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Studies in the Ecology of Breckland

A. S. Watt
Journal of Ecology
Vol. 28, No. 1 (Feb., 1940), pp. 42-70
DOI: 10.2307/2256162
Stable URL: http://www.jstor.org/stable/2256162
Page Count: 32
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Studies in the Ecology of Breckland
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Abstract

The so-called "grass-heath" of Breckland shows much variation in its floristic composition and physiognomy. This variation is causally related primarily to the soil and secondarily to other factors (e.g. grazing by rabbits) still under investigation. The soil is derived from the chalky boulder clay which by progressive decalcification and podsolization shows a series of stages in soil profile development from highly calcareous shallow immature soils to a deep well-marked podsol. Seven stages are recognized and the profile in each described and figured. From each horizon (or at successive depths in the profile) data of the acidity and the content in calcium carbonate, carbon and sesquioxides are graphically presented. From composite surface (0-15 cm.) samples from each type data have been obtained (and tabulated) of the exchangeable calcium, total exchangeable bases, acidity, carbon and nitrogen contents. Tests of acidity on composite surface (0-4 cm.) samples from each plot in each type have been made and the data graphed. The data show that we are dealing with a series in the genesis of a podsol: the increase in soil depth and in acidity, the decrease in calcium carbonate and in base status as well as in the change in the kind of humus from mull to mor and the movement of the sesquioxides confirm this interpretation. The acidity of the surface (0-4 cm.) samples suggest two levels of relative stability separated by a stage of rapid change. The moisture relations in this series of profiles must also vary but no determinations have been made. The first five stages in the series are separated by significant differences in the quantitative estimates of the factors: the last two are essentially the same as the fifth. On the basis of their ecological requirements the species are classed into four groups, calcicole, exacting, calcifuge and tolerant. When arranged in arrays the serial change in the flora from soil stage to soil stage is at once obvious: the calcicoles are confined to the first two stages which contain calcium carbonate and are alkaline, the exacting species have a wider range generally covering the first four: the calcifuges make their appearance in the third stage when the soil becomes acid and the tolerants are indifferent being found in all stages. This primary differentiating basis when considered in relation to the total flora of vascular plants and bryophytes brings out the significant fact that the drift in the number of species from the second stage to the last is due mainly to the progressive elimination of species and not to a radical change in the flora. (The lichen flora on the other hand shows greater specific variation.) The scarcity of calcifuges on the acid soils is probably due to grazing by rabbits and the dryness of the habitat, and dryness is also probably the primary cause differentiating the communities on the first and the second soil stages. The flora is drought resistant or drought evading: there are very few broad-leaved herbs. Physiognomically the outstanding feature is the high percentage of annuals and the increase in the sociability of Cladonia silvatica from the third stage: in the last three stages C. silvatica is the dominant plant. Using narrow ecological requirements and high constancy and high abundance as sifting criteria we find that the first five communities, corresponding to the first five stages in profile development, are significantly different from each other. Vegetationally the last three are essentially the same. They differ however in minor features which suggest the possibility of there being different phases in a cycle of change, causally related to the development of the lichen carpet and to the periodic climatic changes in a region with a climate transitional between oceanic and continental. The vegetation on each soil type is described and characterized. The data given in the paper afford convincing proof that soil is a primary differentiating factor in the "grass-heath" of Breckland.

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