DEMOGRAPHY OF THE ARABLE WEED DIPLOTAXIS ERUCOIDES IN CENTRAL CATALONIA, SPAIN

The demographic performance of a Mediterranean annual arable weed (Dip lotaxis erucoides) related to both environmental conditions and distur bance was studied in an agricultural system disturbed by ploughing at various times in the year. Comparative analysis of survival curves rev eals that mortality rates are dependent on a variety of environmental conditions, namely competition with neighbouring plants, frost, and av ailability of water resources. The demographic response of D. erucoide s may vary depending upon how the unfavourableness of the environment compares with its physiological tolerance. Under environmental conditi ons not exceeding its physiological tolerance, D. erucoides reacts thr ough phenotypic plasticity. However, in arable fields where periodic m echanical disturbance removed all individuals before reproduction, or unfavourable environmental conditions such as frost and drought exceed ed the physiological tolerance of D. erucoides, widespread mortality o ccurred. Persistence then depended on high fecundity, low emergence ra tes, and high year-to-year seed survival. The results of our study sho w that different cultivation treatments will affect the persistence of D. erucoides. Long-term high-disturbance regimes could produce a prog ressive decrease in numbers due to drastic reduction, even the absence , of new fresh seed shed onto the field and consequently a progressive depletion of the seed bank. Similarly, low-disturbance regimes or abs ence of disturbance could produce a rapid decrease in numbers related to an increase of associated species and the low competitive ability o f D. erucoides.