Changes in hill vegetation revealed by selected British soils

Abstract

INTRODUCTION Plant pollen in hill peat is preserved for long periods by the chemical environment, and it reveals the flora they carried. Hill soils include peaty gleyed podzols which also have acidic and preservative environments. Twelve inches of peat overlie a 6 inch thick, grey eluvial (A2)* horizon, leached of iron, over a thin, undulating iron pan on an ochreous illuvial (B2ir) horizon ,. Pollen analysis of these soils in South Wales has given a useful floral sequence,. The distribution of principal pollen species shows that grass (Gramineae) predominates in surface layers, reflecting present vegetation. Below, an increasing proportion of ericaceous (chiefly heather) pollen comes from a recent heathland. Lower down, Pteridophyte spores, chiefly Polypodium, increase. In the deepest soil layers oak (Quercus) pollen is abundant, and in some ericaceous pollen from an earlier heathland. This floral sequence is related to the archaeological time-scale using pollen analyses at archaeological sites,. The ericaceous maximum corresponds to Medieval times; the Polypodium maximum with early Roman and Iron Age times; the oak maximum with the Middle Bronze Age and the lower ericaceous maximum with Neolithic times. Before Neolithic times most soils in South Wales were insufficiently acid to preserve pollen, and there is no record of Mesolithic floral changes. In contrast, peat bogs record a longer floral history, but they are less representative of any particular terrain. Aspect and altitude influence the floral succession. Cold northeast facing slopes favour Ericaceae and birch (Betula) pollen production. Whereas Polypodium was very extensive across the S. Wales uplands, it was much less common at the highest and lowest altitudes. Oak predominates in lowland sites, where grass, heather and other non-tree pollens are uncommon. Hence, in a study of pollen in widely separated British soils it is advisable to keep the sampled environment as constant as possible, so that any differences in the floral sequence from place to place may be attributed chiefly to changes in latitude and distance from the sea. Local accumulations of grains over a pan, over flagstones occurring naturally, or in an archaeological structure, and the consistent correlation of the floral sequence with the archaeological time-scale, confirm that pollen does move down through a soil. Most pollen and spores are about the size of silt/fine sand particles (20-50(x International and U.S.D.A. scales, respectively), the specific gravity is near 1 and they should be easily moved by water flowing through soil pores. Soil particles of the same size are more dense and yet are readily moved in this way. Movement of pollen over the historical time-scale of 1-4,000 years is