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Increments in weed seed size track global range expansion and contribute to colonization in a non-native region

Assessing global variation in phenotypic traits and linking that variation to colonization and range expansion is notably rare in invasion biology. Here, we studied variation in seed size in Centaurea solstitialis, a weed with worldwide distribution. Additionally, we explored how seed size variation affects seedling survival of C. solstitialis under favorable precipitation conditions in Anatolia, an ancestral region, and unfavorable precipitation conditions in Argentina, a non-native region. To that end, we conducted seed collections following dispersal pathways of C. solstitialis in ancestral, expanded, and non-native ranges. Locally, collections followed elevation gradients. Also, we performed a greenhouse experiment with C. solstitialis populations varying in seed size and water additions simulating precipitation patterns in Anatolia and Argentina. Seeds from ancestral populations at low elevation were smaller than those from the rest of study populations. Also, seed size in populations at high elevation in the expanded range, the main source of non-native populations, was similar to that in all, but one non-native population, where seeds exhibited further increase. Increments in seed size thus track range expansion in C. solstitialis. Locally, seed size increased with elevation in all three ranges, suggesting convergent responses to that gradient. Seedlings from larger seeds displayed greater survival than those from smaller seeds only under Argentinean conditions. Consequently, populations with large seeds may have been instrumental for colonizing that non-native region. Our findings suggest that ancient and recent dispersal of large-seeded populations contribute to explain the reported global pattern of seed size divergence and worldwide distribution of C. solstitialis.

Natural weed seed burial: effect of soil texture, rain and seed characteristics

Trials were carried out to investigate natural weed seed burial in a no-till agricultural system. With this aim, 30 important weed species were selected to explore if, and to what extent, seed vertical movement is conditioned by soil texture, cumulative rainfall or by seed characteristics. Without soil tillage, the vertical position of the seeds was only a few millimetres below the surface, but it was dependent on soil texture. Seed burial was much slower (and to lesser depths) in clay soils than in sandy soils. Seeds reached greater depths after 1 year in sandy soil (10% >6 mm) than in clay soils (2% >6 mm). Burial dynamics were also influenced by rainfall and seed weight. The amount of rain necessary to halve weed seeds observable on the soil surface was a function of the soil texture and seed weight; linear regressions between total rainfall amounts and seed weight showed that the slope of the relationship increased as the sand component of the soil decreased (21.1, 35.4 and 39.4 in sandy, loam and silty-clay soil, respectively). A polynomial regression, carried out in sandy soil, between 1000-seed weight and their burial capacity showed an inverse relation ( P >0.05) between these two parameters. Seed shape and coat microsculpture also influenced movement, but only minimally.