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Bent Vad Odgaard

The Holocene vegetation history of northern West Jutland

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  • Geologisk Institut
Holocene sediments of three closed Danish lake basins (Solso, Skånso, Kragso) 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, 6°C. 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 I4C 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 modem 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-tolerancellight-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, rates of palynological change suggest that the
interval between 8OOO 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. Redundancy 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 Culluna-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.
BogserieOpera Botanica
Sider (fra-til)1-171
Antal sider171
StatusUdgivet - 1994

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