THE DYNAMICS OF A THEROPHYTIC COMMUNITY IN RELATION TO SELF-REGULATION, HERBIVORES AND ENVIRONMENTAL VARIATION

The dynamics of a therophytic plant community in Central Spain has been studied, taking into account endogenous and exogenous factors. Using a simplified descriptive resolution a multi-species grass community and four canopy density types have been described in permanent quadrats. Transition matrices expressing the changes of vegetation state have been constructed. The importance of self-regulation was shown by the dependence of successive transitions on each other (highly significative log-linear models). The functioning cannot however be explained through a strict Markovian model because the transition probabilities are not stationary. The external local factors tested (microtopography, compactness and granivory) do not have a consistent effect on the vegetation dynamics, being variable over years, implying the importance of other more general factors on vegetation dynamics. Making transition probabilities dependent on external environmental factors represents a way of further developing projective models of the dynamics of these therophytic systems.