Changes During 3 Years in the Size and Composition of the Seed Bank Beneath a Long-Term Pasture as Influenced by Defoliation and Fertilizer Regime

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Abstract
A 3-yr study was conducted of the changes in the size and composition of the seed bank beneath an Agrostis-Festuca pasture, resulting from regular or infrequent defoliation and moderate fertilizer applications. The initial botanical composition of the plots and the depth distribution of seeds in the soil were also assessed. More than 12,000 seeds m-2 occurred in the uppermost 15 cm of soil at the outset in April 1978. Thirty-three species were represented; the main constituents were Juncus spp. (mainly J. inflexus), Agrostis capillaris, Trifolium repens, Poa trivialis, Deschampsia caespitosa and Ranunculus repens. Generally, the contributions of species to the sward and to the seed bank were only weakly correlated. Juncus spp. were not present in the sward and R. repens was infrequent. Lolium perenne ssp. perenne and Festuca rubra were much more frequent in the sward than as seeds in the soil. A. capillaris was abundant in the sward and in the seed bank. Seeds were present throughout the uppermost 24 cm of soil. About 30-40% of grass seeds occurred in the top 2 cm of soil, 13% in the next 2 cm and there was then a gradual decline with increasing soil depth. In contrast, a relatively smaller proportion of seeds of dicotyledonous species and Juncus spp. occurred in the uppermost 2 cm of soil. Seeds of Juncus did not decline systematically with depth in the uppermost 12 cm of soil. When the input of fresh seed was prevented, grasses declined at 27% per annum but dicotyledonous species at 16% per annum. Many grass species, which made only a small contribution to the seed bank initially, virtually disappeared during the 3 yr of the experiment. When plots were not cut until late June 1978, a large shed of seed (mainly by Cerastium fontanum ssp. glabrescens doubled the size of the seed bank but only transiently. However, about 20% of A. capillaris seeds shed in late summer 1979 became incorporated more permanently. Evidently, in dense undisturbed swards, the chances of seeds of most species becoming incorporated into a permanent seed bank are low. The relative size of the initial seed bank and the seed return in any one season suggested a long life-span for the main component species. The relationship between the seed bank and the flora suggested that the former could be instrumental in changing the latter, should the sward be disturbed sufficiently. Because of the different depth distribution of different components, depth of soil disturbance could also influence the nature of botanical changes. Further work is clearly needed to ascertain the functional significance of seeds beneath permanent pastures under conditions of greater disturbance than in the present investigation.

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