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Studies in the Ecology of Wicken Fen: III. The Establishment and Development of Fen Scrub (Carr)

H. Godwin
Journal of Ecology
Vol. 24, No. 1 (Feb., 1936), pp. 82-116
DOI: 10.2307/2256270
Stable URL: http://www.jstor.org/stable/2256270
Page Count: 37
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Studies in the Ecology of Wicken Fen: III. The Establishment and Development of Fen Scrub (Carr)
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Abstract

The processes of bush establishment in Molinieta and Cladio-molinieta on Wicken Fen are discussed in the light of changes in a permanent belt transect mapped in 1923, 1927, 1931 and 1935, and of numerical analyses of cleared areas of young carr. The most important colonising bush species are Rhamnus frangula, R. catharticus, Salix cinerea and Viburnum opulus, of which the first named is by far the most abundant. An experimental and observational analysis of the factors responsible for the preponderance of seedling Rhamnus frangula over R. catharticus suggests that it is not concerned with differential seed production, germination or effects of soil acidity. The drupes of both species are freely dispersed by birds, but large numbers fall directly to the ground: the stones of both species are eaten by field mice which may in some cases also be dispersal agents, but no differential effect in dispersal seems to be responsible for the great relative abundance of R. frangula seedlings. This is at present attributable to the very great preponderance of R. frangula seed parents, but there is no evidence to show how this condition itself arose. Early stages of carr (scrub) development are recorded in mappings made in 1923-4, 1929 and 1934 of a reserved uncut area of fen. There is clear evidence of the very rapid extension of bush crowns, especially of R. catharticus, with a corresponding killing out of Salix cinerea. Viburnum opulus especially expands by fanning outwards of the main trunks, which bend over and may take root where they reach the ground and produce new erect shoots. It expands similarly when young shoots become prostrate below the shade of larger bushes. Later stages of development of Rhamnus frangula carr have been investigated by permanent transect in the zone of transition to older R. catharticus carr, and some evidence is given to show that R. frangula may eventually all die out and be replaced by carr dominated by R. catharticus. The cause of this displacement seems not to be a direct effect of competition but of widespread fungal die-back of mature bushes of R. frangula. Specific isolation and inoculation experiments indicate that Nectria cinnabarina and a Fusarium species are the fungi responsible for this important seral change. Finally a scheme is developed to show the very great influence upon carr development which the initial chance of dense or sparse colonisation may have.

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