Changing role of water table and weather conditions in diameter growth of Scots pine in drained peatlands

We investigated the impact of water table (WT), monthly temperature, and precipitation of current and previous growing seasons on annual diameter growth of Scots pine in boreal drained peatlands. The data were collected from six sites across Finland. WT was monitored during 5–8 growing seasons depending on site during 2007–2014. The sites contained altogether 19 sample plots, where diameter growth from 339 trees was measured, resulting in 1599 growth observations. Tree-level diameter growth was analysed using mixed linear models, including variables describing tree size, competition, and environment. The higher precipitation in June of the previous year and May of the current year increased diameter growth, whereas the higher temperature in July of the current year and the deeper WT in August of the previous year decreased growth. The results suggest that Scots pine grows better at shallow than deep WT in drained peatlands. This contradicts earlier findings that a deep WT is needed to support tree growth in drained peatlands. We suggest that the development of a mor layer on the peat is changing nutrient cycling and hydrology. The results encourage the avoidance of intensive drainage in forested peatlands, which may also diminish the adverse environmental impacts of peatland forestry.