HUMUS FORM DEVELOPMENT AND SUCCESSION OF DWARF SHRUB VEGETATION IN GRASS DOMINATED PRIMARY PINUS-SYLVESTRIS FORESTS

Because of its role in nutrient and water supply to plants, the humus form is strongly interrelated with the vegetation and is likely to at least partly control vegetation succession. Causal relations between vegetation succession and soil development are generally difficult to study because of the multifactorial nature of ecology. Primary succession of Pinus sylvestris forests on recent inland dunes provides interesting information on succession related humus form development. Humus form characteristics and vegetation composition in these drift sand areas were studied along an age series of Scots pine stands, ranging from 15 to 124 years old, all situated on similar parent materials and lacking antecedent soil formation. From the information thus obtained, successional processes were deduced and the following conclusions were drawn: 1) Chemical variability in the ectorganic profiles under the Scots pine forests concerns a downward decrease of plant available elements as well as allelochemicals, which both may have implications for reproduction and competitive ability of undergrowth species involved in the succession. 2) The concentration of Deschampsia flexuosa roots in the F horizon indicates a pronounced niche differentiation, which enables this species to maintain dominance in the herb layer for a long period of time during succession in pine forests. 3) Facilitation in the succession of the undergrowth of the pine forests seems to emanate from horizon differentiation and associated chemical variability. Dwarf shrubs take advantage of the development of an H horizon, leading to a botanically more variable undergrowth vegetation in older pine forests.