Effects of climate change and human activities on a Swedish raised bog

We determined the effects of climate change and human activities on the species composition of ombrotrophic (rain-fed) peat bogs by analyzing pollen and macrofossils. We used 14C AMS wiggle-match dating (1) to obtain a precise chronology. Thermophilous plant species in the microfossil record provide information on climate change (2). Lime (Tilia), Elm (Ulmus), Beech (Fagus), Hornbeam (Carpinus) and Hazel (Corylus) will show a decline or disappear from the pollen record when climate becomes colder. The macrofossil record also provides information about climate, especially about changing effective precipitation. During wet periods the peat moss Sphagnum section cuspidata and White Beak Sedge (Rhynchospora alba) will expand in the bog vegetation, while hummock forming Sphagna and Heather species (Calluna vulgaris and Empetrum nigrum) will dominate during dry periods. Remains of fungi like Cenococcum geophilum also increase during periods of dryness. Local dryness in a bog could also be caused by human activities. Artificial drainage can cause desiccation of the bog surface, and peat growth can even stop and hiatuses may develop. Pollen grains of cereals, e.g., Rye (Secale), and herbs growing on agricultural fields, meadows and road verges reflect human impact in the region, e.g., Mugwort (Artemisia), Sheep's sorrel (Rumex acetosella), Cornflower (Centaurea cyanus) and Plantain (Plantago) (3). Tree plantations are reflected in high percentages of Pine (Pinus) and Spruce (Picea). Increases of spores of fungi feeding on dung might indicate increased animal husbandry. Figure 1 shows a selection of macro and microfossils in a core from Saxnäs Mosse in southern Sweden. Thirty levels within the 50 cm deep peat monolith were radiocarbon dated (4). The layer from 50 to 30 cm depth was formed during the interval from 1170 ± 13 AD to 1648 ± 11 AD. An increase of human impact around 1300 AD is reflected by Artemisia, Plantago and Cerealia. The synchronous appearance of Sphagnum magellanicum in the bog vegetation might well have been caused by an increased input of soil dust (5). Fagus, Corylus, Carpinus, Tilia and Ulmus decline due to a cooling of the climate during the Little Ice Age (LIA). This coincides with rises in delta 14C which reflect decreases of solar activity (Wolf and Spörer minimum). The layer from 29 to 1 cm depth is much younger and shows negative (after 1950 AD) 14C ages. We used the atmospheric bomb pulse for precise dating of this interval (6-7). Sample depth 29 showed an age of 1959 ± 3 AD. Therefore a time gap of more than 300 years is present between the samples at 30 and 29 cm depth. Climatic conditions cannot explain this hiatus. We found both indicators of wet and dry conditions between 40 and 30 cm depth. This is evidence for secondary decomposition of the bog surface after artificial drainage of the bog. Peat growth started again after 1959 ± 3 AD. The hiatus is a consequence of human activities but the bog vegetation regenerated when artificial drainage stopped.