Factors Influencing the Spatial and Temporal Distribution of Sphagnum Imbricatum Hornsch. Ex Russ. in the British Isles

Abstract

S. imbricatum occurs in abundance as sub-fossil remains in peat mires throughout the British Isles yet today is a very rare species and generally confined to hummocks which are the driest parts of the bogs in which it occurs. This habitat is seemingly incompatible with its frequent Post-glacial occurrence in large expanses in peat strata apparently derived from wet Sphagnum ''lawn'' communities, and also with the usual explanation of the species'' disappearance as being due to the drying out of mire surfaces. Experiments confirm great phenotypic plasticity in the species, especially in relation to habitat water levels, and measurements show good correlation between the degree of ecad variation and differences in this habitat factor. Regression curves for the relationship enable estimates of the water level conditions of growth of the species in the past to be made from measurements of sub-fossil material. These indicate that the plants which formed the great amounts of Postglacial peat were probably semi-aquatic ecads and explain the apparent anomaly between the plant''s past and present habitat preferences. The experimental evidence, however, indicates that falling water-levels alone still cannot fully account for the species'' past abundance and present rarity. Even under conditions of high water level S. imbricatum grows much more slowly than competing sphagna, and the exact conditions under which it thrived in the past remain an enigma.