The Holocene vegetation history of northern West Jutland, Denmark

Abstract

Holocene sediments of three closed Danish lake basins (Solsø, Skånsø, Kragsø) were used for the inference of post‐glacial vegetational dynamics in former heathland areas in northern West Jutland, Denmark. The sites were selected to represent the major geomorphological units of West Jutland. The Holocene history of each lake basin was investigated by mapping of sediment distribution, analysis of loss‐on‐ignition, coarse inorganic matter, humus content, mineral magnetics, δ13C, pollen and selected other microfossils. These techniques were supplemented by plant macrofossil analysis at one site. Holocene terrestrial vegetational development was inferred at each site from analyses of pollen and microscopical charred particles. Chronologies were provided by numerous 14C dates. Stratigraphies of wet ground and terrestrial pollen and spore types were zooned by stratigraphically constrained cluster analysis. Based on the resultant site pollen asemblage zones (site PAZ), regional PAZ were proposed. Using modern analogues, Holocene floristic richness was estimated from pollen richness in the microfossil assemblages. The results support the hypothesis that disturbance is one of the most important mechanisms behind the maintenance of floristic richness. In particular, the response of estimated floristic richness to the intensity of vegetational fires followed the predictions of the Intermediate Disturbance Hypothesis. A period of elevated palynological richness and inferred vegetational disturbance was identified at all sites between 6000 and 5200 BC (calendar years). Using correspondence analysis (CA), the major gradient in the terrestrial pollen sequences was identified as a light‐shade gradient, and CA first axis sample scores were used as a supplement to standard AP/NAP pollen ratios as an indicator of the shade‐tolerance/light‐demand of Holocene terrestrial plant communities. In spite of different vegetational developments since 4000 BC, the timing of major changes towards more light‐demanding vegetation types were broadly synchronous at the three sites. Using chord distance as a dissimilarity index, rales of palynological change suggest that the interval between 8000 and 7500 BC (calendar years) was the period of most rapid vegetational change during the Holocene, both in terrestrial as well as lacustrine ecosystems. While climatic forcing of the rapid events around 8000 BC is hypothesised, the synchronous timing of relatively rapid inferred change in lake and terrestrial vegetation around AD 600 may reflect changes in climate as well as in land‐use. Reducdancy analysis was used to develop a model between fire intensity (inferred from microscopical charred particles) and vegetational response, as reflected by pollen assemblages. Formulated at one site and tested at the two other sites, the model explains regional Colluna‐heathland expansions as a result of vegetational burning. Similarly, declines in heathland cover are explained by lack of maintenance by fire.Regional vegetational development in northern West Jutland is reconstructed and special consideration is given to heathland history. The Holocene heathland development is interpreted as resulting from its importance for grazing. It is hypothesized that on poor soils, Calluna‐dominated heathland was a better grazing resource than grass‐dominated pasture, due to the winter‐grazing offered by Calluna and the low palatibility of dominant grasses on poor soils. This hypothesis is relevant for the explanation of the variation in timing of heathland expansions on the different soil types represented by the study sites.