ASSESSING WHY 2 INTRODUCED CONYZA DIFFER IN THEIR ABILITY TO INVADE MEDITERRANEAN OLD FIELDS

Researchers have suggested that species-community interactions determi ne invasion success. Therefore, it is likely that small biological dif ferences between species interact with habitat characteristics to prod uce distinct patterns of distribution and abundance throughout a new r ange. In this study we test the hypothesis that differences in the dis tribution and abundance of species sharing an identical set of ''ideal weed characteristics'' are explicable in terms of species-specific re sponses to environmental variation within their new range. Using multi factor experiments, we investigated some of the ecological interaction s influencing reproductive success in two very closely related species of annuals having invaded the French Mediterranean region for >150 yr and showing marked differences in their local distribution and abunda nce patterns. We transplanted seedlings of Conyza canadensis (a specie s restricted to recently disturbed areas) and C. sumatrensis (a specie s colonizing early- to mid-successional old fields) at equal densities in three contrasting old fields (6 mo, 4 yr, 17-yr abandonment, respe ctively) during 1991-1992, a growing season with average rainfall. Ind ividual performances (measured as survivorship, reproductive timing, a nd reproductive output) were evaluated with respect to: (1) competitio n with plant neighbors (tested with a weeding treatment), (2) resource availability (tested with nutrient and water addition), and (3) herbi vory (tested with chemical limitation). Manipulated factors interacted in a rather complex fashion to influence survivorship and reproductio n in both species. However, patterns of relative performance were cons istent with relative distribution patterns across Mediterranean landsc apes: C. sumatrensis performed better than C. canadensis in all fields , including the youngest one (6 mo old). Herbivory only slightly affec ted transplant performances. In contrast, competition with plant neigh bors had substantial effects on either Conyza species and may be the m ost important determinant of performance in Mediterranean old fields. The experiment showed unambiguously that the two species differ marked ly in their competitive ability, with C. sumatrensis performing better than C. canadensis in the presence of neighboring vegetation. In addi tion, C. sumatrensis displayed a superior ability to take up and/or to use water and nutrient resources when they become available in compet itive environments. We argue that potential physiological or anatomica l species differences responsible for this differential susceptibility to local resource reduction by neighbors could involve differences in constructional organization, leaf morphology, and reproductive phenol ogies.