THE DECLINE OF HEATHLAND SEED POPULATIONS FOLLOWING THE CONVERSION TOAGRICULTURE

1, The composition and size of the germinable seed populations of undi sturbed heathland soils were compared to those of adjacent farmland wh ich was formerly heathland. The distribution of seeds with soil depth was also examined under the different land use types. 2. In heathland soil, the seed populations of heathland species were very large (17 50 0-33 700 seed m(-2)) and were concentrated in the litter and top 40 mm of the undisturbed soil profile. Repeated ploughing was found to redi stribute the seed population throughout the soil profile. 3. Heathland seed populations exhibited an exponential decay with time under more intensive agricultural management, with an estimated half-life of 10 y ears. This was compared to half-life estimates of 13 years under nearb y conifer plantations. It was thought that the natural decay of heathl and seed populations with time was accelerated by cultivation. Each pl oughing event brings buried seed to the surface where it germinates an d is lost. Despite this, heathland seed populations were found to surv ive for at least 30-40 years under permanent pasture with occasional p eriods of arable farming. 4, The redistribution and loss of heathland seed was accompanied by a large increase in the size of the non-heathl and seed population. This, together with the increased fertility of fa rmland soils, makes the restoration of heathland vegetation from relic heathland seed populations problematical and uncertain.