The effects of changing land use and browsing on aspen abundance and regeneration: a 50-year perspective from Sweden.
Management of trees with high conservation value under altered land use is challenging. This applies to European aspen Populus tremula, a keystone tree species for species conservation in northern forests. Fire suppression in managed forest has reduced niches for sexual regeneration in aspen while levels of browsing have increased with increasing numbers of ungulate herbivores. We combined observational and experimental data from 1953 to 2007 to unravel patterns and causes of changes in aspen abundance in Sweden, the country with the largest forests in the EU. The density of small-sized aspen ramets showed a peak in the early 1970s, followed by a marked decrease. Numbers of moose Alces alces, the most important browser on aspen, showed a similar temporal pattern, but moose numbers peaked 10 years later than aspen. During the same time period, the volume of aspen doubled. The changes in aspen abundance correlate to large-scale changes in forestry including the introduction of clear-cutting practices and extensive clearing of aspen, and cessation of forest livestock grazing and abandonment of marginal farmland. Using exclosures, and controlling for time since disturbance and regeneration status, we monitored aspen demographics for 5 years in an aspen rich landscape. There was an eightfold increase in recruitment rate of established ramets to a height safe from browsing (>3 m) in fenced plots. However, the finite rate of increase, λ, derived from a transition matrix model, was consistently below 1, i.e. the aspen growth rate was negative with or without browsing. This was associated with a decrease in sprouting rate over time. Synthesis and applications. Our results suggest that changes in land use practices are the main cause of changes in aspen abundance at regional and national scales in Sweden during the last 50 years. Restoring regeneration niches, most importantly emulating natural disturbance processes, viz. fire at various spatial scales, and retaining aspen in cleaning and pre-commercial thinnings are the most important management recommendations to secure regeneration of aspen. Protecting established aspen ramets at designated sites from browsing either by fencing or reducing ungulate numbers could be used as complementary management tools.