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Bracken Versus Heather, A Study in Plant Sociology

Alex S. Watt
Journal of Ecology
Vol. 43, No. 2 (Jul., 1955), pp. 490-506
DOI: 10.2307/2257009
Stable URL: http://www.jstor.org/stable/2257009
Page Count: 19
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Bracken Versus Heather, A Study in Plant Sociology
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Abstract

The spatial and temporal relations between Calluna vulgaris and Pteridium aquilinum on a podsolized sand in an area liable to much frost have been examined. The area bears Pteridietum flanked on one side by Callunetum without bracken and on the other by Callunetum with sparse bracken. The relations between the Pteridietum and its flanking Calluneta along their lines of contact are the subject of particular inquiry. On the one side bracken moves into and replaces the Callunetum killing it out by the wave of advance. Bracken is able to do so because of its marginal vigour and the massing of the fronds at their most competitive along a continuous front. But it is not definitely established that this can happen in the absence of grazing by rabbits. On the other flank there is a fairly abrupt change from dominant bracken to dominant heather (with some bracken) without a corresponding change in soil or microclimate. Several lines of evidence show that Calluna is advancing and replacing the bracken. On removing the advancing heather the bracken comes back. The structure of the Callunetum has been described in terms of four phases, pioneer, building, mature, degenerate, as was the Pteridietum with an additional grass-heath phase (Watt, 1947a). Examination of the relations between bracken and heather in the Callunetum show that the bracken is over-dispersed, that this over-dispersion manifests itself in the greater number of fronds in the pioneer and degenerate phases and a smaller number in the building and mature, than if at random, and that there is phasic interdigitation of the two dominants, that is, that the two are in phase. Finally, the interpretation of the phenomena is based upon knowledge of the life history of the plants and of the structure of the communities they form.

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