TreeMig: A forest-landscape model for simulating spatio-temporal patterns from stand to landscape scale

Landscape patterns result from complex endogenous dynamics and heterogeneity of environmental drivers. Landscape models are appropriate tools for analysing such patterns. We present the dynamic, spatially explicit, grid-based and spatially linked forest-landscape model TreeMig. In each grid cell, the forest dynamics is simulated with a multi-species, height structured forest model, based on growth, competition and death of the trees in each height class. Within-cell heterogeneity is accounted for by assuming that trees are randomly distributed resulting in Poisson distributions of tree densities and light. Reproduction is modelled explicitly by seed production, seed bank dynamics, germination and sapling development. The forests in the different cells interact spatially through seed dispersal. The model is flexible and can be applied on a range of spatial scales, from single stands to entire regions. The model's ability to generate patterns was tested in two case studies with different spatial resolutions. The first case study shows simulations of pattern formation by endogenous dynamics on a small spatial scale. The simulations are conducted under spatially and temporally homogenous environmental conditions, initialized with seeds of all species in the centre cell. The simulation shows transiently several types of patterns in the species biomasses: circular standing waves, patch structures and homogenous spread of dominant species. In the second, large-scale case study, the tree species’ spread since the last Ice Age in the Alpine region of the Valais is simulated, under temporally and spatially heterogeneous environmental conditions. The simulated spatio-temporal pattern consists of immigration waves of new species into empty or already forested areas, fast die-backs after sharp, strong temperature decreases, slower recolonization after temperature increases and spatial separation of the species according to the environmental conditions. The simulation indicates that the environment forms the basis for the endogenous dynamics, primarily migration and competition, which play a particular role during the transient phases after drastic changes in the boundary conditions (immigration and climate change). We conclude that the forest-landscape model TreeMig is able to produce landscape patterns resulting from both, endogenous dynamics and exogenous drivers and is suitable for a range of different applications.