Large ungulates, landscape dynamics, and forest succession in a changing climate
Biome boundaries and their dynamics in a changing climate are influenced by temperature, disturbances, and physiographic factors such as topography, water bodies, and soils across the landscape. A spatially variable mosaic of above- and below-ground trophic factors are also superimposed on existing abiotic factors. These include interactions among large ungulates, their predators, vegetation, and earthworms. These complex interactions in turn cause spatially variable impacts on direction of forest succession. Here we provide a case study of white-tailed deer and earthworms as factors that can influence transition from boreal forest to temperate forest as the climate warms in the western Great Lakes region of Minnesota, Wisconsin and Michigan, USA. Questions to be answered by synthesis of existing studies include: (1) Under what conditions can ungulates speed up or impede transitions from one biome to another in a warming climate? And (2) How might ungulates and soil organisms such as earthworms interact to create novel communities?
Deer generally prefer to browse seedlings and saplings of temperate species such as maple and oak rather than boreal spruce and fir. Deer densities vary at spatial scales of 100s to 1000s of square km, due to variation in predator density (wolves), human fragmentation of the landscape, climate, disturbance and physiography. In local areas with high ungulate densities, invasion of temperate tree species into the understory of boreal forests may be impeded, possibly leading to an abrupt transition later on when threshold temperatures detrimental to boreal species are reached. In other nearby areas with lower ungulate densities, temperate species may expand into boreal forests at a higher rate, gradually replacing the boreal forest as the climate warms. European earthworm invasion can also reinforce or counteract the impacts of deer and climate in complex ways and create a novel set of successive filters on future plant community composition. Temperate forests replacing the current boreal forest are likely to be different than current temperate forests, rather than a simple northward shift of the existing temperate plant communities. For example, drought and worm tolerant red maple is likely to be the dominant maple rather than sugar maple, while worm and deer tolerant Pennsylvania sedge could replace existing dominant herb species. Highly patchy and variable transitions from one biome to another are likely to occur due to complex trophic interactions among ungulates, their predators, physiographic factors, and earthworms.