LONG-TERM EFFECTS OF WATER-LEVEL DRAWDOWN ON THE VEGETATION OF DRAINED PINE MIRES IN SOUTHERN FINLAND

1. The effect of water level drawdown after drainage of mires for fore stry was studied by comparing the vegetation on undrained pine-mire si tes with that of sites drained 3-55 years earlier. The plant communiti es were analysed with respect to the following environmental variables : drainage age, total nutrient contents of surface pear, and tree stan d characteristics. Canonical correspondence analysis was used to relat e the environmental variables to data on the botanical composition of vegetation. 2. Two main gradients were found in the data. The first or dination axis clearly relates to a gradient in forest vegetation succe ssion and the second axis to a gradient in peat nutrient level and pH. 3. The secondary succession towards forest vegetation started soon af ter drainage and proceeded most rapidly in the most nutrient-rich site types. This led to a more uniform vegetation composition between the site types. 4. Original mire species reacted differently to the changi ng post-drainage environment. Tall sedges (Carex lasiocarpa, C. rostra ta) disappeared soon after drainage. The coverages of the mire dwarf s hrubs gradually decreased with increasing tree stand volumes; Betula n ana appears to have been the most sensitive species. The coverages of the Sphagna studied appear to have decreased in response to increasing tree stand shading in the order: S. fuscum > S. recurvum complex > S. magellanicum > S. russowii. 5. The development towards forest vegetat ion on mires may diminish the regional (gamma-) diversity on forest-do minated landscapes, even if the species (alpha-) diversity on individu al sites is little affected. 6. Long-term vegetation changes after art ificial water level drawdown, emphasized in this study, can be used to mimic the effects of the predicted global climatic warming on mire ve getation.