Commercial first thinnings in peatland forests should be made later

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ANNUAL REPORT 2005

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Photo: Erkki Oksanen

Commercial pre-harvesting should be later than is the current practice, if the source stand volume is closer to 150 m³ per hectare. This would improve the profitability of harvesting, maintain the tree stand growth rate and tree numbers would remain adequate to keep the water level sufficiently low. These results were reported in the final report of the SUO Research Programme (Ecologically and economically sustainable forestry on drained peatlands).

According to the 9th National Forest Inventory (NFI 9), the annual growth in drained peatland forests was approximately 24 million m³ and the annual total removal approximately 9 million m 3 . Through forest improvement practices annual growth has increased by approximately 15 million m 3 , which means that the growth rate of drained peatland stands has become almost triple that of the pristine state.

The studies of the SUO programme confirmed the close relationship between the growth of Scots pine and nitrogen concentration of peat and the connection between nitrogen and the degree of decomposition of the peat layer. The nitrogen status of the peat layer can be easily determined in situ by the degree of decomposition.

The technical quality of peatland stands is good, especially in developed stands of Norway spruce. Root and butt rot, which is common in spruce stands on mineral soils, does not occur in peatland stands. However, the roundwood removals as well as stem and log diameters are smaller and the occurrence of dead branches is greater than in mineral soil stands of Norway spruce. The occurrence of butt sweep, reaction wood and stem crooks are common in first-generation Scots pine forests on drained peatlands.

The high bark content, low wood density and small pulp yield in pulpwood harvested from the first commercial thinning of peatland stands increase the production costs of pulp. Still, as raw material for pulp production on the whole, peatland pulpwood has the same quality as wood grown in upland forests.

The problems associated with harvesting in peatland forests are not so much technical as economic ones. The problem of forest transportation is soil bearing capacity. Cost-wise, a mid-sized forwarder is the most sensible solution. As for harvesters, small machines of less than 12 tons are suitable for drained peatlands, because their hourly operating costs are reasonable.

After ditch network maintenance, a major part of the loads of suspended solid material and nutrients to the water-courses can be prevented by using sedimentation ponds and overland flow areas. Leaching of phosphorus to water-courses can be reduced by performing ditch network maintenance simultaneously with major cuttings and by using compound fertilizers containing aluminium and iron.

Additional information: www.metla.fi/hanke/3269/index-en.htm and www.metla.fi/ohjelma/suo/index-en.htm

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