Allelopathy Among Some British Grassland Species

Abstract

Allelopathy, the influence of one plant on another by means of chemical substances, has been extensively studied, and there is now strong evidence that this type of interaction does occur (Whittaker & Feeny 1971; U.S. National Committee for the International Biological Programme 1971; Grodzinsky 1973). Most often investigation of a particular species for possible allelopathic activity has started because of field observations; the species may tend to be surrounded by a bare zone, to have few other species growing beneath it, or in some other way shows an ability to suppress other species which is not expected from its size, density of foliage and other morphological characters. However, the possibility should also be considered that there are other allelopathic interactions where the effects are less extreme, and would not therefore be detected by preliminary field observations, but are nevertheless large enough to alter the balance between species. There are several published reports of species being found to contain toxic materials, when field observations on the species suggest no special toxicity (Welbank 1963; Grant & Sallans 1964; del Moral & Cates 1971). Del Moral & Cates (1971) tested forty species and found toxic materials in every one. All of these studies, however, concerned substances which were extracted from chopped-up plants or which were exuded by dying plant parts; none of them demonstrated the exudation of toxic substances from intact, healthy plants. In many studies of allelopathy the test plants to which the extracts or exudates are applied have been chosen for their convenience rather than their ecological relevance. For example, in the study by del Moral & Cates (1971), the plant volatiles or extracts were applied to seedlings of barley, Bromus tectorum L. and Pseudotsuga menziesii (Mirb.) Franco. Of these three only P. menziesii is native to Washington State, the area from which the donor species originated. The argument implicit in such a choice of convenient species is that all species will respond to the exudates in the same way. This is a very basic assumption and merits investigation. In the research described here, eight species were chosen which give no particular indication, from field observations, that they are involved in allelopathic interactions. The species, four grasses and four dicotyledons, were chosen because they are common in permanent neutral grassland in Britain, and all eight can be found growing together. In order to test whether the exudates have different effects on different species, exudates from each of the eight species were applied to each of the eight, in a factorially arranged experiment. The experiment aimed to study the effects of root exudates from plants young enough to have few dying roots. Leaching from above-ground parts was minimized. There has been little previous work on allelopathy among British grassland species.