This review summarises the knowledge about the ecology, breeding and management of hybrid aspen (Populus×wettsteinii Hämet-Ahti=P. 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.

At northern latitudes a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We studied several growth and functional traits of hybrid aspen (Populus tremula L.×P. tremuloides Michx.) in response to elevated atmospheric humidity (on average 7% over the ambient level) in a free air experimental facility during three growing seasons (2008–2010) in Estonia, which represents northern temperate climate (boreo-nemoral zone). Data were collected from three humidified (H) and three control (C) plots, and analysed using nested linear models. Elevated air humidity significantly reduced height, stem diameter and stem volume increments and transpiration of the trees whereas these effects remained highly significant also after considering the side effects from soil-related confounders within the 2.7 ha study area. Tree leaves were smaller, lighter and had lower leaf mass per area (LMA) in H plots. The magnitude and significance of the humidity treatment effect – inhibition of above-ground growth rate – was more pronounced in larger trees. The lower growth rate in the humidified plots can be partly explained by a decrease in transpiration-driven mass flow of NO₃⁻ in soil, resulting in a significant reduction in the measured uptake of N to foliage in the H plots. The results suggest that the potential growth improvement of fast-growing trees like aspens, due to increasing temperature and atmospheric CO₂ concentration, might be smaller than expected at high latitudes if a rise in atmospheric humidity simultaneously takes place.