Thinning 2020
SMEAR II stand history
SMEAR II station is located to a stand established by sowing after prescribed burning conducted in 1961. The stand in question, the neighboring stands and in practice all stands within the 200 m radius from the main mast have been mostly outside of the routine forest management since the establishment of the SMEAR station more than 20 years ago. The only significant exception for this was the first thinning for some parts of the study stand and neighboring stands, conducted in 2002. However the effect of that thinning was mild and there are hardly any signs of that left. Most of the stands within the 200 radius were rather dense and the ratio between the living crown and tree height was decreasing quite rapid in time. Individual trees suffered from unfavorable light conditions and in some parts of the study area dying/dead trees were becoming more common. Overall, trees showed signs of decreased vitality, and especially obvious this was for suppressed, sub-dominant and co-dominant trees. Most of the stands located farther than 200 m away from the main mast have been managed by Metsähallitus, following typical routines and timing applied in Finnish commercially managed forests.
Aim of the thinning
The aim of the thinning was to return the SMEAR II forest stand structure to resemble typical Finnish commercially managed forest. The target area for thinning covered forests within 400 m radius from the SMEAR II main mast. The stands that didn’t require thinning (were already sparse enough and won’t need thinning within the next 10 years) were left intact. In practice about 80 % of forest area within 200 m radius from the mast (in total about 12,5 ha) was thinned, as well as 10 ha outside of the 200 m radius. In total the thinning area covered 40 % of the forest area within 400 m radius around the main. For the thinned stands, the aim was to reduce the stand basal area by about 40 %, close to the recommended lower limit of stand basal area for forest at this stage of development.
Small area (about 0.75 hectare) close to the study stand was left intact, to ensure that there is suitable control area for studies comparing the effects of thinning on ecophysiological processes.
Timing of the thinning
Clearing of understorey trees (small, non-commercial trees) from stands to be thinned took place in spring 2019. The thinning itself took place in February-March 2020. Small area (about 1 hectare) in the middle of the study stand was thinned already in December 2019 and January 2020. Wintertime was chosen in order to minimize root damages caused by heavy machines.
Thinning method
Clearing of understorey trees was conducted manually with a brush saw. The general instruction for clearing was to remove all understorey trees, except juniper (Juniperus communis L.). It’s worth noticing that the stands that were not in need of thinning were left intact in clearing of understorey.
The thinning was mostly conducted with normal harvesters and forwarders, except where the measurement infrastructure or other sensitive objects required the usage of special methods. For those areas the thinning was conducted manually, with a chainsaw.
For the majority of thinning area the thinning was classified as thinning from below, where most of the removed trees are suppressed or sub-dominant, and few co-dominant or dominant trees are cut. This type of thinning is the most common approach in case of even-aged forests if Finland.
Some parts of the forest within 400 m radius had already achieved the condition enabling final cutting. However, no final cuttings were conducted, and instead thinning from above or quality harvesting was applied.
Scots pine was favored in selection of remaining trees. This means that the thinning decreased stand heterogeneity, when the proportion of other species than Scots pine was reduced. However, dominant tree species didn’t change at any of the thinned stands. Because Scots pine was already dominant species around the measurement station, majority of the cutting yield consisted of pines.