Long-term effects of water level drawdown on the vegetation of drained pine mires in southern Finland.
The effect of water level drawdown after drainage of mires for forestry was studied by comparing the vegetation on undrained pine [Pinus sylvestris]-mire sites with that of sites drained 3-55 years earlier. Some of the sites had been fertilized to increase forest growth. The plant communities were analysed with respect to the following environmental variables: drainage age, total nutrient contents of surface peat, and tree stand characteristics. Canonical correspondence analysis was used to relate the environmental variables to data on the botanical composition of vegetation. Two main gradients were found in the data. The first ordination axis clearly related to a gradient in forest vegetation succession and the second axis to a gradient in peat nutrient level and pH. The secondary succession towards forest vegetation started soon after drainage and proceeded most rapidly in the most nutrient-rich site types and fertilized sites. This led to a more uniform vegetation composition between the site types. Original mire species reacted differently to the changing post-drainage environment. Tall sedges (Carex lasiocarpa, C. rostrata) disappeared soon after drainage. The coverages of the mire dwarf shrubs gradually decreased with increasing tree stand volumes; Betula nana appears to have been the most sensitive species. The coverages of the Sphagnum spp. studied appear to have decreased in response to increasing tree stand shading in the order: S. fuscum > S. recurvum complex > S. magellanicum > S. russowii. The development towards forest vegetation on mires may diminish the regional (gamma-) diversity on forest-dominated landscapes, even if the species (alpha-) diversity on individual sites is little affected. Long-term vegetation changes after artificial water level drawdown, emphasized in this study, can be used to mimic the effects of the predicted global climatic warming on mire vegetation.