Impact of drainage on vegetation of transitional mires in Estonia

Abstract

The extent of drainage impact was studied in five transitional mires along a hydrosequence. The principal environmental variables driven by drainage are the minimum water level in the peat layer, together with the dry matter and total-N contents of peat. The drawdown effect of a cutoff ditch (intercepting surface and subsurface flow) on the minimum water level extends to a distance of 250–320 m. Peat water levels above -50 cm are associated with a sharp increase in Sphagnum cover, and the percentage of bog-specific species increases markedly when the minimum water level is higher than -25 cm. The girths and heights of trees and canopy closure of the tree layer decrease rapidly up to a distance of 200 m from the cutoff ditch. Negative impacts of the cutoff ditch on the canopy cover and average height of trees, estimated using LIDAR data, can be followed for distances up to 400 m and 350 m, respectively. The density and height of shrub stems start to increase at a distance of 16 m and continue to increase up to 400 m from the cutoff ditch. The percentage of bog-specific species increases up to a distance of 100 m, whilst the percentage of fen-specific species begins to decrease remarkably at a distance of about 200 m from the cutoff ditch.