The effect of vole population cycles on the boreal forest ecosystem dynamics

The boreal forest biome covers some 11% of the Earths terrestrial surface and comprises about 25% of the Earths closed canopy forests. Despite its low species diversity compared to many other biomes at lower latitudes, the boreal forest has attracted a substantial attention because of its important role in timber production and the global carbon cycle, but also as a provider of local ecosystem services for people living in the boreal zone. We have studied some conspicuous geographic ecosystem differences between North America and Northern Europe (i.e. Scandinavia), and why these should exist at the first place. The boreal ecosystems of N-Europe and N-America provide a good case for a first cross-continental comparison, because these two regions have been subjected to intense ecological research over many decades. In both regions, the dominant trees are conifers, especially spruce trees. However, beyond the trees the visual vegetation difference between regions are striking. The layer of deciduous tall shrubs that is predominant in N-America, is virtually missing in N-Europe, where a layer of dwarf-shrubs predominates. Other aspects of the food web also contrast strikingly between the two regions, and most differences are due to different key herbivores. In N-America, the snowshoe hare exhibits a high-amplitude 9-10-year population cycle. In N-Europe, a guild of small rodents exhibit a guild-wide 3-5-year abundance cycle. In the boreal forest of N-Europe recent human impacts have shaped forest stand structure, the distribution of successional stages and some aspects of the structure of the food web that does not have N-American counterparts. The anthropogenic impacts in Europe include increases of meso-predators (especially red fox) and ungulates (especially moose) resulting from release of top-down limitation from apex predators (especially wolves) as well as bottom-up boosts from land use (especially forestry and agriculture). However, we contend that the fundamental regional differences we have highlighted: which is the contrasting understory vegetation and key-stone herbivore-predator interactions that drives ecological dynamics with different pulse rates in the two systems, does not result from different degrees of human intervention. Instead, we propose that these differences are more ancient and caused by different winter climates that mainly act bottom-up by first shaping the understory vegetation, and then second and third the dominant herbivores and predators. Winter climate, through snowpack characteristics, also has a direct influence on the key predator-prey interaction type and strength that contributes to shaping the dynamics, such as population cycle period and amplitude. For the Scandinavian boreal forest we have studied vole population cycles in more detail to understand how this affect the boreal forest ecosystem dynamics. We have made a conceptual model describing how winter survival pockets, potentially with good food conditions, will cause the aggregation of females. These aggregated females will be social and amicable and reproduce well during winter. This results in the peak phase which collapse through an interaction between predators, dispersal and social factors. As the population increases, predation will after a delay also increase. The death of dominant males will cause a turnover of males resulting in infanticide and other social death events. The disrupted social system will increase dispersal and make animals more prone for predation. This conceptual model is confirmed by individual based modelling where we show that only in a situation including both intrinsic factors (sociality and dispersal) and predation, yield cycle periods, amplitudes and autumn population sizes closest to those observed in nature. As snow conditions is often expected to be the cause of disappearing cyclic dynamics in the Norther Hemisphere we used an altitudinal gradient as a proxy of variations in winter weather conditions. Results suggest higher population amplitudes during the summer season at high altitudes than low altitudes. We suggested that this is due to better winter survival and snow conditions at high altitudes, which give rise to the aggregration of social female groups with higher recruitment in spring. We confirmed that rodents had a higher survival at high altitudes compared to low altitudes. However, we observed rodent populations crashing the winter 2014/15 at all altitudes independent of snow conditions. Only populations that received supplemental feeding in an experimental setting survived this winter. Even attempts to improve rodent habitat with straw did hot help survival. The effect of food on winter dynamics has to be further investigated.

In this proposal, we build a highly prestigious scientific network by linking Scandinavian ecological environments with the distinguished Canadian group working on the boreal forest ecosystem in North America. We will study the role of vole multiannual po pulation cycles on the boreal forest ecosystem dynamics. We will manipulate vole cycles in large scale natural experimental plots (16 km2). Treatment groups will consist of: (1) low non-cyclic vole densities; (2) high non-cyclic vole densities; and (3) co ntrols, where vole populations are expected to fluctuate naturally. This is the first time a dynamic phenomenon such as vole cycles is manipulated at such a large spatial scale. The large scale manipulation gives a unique opportunity to resolve questions related to a natural food web at the landscape level, including the response of wide-ranging predators such as mustelids, fox and owls. Our approach will resolve how the conspicuous vole cycles may be a driver of the boreal forest ecosystem dynamics and w hether temporal heterogeneity enhances species diversity similar to how spatial heterogeneity does; it will improve our understanding of how the Fennoscandian boreal forest ecosystem will react towards anticipated changes in vole cyclicity through global warming; and reveal why the Fennoscandian and North American boreal forest ecosystem dynamics are so different. We will use ecosystem models, such as the Ecopath trophic mass-balance modelling framework, to compare the dynamics of these two continents b oreal forests. Because we are manipulating ecosystem dynamics at broad scales and comparing these dynamics on different continents, our research will provide a base for a broader and more predictive evidence-based management. The project will also contrib ute to general questions in ecology, including what factors contribute to variation in community structure and function and their cascading effects on natural systems.