ORIGINAL ARTICLE Depth of water table prior to ditch network maintenance is a key factor for tree growth response S. SARKKOLA 1 , H. HO ¨ KKA ¨ 2 , E. AHTI 1 , H. KOIVUSALO 3 & M. NIEMINEN 1 1 Finnish Forest Research Institute, Southern Finland Regional Unit, Vantaa, Finland, 2 Finnish Forest Research Institute, Northern Finland Regional Unit, Rovaniemi, Finland, and 3 Department of Civil and Environmental Engineering, Aalto University School of Engineering, Aalto, Finland Abstract In boreal-drained peatland forests, tree growth is retarded by the gradual deterioration of drainage ditch networks. In order to avoid the development of suboptimal growth conditions, ditch network maintenance (DNM) operations (ditch cleaning and/or complementary ditching) are annually conducted on an area of about 70,000 ha in Finland. The previous studies indicate that the depth of the water table prior to DNM may influence the magnitude of the growth response to DNM. Tree growth does not necessarily increase after DNM at sites with large stand volume and subsequent low water levels due to tree stand evapotranspiration. We investigated how the pre-treatment water table depth (pre-WTD) in late summer relates to the growth of Scots pine (Pinus sylvestris L.) stands after the DNM operation in 12 field experiments. The increase in mean annual volume growth caused by DNM was negatively related to the pre-WTD, with the highest growth response in stands where the pre-WTD was less than 2530 cm. DNM did not clearly increase stand growth in sites where the pre-WTD was more than 3540 cm below the soil surface. There was a high variation in growth response to DNM between the water levels from 20 to 35 cm below the soil surface, indicating that factors other than water table depth are also needed as decision criteria for assessing the appropriate timing of ditch network maintenance. Keywords: Scots pine stand, boreal forest, peatland, drainage, ditch network maintenance. Introduction The water table level in pristine boreal peatlands is typically near the soil surface and its fluctuation is mainly governed by precipitation and evapotran- spiration (Valgma, 1998). The pre-condition for economically profitable timber production in boreal peatlands is that the water table level resides below the tree root systems during most of the growing season. A sufficiently low level of the water table can be ensured by constructing an artificial drainage network. For coniferous tree stands, previous studies suggest that the depth of the water table level should be deeper than at least 30 cm below the peat soil surface during the latter half of the growing season for optimal tree growth, but the minimum sufficient water table depth varies largely among the studies, down to the depth of 70 cm below the peat soil surface (Huikari & Paarlahti, 1967; Paavilainen, 1966; Paavilainen & Pa ¨iva ¨nen, 1995; Vompersky & Sirin, 1997). The amount of air filled pore space in the peat is the key factor for the functioning of the tree root systems and their metabolism (Kozlowsky & Pallardy, 1997). The air filled pore space in the peat is not only determined by the level of the water table but also by the peat characteristics, such as decomposition rate and water retention character- istics (Magnusson, 1994; Vompersky & Sirin, 1997). The study by Paavilainen (1967) reported that when the air filled porosity is less than 10%, the excess moisture begins to hamper tree growth. Schwa ¨rzel et al. (2006) studied peat moisture dynamics and evapotranspiration in a peat lysimeter and suggested that poor aeration started to lower the evapotran- spiration from its potential rate at pressure head values above 15 cm. Depending on the water retention characteristics of peat (e.g. Pa ¨iva ¨nen, Correspondence: S. Sarkkola, Finnish Forest Research Institute, Southern Finland Regional Unit, P.O. Box 18, FI-01301 Vantaa, Finland. E-mail: sakari.sarkkola@metla.fi Scandinavian Journal of Forest Research, 2012; 27: 649658 (Received 15 July 2011; accepted 24 April 2012) ISSN 0282-7581 print/ISSN 1651-1891 online # 2012 Taylor & Francis http://dx.doi.org/10.1080/02827581.2012.689004