Detecting drainage events and drainage affected zone: A dendrochronological study at Tellissaare raised bog, Estonia

Mires are an important part of northern and boreal landscapes. Many peatlands have been drained in Estonia either for peat extraction, forestry or agricultural purposes. The remaining mires often have drainage ditches in their peripheral area, affecting the hydrology and vegetation. The main objective of this study was to quantify the effect of drainage on the radial growth of trees growing in the mire margin and to assess if a drainage-induced change in the radial growth of bog pines can be used as an indicator to determine the width of a buffer zone for the preservation of the natural interior of the mire. Pines growing at different distances from the ditch were sampled by increment borer, and their radial growth was analysed. We found a time lag in the effect of drainage on tree growth towards the interior of the mire. Detecting drainage events is complicated as the effect is mixed with the climate signal which varies also with drainage intensity. Trees growing closer to the ditch were mostly negatively affected by late autumn temperatures of the previous year, while trees in the unaffected area of the mire were more affected by winter temperatures. The positive spring and summer precipitation signal intensified towards the pristine core of the mire. A significant increase in radial increment of pines was observed near the drainage ditch up to 40 metres; however, a weaker effect of drainage was still detected at a distance of 90 metres from the ditch. The drainage effect on the radial growth disappeared at a distance between 190 and 440 metres from the ditch. This indicates that the buffer zone width to protect the mire core from forest encroachment due to anthropogenic drainage should be at least 190 metres wide. Key words: dendroecology; climate–growth relationships; disturbance; peatland; Pinus sylvestris L.