1 A zero-dimensional model of local atmosphere-vegetation interaction is presented. The model includes essentials of water related two-way feedbacks, such as the influence of vegetation on evapotranspiration, and the impact of temperature and drought on biomass growth and mortality. The simple model serves as a framework for the preliminary investigation of vegetation related feedbacks under climate change scenarios. 2 Model simulations for a mid-latitude forest area for an increasing external forcing indicate a transient growth of biomass up to a critical forcing, where drought stress begins to dominate the response. Beyond, biomass decreases, reinforced by the reduced evapotranspiration of a diminished vegetation, leading to an additional temperature increase (biomass- evapotranspiration feedback). 3 The implementation of an additional feedback loop based on the hypothesis that drought stress implies not only a reduction in above-ground biomass, but also a net reduction in roots and therefore a reduction of the amount of water accessible to the plants for transpiration, leads to the occurrence of a second stable state in the atmosphere-vegetation system. In the bistable regime, a moderate perturbation can trigger an abrupt change of state. 4 The present conceptual investigations underline the importance of a dynamic vegetation subsystem in transient climate change, and stress in particular the possible role of feedbacks related to root dynamics.
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