The decline of heathland seed populations following the conversion to agriculture.
The composition and size of the germinable seed populations of undisturbed heathland soils in SE Dorset were compared to those of adjacent farmland which was formerly heathland. The distribution of seeds with soil depth was also examined under the different land use types. In heathland soil, the seed populations of heathland species were very large (17 500-33 700 seeds m-2) and were concentrated in the litter and top 40 mm of the undisturbed soil profile. Repeated ploughing was found to redistribute the seed population throughout the soil profile. Heathland seed populations exhibited an exponential decay with time under more intensive agricultural management, with an estimated half-life of 10 years. This was compared to half-life estimates of 13 years under nearby conifer plantations. It was thought that the natural decay of heathland seed populations with time was accelerated by cultivation. Each ploughing event brings buried seed to the surface where it germinates and is lost. Despite this, heathland seed populations were found to survive for at least 30-40 years under permanent pasture with occasional periods of arable farming. The redistribution and loss of heathland seed was accompanied by a large increase in the size of the non-heathland seed population. This, together with the increased fertility of farmland soils, makes the restoration of heathland vegetation from relic heathland seed populations problematical and uncertain.