Principles of modelling vegetation dynamics with regard to climate changes

Principles of modelling vegetation dynamics in the successional time scale are discussed. Global models of biosphere predict shifts of bioclimatic zon es in response to climate changes by means of corresponding shifts in a glo bal space of climatic indices designed in a special way. On the contrary, t he localization principle lays a local geobotanic knowledge into the founda tion of the models, namely, the knowledge of conceptual scheme for; and reg ularities in, those plant successions which proceed in the area under study . The Markov-property principle -(postulate) arises once a particular schem e of succession is determined, thereafter random Markov chains serve a conv enient tool to describe in formal terms how phytocoenoses change in time. T he ergodicity principle (hypothesis) is used whenever the behaviour of a te mporal (successional) series is judged from the analogy with that of the sp atial (ecological) one or vice verse, and this kind of reasoning is general ized in the fundamental mathematical notion of ergodicity. The homogeneity principle, i.e. independence of time in the transition probabilities of the chain, gives rise to a mathematical property in common of such models, nam ely, convergence to a stable limit distribution, thus meeting the major par adigm of succession theory: regular movement from pioneer stages to the sta ble (poly)climax one. However, the traditional time-homogeneous models (wit h constant transition probabilities) accept actually the hypothesis of envi ronmental invariance, which ran hardly be accepted when the matter is about long-term prediction in the successional time scale. Models of new generat ion, the inhomogeneous chains, in which the transition probabilities are de signed as functions of the key factors of the environment influencing the c ourse of succession, now loose the algebraic elegance of their homogeneous prototypes But gain sensivity to real (climatic, in particular) parameters of the environment. The problem to model successions under global changes i s thus reduced to the task to scale the global climate scenario down to the level of local key factors. In the gnosiological perspective, these models signify a move from phenomenology to the causality principle, which the ho mogeneous description has been a priori ruling out. The theoretical princip les are exemplified with a practice in modelling successions in the forest- steppe zone.