Late-Holocene human impact and climate change near Umea

In 2003, peat cores were sampled from sites positioned on a north-south transect through Sweden and northern Germany. Only bogs with a similar species composition were selected. The aim of our research is to compare the changes in regional and local species composition with carbon accumulation over time and along the N-S transect. The local and regional vegetation was reconstructed using macrofossil and pollen analyses. Bulk density, C/N, degree of humification and loss on ignition were measured and carbon accumulation rates were calculated. A precise chronology was established by wiggle-matching 14C AMS measurements.

The most northern research site, Lappmyran, is situated north-west of Umeå, close to Degerö Stormyran. Our record of Lappmyran (LPM) is continuous and dates back to ca. 750 AD at 72 cm depth. The regional vegetation changes have been highly influenced by changing intensities of human impact. Until the 14th century, the human impact on the landscape was of minor importance due to the low human population density. In the following centuries, human impact increased: cultivation of cereals began in this area, grazing of livestock in the forest intensified and the pre-industrial forestry opened the forest. Agriculture had its maximum extent from the beginning of the 19th century until the middle of the 20th century. At the same time, industrial forestry resulted in the deforestation of large areas. Later, in the middle of the 20th century, the area used for agriculture decreased, probably as a result of industrialisation. These anthropogenic changes are clearly reflected in the pollen-diagram and mask possible climate signals. Thermophilous trees, which may be used as climate indicators, do not have natural stands in the Umeå region. Occasionally though, pollen of thermophilous trees was recorded owing to long distance transport. Pollen of Quercus was even found regularly in the deeper part of the core. At about 50 cm depth, ca. 1750 AD, Quercus disappears from the record until ca. 1940 AD. A cooling of the climate may have caused a southward movement of the distribution range limit of Quercus. It is possible that the increased distance became too great for long distance transport, until the climate warmed in the 20th century and Quercus pollen was again deposited in Lappmyran.

The change in Sphagnum composition and the appearance of Rubus chamaemorus at 46 cm depth, about 1800 AD, may have been caused by a change in climate or the internal dynamics of the raised bog. The dominant wet growing Sphagnum Section Cuspidata (S. cuspidatum and S. angustifolium) was replaced by the hummock forming Sphagnum Section Acutifolia (S. fuscum) suggesting a change to a drier environment. We took surface pollen samples on a hummock-hollow-hummock transect to map the pollen dispersal of the insect-pollinated Rubus chamaemorus. If the pollen of R. chamaemorus is well dispersed, we would expect to find pollen in both hummock and hollow surface samples. In that case, our palaeo data would suggest a change in climate because we did not find R. chamaemorus pollen before. If R. chamaemorus has a poor pollen dispersal, we would only find pollen in the hummock surface samples (where it is growing). This is what we found in the experiment. Our palaeo macrofossil data show hollow-type vegetation in the deeper part of the record. Due to the poor dispersal of R. chamaemorus we cannot conclude whether R. chamaemorus was present or absent in Lappmyran before 1800 AD. On this basis we cannot answer if climate or internal dynamics caused the shift in Sphagnum species composition.