Estimation of the Seed Bank of a Chalk Grassland Ley Established on Former Arable Land

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Abstract
(1) The size, composition and seasonal and depth-related variations in the soil seed bank of a former chalk grassland ley were determined at monthly intervals from March 1981 to October 1982. (2) The number of species germinating showed little seasonal fluctuation. Fewer grass and perennial forb species germinated from soil deep in the profile than from soil near the surface. Equal numbers of annual and perennial species germinated from a depth of 0-4 cm, but more annual than perennial species germinated from soil sampled lower in the profile. (3) Numbers of seeds germinating peaked in soils samples collected in mid-summer to winter, after seed dispersal; 83% of grass seedlings and 52% of forb seedlings emerged from 0-4 cm. The corresponding figures for annual and pernnial species were 38% and 67%. (4) Mean percentage emergence of grasses and perennial forbs declined significantly with depth, reflecting a lack of long-term viability combined with a lack of soil inversion in recent years. Biennial (= short-lived perennial) forbs behaved similarly. Mean percentage germination of annual forbs did not alter significantly with depth, reflecting incorporation during previous annual cultivation cycles and considerable longevity. Poa annua showed a depth-related germination pattern which was consistent with that of perennial grasses rather than annual forbs. (5) The seed bank contained many annual weed seeds, but very few seeds of species characteristic of chalk grassland. These seeds were all of minority components of chalk grassland vegetation. (6) Management aimed at re-establishing chalk grassland vegetation should avoid soil disturbance, which would release many annual weed seeds of arable cultivation from enforced dormancy in the seed bank. Growth of such weeds may be enhanced following soil improvement during the years of cultivation, leaving little opportunity for less competitive chalk grassland perennial species with small seed banks to become established.