Following the detection of multimodality of the boreal forest's tree-cover distribution, the existence of areas with potentially alternative tree-cover states has been shown. These areas encompass ~1.1 million km2, and correspond to possible transition zones with a reduced resilience to disturbances. Identifying which are the main factors underlying their existence, is important to project future change of natural vegetation cover and its effect on climate.
To explain the multimodality found in the data and the asymmetry in tree-species distribution between North America and Eurasia, we develop a conceptual model based on tree-species competition with stochastic disturbances, and use it to simulate the sensitivity of tree cover to changes in environmental factors. We include different Plant Functional Types based on survival adaptations, and force the model with remotely-sensed data on temperature, soil moisture, permafrost distribution, and precipitation. Furthermore, to project possible future changes of natural vegetation cover, we employ our conceptual model to simulate the dynamics of multistable zones under environmental conditions from the RCP2.6 and RCP8.5 scenarios.
Results/Conclusions
We find that multimodality and multistability can be explained through our conceptual competition model by including tree species with different adaptations to environmental factors and stochastic disturbances. Moreover, the model is able to reproduce the asymmetry in tree-species distribution between Eurasia and North America. We find that changes in permafrost could be associated with phenomenological bifurcation points of the model, corroborating the importance of permafrost in a changing climate. Finally, we show how the extent of multistable areas could increase or decrease depending on anthropogenic climate change under the selected representative scenarios.
We conclude that multistability of the tree cover in the boreal region can emerge through competition between species subject to periodic disturbances. Changes in permafrost thaw and distribution could be responsible for the asymmetry in tree-species distribution between North America and Eurasia. Under the RCP2.6 scenario, anthropogenic climate change could significantly increase the extent of multistable areas, with the potential for shifts in tree-cover state and dominant species.