The interpretation of some post-Glacial vegetation diversities at different Lake District sites

Abstract

The lake sediments which have been described by Mr Mackereth contain pollen and other plant remains which record the broad outlines of vegetational history. Many pollen diagrams are now available from the one ecologically homogeneous area of the central Lake District; their similarities record the effects of a single type of climate on an area of uniformly ancient rock, while their diversities record the individual history of each drainage basin. The large river valley lakes such as Windermere represent an integration of the many small drainage basins of which the river systems are made up; a simpler ecological picture is presented by the small lakes or tarns. Mackereth’s hypothesis, based on purely chemical evidence, that the lake sediments are derived from a series of soils washed in from the drainage basins, is supported by all the pollen evidence. Two deductions are made from this accepted assumption—first, that radiocarbon assay of the lake muds would be of comparatively little value, since the organic matter in any sample may have originated from a soil and be much older than its date of incorporation in the lake mud, and secondly that the presence of derived pollen from soils must be considered in interpretation of the pollen diagrams. The outline of the ecological history of the Lake District attempted here must therefore be built up by synthesis and comparison, but, as yet, in the absence of radiocarbon dates. The advantages of lake sediments for post-Glacial investigation—to set against their limitations—are that there is no overwhelming local pollen component as in fen or bog peats, and that stratigraphical changes in the sediments provide evidence for major changes in erosion rate, which can be correlated with vegetation changes shown in the pollen diagrams—as can chemical changes related to soil changes in the drainage basins.