Occupation of Biological Space During Seedling Establishment

Abstract

gains access to resources necessary for its further growth and development. The term 'biological space' is used to cover this complex of resources. Such a simplification assumes that the requirements of individuals within a species are essentially similar. The object of this paper is to show the importance of the position of an individual in the overall ranking of emergence times, and its spatial separation from neighbours, in enabling it to occupy space to the exclusion of its neighbours. Most information on the performance of an individual within a population has come from work with mixed populations and this literature warrants consideration. A number of studies have been made on the temporal and spatial components of seedling interference, but the interaction of the two, interference in the dynamic context, has received less attention. The relative times at which different species are introduced into a mixture has been shown by a number of authors to produce dramatic changes in the outcome of interference between the components. When Sagar (1959) sowed S23 Lolium perenne and Plantago lanceolata simultaneously, Lolium contributed 80 % of the total plant dry weight at harvest. If Lolium were introduced 3 weeks before Plantago, the proportion contributed by Lolium rose to 90 %, but, sown 3 weeks after Plantago, Lolium contributed only 6.4 % of the total dry weight. A similar result was reported by Harper (1961) using two Bromus species. When B. rigidus and B. madritensis were sown simultaneously, the former contributed over 75 % of the final dry weight of the mixture. If the introduction of B. rigidus were delayed for 3 weeks, then the contribution by this species dropped to approximately 13 % of the total dry weight. Williams (1962) has repeated this effect in an agronomic context. He found that Chenopodium album reduced the yield of barley by 20 0 when the two were sown at the same time, but delaying barley sowing by 7 weeks allowed Chenopodium to reduce the yield of the barley by 45 %. The same effect with one species was shown by Black & Wilkinson (1963) using subterranean clover. They delayed planting alternate seeds within a grid pattern by 0, 2, 4 and 8 days, thus considerably increasing the range of emergence times within swards. By recording the final yield of each individual, they showed that a 5-day delay in the emergence of an individual reduced its final weight by 50 %, and a delay of 8 or 9 days, by at least 75 %. These differences could not be accounted for solely by differences in growing times between emergence and harvest. As early as 1938, Weaver & Clements described how an advantage gained by an individual over its neighbours was likely to be maintained or accentuated during subsequent