Study site and data acquisition
The study area consists of three study sites dominated by Scots pine (Palomäki, Pollari, and Vesijako) (Figure 2), established and maintained by Natural Resources Institute Finland (Luke). All study sites are located in southern boreal forest zone and characterized as mesic heath forest (i.e., Myrtillus forest site type according to Cajander (1913)). Nine rectangular sample plots (size from 900 and 1200 m2) were placed at each study site resulting in a total of 27 sample plots. At the time of the establishment (Palomäki in 2005, Pollari and Vesijako in 2006), the stand age was 50, 45, and 59 years for Palomäki, Pollari, and Vesijako, respectively. The first thinning (removal ~30% of stems) had been carried out for all study sites in the early 1990s.
The experimental design of the study sites includes two varying levels of thinning intensity (i.e. moderate and intensive) and one plot at each study site remained as a control plot where no thinning has been carried out since the establishment of the sites. The remaining relative stand basal area after moderate thinning was ~68% of the stocking before thinning and intensive thinning reduced the stocking levels down to 34%. Suppressed and co-dominant as well as unsound and damaged (e.g. crooked, forked) trees were removed (i.e. thinning from below) in both thinning intensities. Other thinning treatments were also carried out but here we concentrated on 3 plots with moderate, 3 plots with intensive, and 3 plots with no treatment since establishment.