Résumé
1. During a 2-year study, the spatial and temporal dynamics of clonal growth, flower and fruit production, and production and survivorship of seedlings were compared in two species of Lathyrus in south-western France. Both are perennials that have a `phalanx' growth form, with a high density of closely packed ramets. 2. Transects of contiguous quadrats were established in one population of L. latifolius (at an altitude of 150 m a.s.l.) and in two populations of L. sylvestris (520 m and 750 m a.s.l.). In each transect the number, distribution and survivorship of ramets were measured at monthly intervals, together with their flowering and fruiting activity; the number and distribution of seedlings and seedling survivorship were also measured. 3. Temporal dynamics of ramets in all sites followed the same general pattern. In late autumn and winter, following the summer flowering season, plants produced a new cohort of ramets. This was followed by a very large cohort of new ramets produced in late winter and spring. These ramets grew only vegetatively and flowered very rarely. Flowering and fruiting were largely restricted to smaller cohorts of ramets, produced in late spring and beginning of summer. Mean life expectancy of these cohorts was significantly lower than that of all other cohorts. 4. Temporal dynamics of seedlings were similar to those of ramets, except in the high-altitude site for L. sylvestris, where the maximum number of seedlings appeared near the end of May, about 1 month before the peak in ramet production. Most seedlings survived no longer than 3 months; no surviving seedling in the transects had produced flowers by the end of the study. 5. New ramets showed a clumped distribution at all scales. Most new ramets arose in tufts of other ramets from a single tap-root, or from daughter ramets very close to younger parts of the plant. The distribution of seedlings within transects was similar to that of ramets. In fact, seedlings only appeared in quadrats with tufts of ramets, or in adjacent quadrats. 6. Except in the highest-altitude site, temporal patterns of vegetative growth in both species allowed the opportunistic growth of ramets throughout the year, even in midwinter during bouts of warm weather. Ramets initiated in winter did not flower. Winter ramets had longer life expectancies at birth than cohorts produced at other times of the year. 7. Demographic traits of Lathyrus are discussed in relation to their consequences for genetic structuring observed in populations of these species.