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High-resolution reconstruction of wetness dynamics in a southern boreal raised bog, Finland, during the late Holocene: a quantitative approach

Environmental Change Research Unit (ECRU), Department of Biological and Environmental Sciences, P.O.Box 65, 00014 University of Helsinki, Finland, Department of Geology, P.O. Box 64, 00014 University of Helsinki, Finland, minna.valiranta{at}helsinki.fi

Atte Korhola

Environmental Change Research Unit (ECRU), Department of Biological and Environmental Sciences, P.O.Box 65, 00014 University of Helsinki, Finland

Heikki Seppä

Department of Geology, P.O. Box 64, 00014 University of Helsinki, Finland

Eeva-Stiina Tuittila

Department of Forest Ecology, University of Helsinki, Finland

Kaarina Sarmaja-Korjonen

Department of Geology, P.O. Box 64, 00014 University of Helsinki, Finland

Jukka Laine

Parkano Research Unit, Finnish Forest Research Institute, Kaironiementie 54, 39700 Parkano, Finland

Jukka Alm

Joensuu Research Unit, Finnish Forest Research Institute, P.O. Box 68, 80101 Joensuu, Finland

A high-resolution plant macrofossil analysis was applied to investigate wetness dynamics in a southern Finnish boreal bog, Kontolanrahka, during the last 5000 years. The chronological control and the age—depth model were based on 40 AMS radiocarbon dates. Pollen analysis provided information on regional-scale vegetation changes. Macrofossil analysis revealed prominent changes in vegetation assemblages during the late Holocene, indicating fluctuations in water-table. The reconstruction suggests that at the coring point, which nowadays is a relatively wet lawn, habitat type has repeatedly varied between transient communities similar to those currently represented in dry hummocks, very wet lawns and even hollows. In order to quantify historical changes in water-table, Generalized Additive Models (GAM) were used to investigate the current relationships between surface plant species and water-table depth. Modern water-table measurements and a survey of associated plant communities along moisture gradients provided data for GAM-analyses. The plant species showed unimodal distributions with apparent optima and narrow tolerances along the water-table gradient. A transfer function for water-table reconstruction was created by calibrating plant macrofossil records against the modern vegetation/water-table relationship using the weighted averaging partial least squares (WA-PLS) regression method. The quantitative water-table reconstruction for the late Holocene showed that the water-table depth had varied between 38 and 2.5 cm, the root mean square error of prediction being 3 cm. The detected historical wet and dry shifts were compared with other similar data from Finland, Sweden and Estonia, and from Western Europe. Despite some similarities, especially during the last c. 1000 years, noticeable differences in timing and duration occur, suggesting they may not have been driven only by climate, but also by local factors, including surface fires.

Key Words: Plant macrofossils • pollen • peatland dynamics • boreal forest • wetness • hydrological changes • late Holocene • quantitative reconstruction • Finland.

The Holocene, Vol. 17, No. 8, 1093-1107 (2007)
DOI: 10.1177/0959683607082550


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