No abstract

European gorse (Ulex europaeusL.) is a spiny shrub that grows spontaneously in the understorey of forests and heathlands in western Europe. Gorse is a pioneer species and forms large seed banks that can persist for a long time while buried deeply in the soil. Although many studies have been conducted on gorse seed banks in invasive contexts and in scrubland ecosystems, few data are available on forests in a native context. The aim of the present study was thus to report on the variability of seed-bank density in ‘critical’ stages in the forest management of pine stands (five stands) in south-western France. We examined variations in the number of gorse seeds as a function of soil depth but also of the presence and abundance of adult gorse in the understorey. Seed-bank density did not show a clear decrease in seed number with pine stand age, principally because gorse also appears to be able to establish itself in mature pine stands, probably thanks to local disturbances. In the pine stands in our study, the presence and abundance of seeds in the soil appeared to depend mostly on the presence of adult gorse as seeders in the understorey. Finally, we observed that, contrary to what has generally been found in scrubland ecosystems, most gorse seeds were located in the 5–10 cm soil layer rather than in the 0–5 cm soil layer. This depletion of the first 5 cm may be linked to seed germination that was not compensated for by the production of new seeds.

Background and aims-Large, persistent seed banks contribute to the invasiveness of non-native plants, and maternal plant size is an important contributory factor. We explored the relationships between plant vegetative size (V) and soil seed bank size (S) for the invasive shrub (Ulex europaeus) in its native range and in non-native populations, and identified which other factors may contribute to seed bank variation between native and invaded regions.  Methods-We compared the native region (France) with two regions where Ulex is invasive, one with seed predators introduced for biological control (New Zealand) and another where seed predators are absent (La Réunion). We quantified seed bank size, plant dimensions, seed predation, and soil fertility for six stands in each of the three regions.  Key results-Seed banks were 9 to 14 times larger in the two invaded regions compared to native France. We found a positive relationship between current seed bank size and actual plant size, and that any deviation from this relationship was probably due to large differences in seed predation and/or soil fertility. We further identified three possible factors explaining larger seed banks in non-native environments: larger maternal plant size, lower activity of seed predators and higher soil fertility.  Conclusions-In highlighting a positive relationship between maternal plant size and seed bank size, and identifying additional factors that regulate soil seed bank dynamics in non-native ranges, our data offer a number of opportunities for invasive weed control. For non-native Ulex populations specifically, management focusing on 'S' (i.e. the reduction of the seed bank by stimulating germination, or the introduction of seed predators as biological control agents), and/or 'V' (i.e. by cutting mature stands to reduce maternal plant biomass) offers the most probable combination of effective control options.