Abstract
Drainage ditches have been dug in peatlands and paludified forests in order to enhance forest growth in an area of 4.7 M ha in Finland. Because of peat subsidence, bank erosion, sedimentation, and ingrowth of vegetation ditches deteriorate with time. In this study the shallowing of ditch depth over time was investigated on the basis of country-wide peatland inventory data measured repeatedly up to four times. Mixed linear models were constructed separately for original ditches and maintained ditches (cleaned once or twice). After 20 years the ditches were 20-30 cm shallower than right after the digging. Time since digging was the most important variable explaining the shallowing for both original and maintained ditches. Other variables explaining the ditch shallowing were the digging method (excavator, plow), ditch bed slope, location, and peat layer thickness. The average development of maintained and original excavator ditches was very similar. The results can be used in assessing decision making concerning ditch cleaning.
Highlights
In Finland, the intensive drainage of peatlands and paludified forests aiming to enhance the growth of tree stands started in the 1930s and ended in the mid-1990s when the digging of ditches on pristine peatlands virtually ceased
The digging method influenced the model intercept in a way that ditches made by plough were clearly shallower than those made by excavator (Table 3, Figure 1), but there was no significant difference in the slope
The long-time span covered by this study indicated that the rate of shallowing of original ditches is faster for the first ten years but slows down after 35 years (Figure 1)
Summary
In Finland, the intensive drainage of peatlands and paludified forests aiming to enhance the growth of tree stands started in the 1930s and ended in the mid-1990s when the digging of ditches on pristine peatlands virtually ceased. The soil water table level was lowered which enabled a higher decomposition and nutrient supply, and improved tree growth. Lauhanen et al (1998), Lauhanen and Ahti (2001), Hökkä and Kojola (2003), and Ahti et al (2008) have shown that after maintenance of the ditches, a growth response of trees is to be expected This response varies depending on the soil water conditions before the DNM operation (Sarkkola et al 2013), geographical location of the tree stand (Ahti et al 2008), stand stocking, and site quality (Hökkä and Kojola 2003). The necessity of deep good quality ditches for good tree growth has been questioned in recent studies (Sarkkola et al 2012, 2013), and the biological drainage mediated by the water use of the tree stand has been emphasised (Sarkkola et al 2010, 2013)
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