Identifying and managing the ecological risks of using introduced tree species in Sweden's production forestry

Introduced tree species are increasingly being considered for use in production forestry due to production targets, and demand for a diversity of wood products. However, prior to expanding their use, active consideration needs to be given to the breadth of potential ecological consequences associated with each introduced tree species. Ecological consequences include the invasion and modification of sensitive ecosystems, changes in habitat provision for native taxa, altered risk of pest and pathogen outbreaks, and hybridization with native con-generics. Here we review the scientific literature to assess the potential ecological consequences from expanding the use of introduced tree species within Swedish forestry. We use an interdisciplinary approach to evaluate ecological risks, and our assessment is based on the scenario that a proportion of Norway spruce (Picea abies) monocultures in southern Sweden will be replaced by monocultures of Sycamore maple (Acer pseudoplatanus), Douglas fir (Pseudotsuga menziesii), hybrid aspen (Populus tremula tremuloides), or hybrid larch (Larix eurolepis/L. marschlinsii). Our results highlight that univariate consideration of the ecological consequences of exotic tree species can be highly misleading, due to the complex suite of costs, benefits, risks and uncertainties that each tree species brings to the region of introduction. We discuss our results in relation to conflicting management goals, and the lack of reversibility of some adverse ecological outcomes. We also highlight the need for assessments of ecological risk to facilitate evidence-based decision making by stakeholders. Our results provide a foundation for adaptive management programs aiming to limit the extent to which introduced tree species used in production forestry are accompanied by adverse ecological impacts.