Regeneration of deciduous trees – Variable success of moose exclosures in central Sweden

Abstract

Repeated browsing by moose (Alces alces L.) may prevent tree saplings from reaching the canopy. Due to the large population of moose in Sweden and their preference for certain deciduous tree species (Populus tremula L., Salix caprea L., Sorbus aucuparia L.), there is concern for insufficient replacement of mature trees of these species, with consequent negative impacts on the associated biodiversity. To aid recruitment of deciduous trees, forest companies and conservation authorities have set up a number of exclosures in central Sweden during the last 20 years. The aim of this study was to evaluate the outcome of these exclosures, in terms of 1) the number of saplings, 2) the proportion that were browsed, and 3) the height of the tallest individuals. Study areas included 7 forest stands with fences (3 of which were burned prior to fencing) and 3 stands with log barriers. Three wind throw areas were also included as a type of natural exclosure. Controls were placed outside each exclosure. Due to the heterogeneity in the study design, each forest stand was treated as an individual study in the analysis. The most evident result was that 8 out of 11 fences were currently broken, and the rest had been repaired. Overall, there was great variation in how successful the exclosures were, but there were some trends: The number of saplings of birch, aspen and rowan was sometimes high, whereas sallow was always present in low numbers. Birches were few in the wind throw areas. The proportion of browsed individuals was generally quite large both inside and outside the exclosures, and increased with age. The overall proportion browsed across all study areas appeared to be similar for aspen, sallow and rowan, whereas birch seems to be of relatively minor interest. There was also a tendency for all species to have a higher proportion of browsed individuals outside the exclosures, where the variation was also slightly smaller. In general, there was a tendency toward taller saplings inside the exclosures, at least for aspen and rowan, and there was no consistent difference between exclosure types. Rather, there seemed to be a general increase in sapling height with age. The positive trend for especially aspen and rowan inside as compared to outside the exclosures stems mainly from the two oldest fenced sites which are both located in production stands, and might thus be a result of the fencing or an artefact of previous clearings. The small size of the log barriers limits their potential, and wind throw areas are not an alternative for human construction. Fences can potentially be successful in limiting moose browsing and enabling recruitment of deciduous trees, but it requires regular monitoring and maintenance.