short-rotation forest stands. Stand A was a regularly spaced plantation on clay soil. Stand B contained lysimeters, which were inserted in the soil and filled with either clay soil or washed sand. Both stands were irrigated and fertilized daily, to provide near-optimum conditions with respect to water and nutrient availability. The calculations were based on morphological studies and observations in minirhizotrons. Mean fine-root ages of growth and decay phases were calculated from third-order polynomials, and by summing up those phases and adding a short stationary phase, turnover time was obtained. Calculated fine-root turnover rates were 4.9–5.8 year–1 in the plantation and 4.8–8.1 year–1 in the lysimeters. No significant difference in turnover rates was detected between clay and sand substrates. Soil temperature had a significant effect on the decay phase, and in the calculations the data were weighted by soil temperature intervals. The importance of observing fine roots throughout the year is stressed.