Figure 2. Location of the three study sites namely Palomäki, Pollari, and Vesijako and vegetation zones in Finland (left) and study sites on top of satellite imagery © 2020 TerraMetrics.
The plots were measured and thinning treatments performed at the time of the establishment (in 2005 at Palomäki and in 2006 at Pollari and Vesijako). The most recent field measurements were carried out in October 2018 and in April 2019. Tree species, crown layer, DBH from two perpendicular directions, and health status were recorded from each tree within a plot (i.e. tally trees). At the time of the latest measurements, the proportion of Norway spruce and deciduous trees (i.e.Betula sp and Alnus sp) from the total stem volume of all trees within the 9 sample plots was 3.02% and 0.07%, respectively. Approximately half of the trees (n=318) were selected as sample trees from which tree height, crown base height, and height of the lowest dead branch were also measured. Height of the tally trees was estimated using an allometric model calibrated for each sample plot with the information from the sample trees. Stem volume of individual trees was produced by using the existing nationwide, species-specific volume equations with DBH and height as predictors (Laasasenaho 1982) to obtain corresponding information from different field measurements. Plot-level attributes before and after thinning treatments are presented in Table 1, and the development of tree-level attributes for each thinning treatment can be found in Table 2.
Table 1. Mean stand characteristics by treatments before and after thinning and thinning removal. N = stem number per hectare, G = basal area, Dw = mean diameter weighted by basal area, Hw = mean height weighted by basal area, H100 = dominant height, and V = volume.