Survival of Ulex europaeus seeds in the soil at three sites in New Zealand

Abstract

Seed banks are a major factor in the persistence of perennial, woody, leguminous weed species. The longevity of seeds of Ulex europaeus, (gorse) in the soil at three sites was determined, and the implications of these results for the selection of biological control agents for the weed are discussed. Locally collected seeds were buried in bags at depths ranging from 2.5 cm to 15 cm at three climatically distinct sites. Seeds were recovered and germinated over a 10-year period to estimate the dormancy characteristics and viability of U. europaeus seed banks at the three sites. The decline in the seed bank conformed to a linear model using an exponential transformation. Seeds survived longer when buried deeper, but this influence was weak. The main determinant of seed survival was the time elapsed since burial. Results indicate that the dynamics of the seed bank vary from site to site. Thus, the view that U. europaeus seeds last almost indefinitely in the seed bank may be exaggerated, at least in some sites. At two sites, the number of viable seeds buried at a depth of 5 cm declined to 10% of the original number within 10 years of burial and to 1% within 20 years. By contrast, data from the third site suggested that in some places seed could survive many decades in the seed bank. Almost all seeds recovered from the soil were viable, and losses from the seed bank were probably due to germination. Local climatic conditions or local genotype may explain the variation in seed longevity between sites, although this study could not examine the relative importance of these factors. Seed-feeding biological control agents reduce U. europaeus seed rain, and these may influence the population dynamics of the weed within several decades in areas where the rate of seed decline in the soil is rapid.