Vegetatio 71: 33-39, 1987 © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands 33 Temporal changes in therophytic communities across the boundary of disturbed-intact ecosystems* E D. Pineda l, M. A. Casado 1, B. Peco 2, C. Olmeda 1 & C. Levassor 2 Departments of Ecology, University Complutense I and University Aut6noma 2, Madrid-28040, Spain Accepted 9.2.1987 Key words: Ecosystem: boundary, Hysteresis, Meteorological influence, Monte Carlo simulation, Pasture: disturbed, Pasture: Mediterranean, Succession, Therophyte Abstract External control processes cause continual compositional and structural readjustments of Mediterranean pasture ecosystems. Such control processes include herbivore grazing, meteorological fluctuations and tradi- tional management activities, which determine the "stable" environment where the succession occurs. Tradi- tional management in this ecosystem frequently involves periodic ploughing or controlled fires. Experimental disturbances were applied to pastures of different maturity. Recovery was studied by relating information gathered for each disturbed system to successional age. The boundary between original systems of differing ages and the newly created systems was studied to compare the space-time evolution of therophytic communities. Permanent transects perpendicular to the disturbance boundaries and containing many small plots were sampled during consecutive years. Sampling plots located on both sides of the boundaries were classified into communities, in order to detect the space-time pasture evolution in successive years. Annual conditional probabilities were calculated for transitions between the recognised communities. During succession different strategies were detected in re- sponse to meteorological variations. In pioneer successional stages, substitutions of one community by an- other in the same space seem to be random. However, greater determinism was detected in more mature pastures, where, in addition, communities' abundance does not respond to meteorological change. * Nomenclature follows T. G. Tutin et al. 1964-1980. Flora Europaea. Cambridge University Press, Cam- bridge. Introduction Vegetation occurring in fluctuating environments presents peculiar successional characteristics. Vari- ous plant responses to such fluctuations are su- perimposed on the trend of temporal change (Dru- ry & Nisbet, 1973; Peco et ai., 1983a). A simple ecosystem such as a therophytic pasture is particu- larly suitable for the study of these phenomena in a Mediterranean environment, as it is characterized by both land management techniques - periodic ploughing and controlled fires - and imerannual meteorological fluctuations. Therophytes show clear, rapid responses to such phenomena (Peco et a[., 1983a, b). Several significant questions may be posed con- cerning these changes: I. Is it possible to recognize regular vegetational succession when fluctuating environmental con- ditions are superimposed?