Studies on the feasibility of re-creating chalk grassland vegetation on ex-arable land. I. The potential roles of the seed bank and the seed rain.
The potential of the seed bank and the seed rain to promote the re-establishment of chalk grassland vegetation was investigated on an ex-arable site which had not been cultivated for 10 years. Comparisons are drawn with the composition of the seed bank as recorded in a study undertaken close to the current site 6 years after cultivation ceased. The seed bank comprised 46.6% grass seeds, 38.6% perennial forbs, 8.4% biennial forbs and 6.3% annual forbs. In comparison, annual forbs had accounted for 49.5% of the seed bank 6 years after cultivation ceased. The seed bank was concentrated near the top of the soil profile and grass seeds showed a more marked decline in abundance with depth than forb species. However, common annual forb species mostly germinated from the lower soil strata. The common grasses and perennial forbs were species with wide ecological amplitudes, characteristic of fertilized, neutral grassland. Only 20 of the 68 forb species recorded in the seed bank were characteristic components of adjacent ancient chalk grassland. These species accounted for less than 20% of the total forb seed bank. Only two out of 11 recorded grass species were characteristic of the ancient chalk grassland, and these accounted for only 0.3% of all grass seedlings. The grass component of the seed bank was dominated by Agrostis stolonifera. The species richness of the seed bank has increased in recent years due mainly to acquisition of seeds of non-annuals. However, species characteristic of the ancient chalk grassland have made little contribution either to the seed bank or to the vegetation growing on the site. Those chalk grassland species which were most abundant in the seed bank tended to be short-lived species and they occurred mainly at the margins of the ex-arable site, close to the adjacent chalk grassland. Even here they rarely accounted for more than 20% of the germinable seed bank. They were strongly concentrated at the soil surface, indicating their deposition since cultivation ceased. Agrostis spp., Phleum pratense and Holcus lanatus accounted for over 50% of the recorded seed rain. Of the commonly trapped species, analyses of mean dispersal breadths indicated that forb species characteristic of the adjacent chalk grassland would be comparatively slow invaders of ex-arable habitats. The vegetation on transects across the ex-arable site contained few of the species which occurred in the adjacent old chalk grassland. Chalk grassland species were more abundant in vegetation at the margins of the ex-arable site, but even here similarity indices between the ex-arable vegetation and the chalk grassland vegetation were normally below 25%. The slow invasion of species from the adjacent chalk grassland into this ex-arable site, which is ideally placed for their colonization, suggests that seeds of such species will often need to be artificially introduced to prevent ex-arable sites becoming dominated by fast-growing more weedy species. Further management would also be necessary to prevent more weedy species subsequently invading and eliminating the chalk grassland species.