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Journal Article

Topographical Gradients of Chalk Grassland

Franklyn Perring
Journal of Ecology
Vol. 47, No. 2 (Jul., 1959), pp. 447-481
DOI: 10.2307/2257376
Stable URL: http://www.jstor.org/stable/2257376
Page Count: 36
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Topographical Gradients of Chalk Grassland
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Abstract

1. The choice of sites at which to study climatic and edaphic gradients is limited to those where the independent variables can be estimated. 2. Standard methods of soil and vegetation analysis were used except that N/100 solutions of sodium acetate/acetic acid proved to be a more satisfactory extractant in exchangeable phosphate determination on chalk soils. Weak solutions (N/25 ammonium nitrate) were also used in the determination of exchangeable K, Na and Ca. 3. pH is related to slope but not to aspect, nor is it altered by grazing. 4. Total carbonates are related to slope and aspect. Higher carbonate values were recorded on S.W. slopes than on equivalent N.E. ones. Values are very scattered on slopes over 25⚬, probably because of soil instability. 5. Organic carbon is affected by slope, aspect and grazing. Higher values are found on S.W. or ungrazed sites than on N.E. or grazed sites. 6. Exchangeable phosphate is affected by slope and grazing, but not by aspect. Shallow ungrazed sites give the highest values. 7. Exchangeable potassium values are strongly affected by aspect, slope and grazing. The highest values are found on steep, ungrazed S.W. slopes, the lowest on shallow, grazed N.E. slopes. 8. Exchangeable calcium values are highest on northerly slopes between 10⚬ and 18⚬. Values are lower on shallower, partially leached slopes and lower on soils of steeper slopes which are more juvenile. 9. Colour, depth and mechanical analysis values are all correlated with topography. 10. Comparisons between regions in which the dip of the strata is negligible and an area such as the East Riding where it is severe (N.W. to S.E.) indicate that this may be important for pH and the mobile ions potassium and calcium, but not important for organic carbon and an immobile ion such as phosphate. 11. Species are divided into five groups according to their distribution pattern in relation to topography in north Dorset. These range from Group I which includes species confined to steeper slopes of aspects between S.E. and W., to Group V containing those which reach their maximum cover on slopes between N.W. and E. A number of species which do not fall into these primary groups are discussed; there are three types--putative calcicoles, putative calcifuges and species apparently distributed at random. 12. That the main axis of distribution is S.W. to N.E. agrees well with theory. However, though the main centre of the `xerophytic' element is on steep S.W. slopes, the centre of the `mesophytic' element is on shallow N.E. slopes. The probable reason why the centre is not on the slopes of 30⚬ which receive the minimum insolation is that the relative water-holding capacity of such slopes is only half that of those of only 10⚬. 13. The more `xerophytic' element includes most of the species which are characteristic of chalk grassland, whilst the `mesophytic' element is largely composed of species of neutral grassland. 14. The dip of the strata in East Riding appears to affect the distribution of certain species, particularly putative `calcicoles' and `calcifuges'. The majority of species, however, are still distributed in relation to a S.W. to N.E. axis. Soil water movement may redistribute mobile ions during the winter but does not disturb the humidity regime in summer when the distribution of most species is clearly related to the insolation received by different aspects.

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