Impact of drainage on raised bogs in Estonia

Abstract

Drainage is the most important single factor affecting mires globally but its effects vary depending on drainage type, climate and wetland ecosystem. Sixteen transects in fifteen drained raised bogs in the hemiboreal zone of Estonia were examined to determine the spatial effects of drainage on vegetation and environmental factors. We found that the effects of drainage along a hydrosequence depended on the drainage type; i.e., whether it took the form of a cutoff ditch intercepting surface and subsurface water flow around the perimeter of the bog massif, or the drainage ditch crossed the central part of the bog. Some drains of the latter type were still functioning whereas old hand-dug examples seemed to be derelict and inactive. For both of the active drainage types, maximum water levels stabilised within 25 m of the ditch but drawdown effects on minimum and average water levels extended to 450 m. The vegetation variables that were most sensitive to drainage were the number and the percentage of bog-specific species, the number of ground vegetation species, the total cover of Sphagnum species, and the height and canopy cover of trees. Trees were rare when the minimum water level was higher than -20 cm, and the number of bog-specific species increased rapidly as the minimum water level rose towards the surface from -90 cm. Total cover of the shrub layer decreased and total cover of Sphagnum species increased almost linearly with a rise of minimum water level up to 40 cm, whereas the total cover of field layer species stabilised when the minimum water level was at -100 cm. Total cover of Sphagnum species in the field layer increased over distances of up to 250–300 m from the drainage ditch and the percentage of bog-specific and fen-specific species stabilised 190–300 m from the ditches.

Keywords

• hydrosequence
• indicator species
• peatland
• water level
• vegetation change