EARLY PLANT SUCCESSION ON FORMER ARABLE LAND

Since 1988, the removal of land from arable production under the set-a side scheme has formed a significant part of EU agricultural policy pr imarily aimed at reducing food surpluses. Set-aside can also offer a n umber of environmental benefits, particularly as a wildlife resource. The rate and type of vegetation development on set-aside will determin e the overall conservation value. Management will often seek to accele rate or decelerate the successional process to produce a particular sp ecies assemblage that is either annual or perennial dominated. This st udy examined models of early succession on an area removed from an exp erimental arable rotation at Silwood Park, UK and considered the inter action between early-colonizing annual species and later-colonizing pe rennial species by using plant removal and addition experiments. Remov al of all annual species had no effect on perennial performance during the two years of the experiment. The removal of perennial species inc reased annual recruitment in the first year, but had no effect in the second. Consequently, at natural densities, there is only a weak net i nteraction between annuals and perennials and they are considered to b e tolerant of one another. Enhancement of annuals by the addition of P oa annua and Capsella bursa-pastoris by seeds to plots delayed perenni al recruitment in the first year of the experiment, but had no effect on perennial performance in the second. Facilitation by early-colonize rs as a mechanism of species replacement was thus discounted. Perennia l establishment was significantly increased in both years of the exper iment following addition of Holcus lanatus and Trifolium repens by see d. Exclusion of insect herbivores by chemical insecticides did not alt er the underlying tolerance-based successional mechanism. With regard to set-aside management for conservation purposes, the experiment conf irmed that planting later-colonizing species will accelerate successio n, but increasing the abundance of annuals will not retard succession. (C) 1998 Elsevier Science B.V. All rights reserved.