Finnish Forest Research Institute  Metla

Press release 23.05.2006

The contribution of drained peatland forests to the Finnish forestry is rapidly increasing

According to the latest inventory of the Finnish forests, already one quarter of the total annual yield comes from peatland forests. The final report of the SUO research programme (Ecologically and economically sustainable forestry on drained peatlands; in Finnish), reveals that in many respects, peatland forestry is more challenging than traditional forestry on mineral soil sites. However, the harvesting problems in drained peatland forests are economical rather than technical.

According to the 9th National Forest Inventory (NFI9), the annual growth in drained peatland forests was approximately 24 million m3 and the annual total removal approximately 9 million m3. Through forest improvement practices annual growth has increased by approximately 15 million m3, which means that the growth rate of drained peatland stands has become almost triple that of the pristine state. During the next two decades, the share of drained peatland forests will be approximately 25 percent of the total harvesting potential of the Finnish forests.

The amount of available nitrogen is decisive for the level of the potential wood yield on a site. The studies of the SUO programme confirmed the close relationship between the growth of Scots pine and nitrogen concentration of peat. The connection between nitrogen and the degree of decomposition of the peat layer was also confirmed in the studies. Accordingly, the nitrogen status of the peat layer can be approximated by the degree of decomposition, which is easily and cheaply determined in situ. The studies also provided new information about the limit values of peat’s nitrogen concentration and temperature sum, below which fertilization no longer increases stand growth. The results can be utilized when choosing areas for ditch network maintenance especially in northern Finland. The temperature sum affects the amount of nitrogen available for trees: for equal tree growth, the total nitrogen concentration in peat must be higher in the north than in the south.

During the SUO programme extensive peat sampling was performed on the NFI permanent plots. The material represents a cross section of the nutrient stores in peatland forests. In the future, the NFI vegetation data, as well as stand and site description data can be linked to the nutrient stores of peat.

Nutrient deficiencies (phosphorus, potassium) were found to limit tree growth on nitrogen-rich blueberry and bilberry-type transformed mires. Fertilization experiments confirmed that phosphorus application improves the growth rate and nutritional status of a tree stand for a long period of time (over 30 years). When applying water-soluble potassium, the duration of the fertilization effect was only 15-20 years, while the effect continued longer, for at least 20-25 years, with slowly dissolving potassium fertilizers (biotite). The higher the nitrogen content of the peat, and the more southerly the site was located, the more fertilization with phosphorus and potassium increased growth and the higher was the profitability of the fertilizer application: in stands that suffered from nutritional deficiencies, the growth rate more than doubled compared to unfertilized control. Wood ash significantly improved the growth rate of Scots pine stands on drained peatlands as well.

The technical quality of peatland stands is good, especially in developed stands of Norway spruce. Especially density, strength and stiffness properties of wood are good. Root and but 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 but sweep, reaction wood and stem crooks are common in Scots pine forests on drained peatlands. Dead branches are also more common on drained peatlands than in mineral soil stands. During the first generation, wood is not as even in quality as on mineral soils, because wood formed before drainage is different to that formed after. As for pulpwood from the first commercial thinning of peatland stands, high bark content, low wood density and small pulp yield increase the production costs of pulp. As raw material for pulp production on the whole, peatland pulpwood has the same quality than 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. A light crawler is expensive due to the smaller load carrying capacity and long haulage distances. As for harvesters, small machines of less than 12 tons are suitable for drained peatlands, because their hourly operating costs are lower. The cutting strip method, allowing a larger strip road spacing, can be applied when using small harvesters.

The results of the SUO programme have shown that after ditch network maintenance, 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 preventing the groundwater level from rising during major cuttings, i.e. by simultaneous ditch network maintenance. Phosphorus leaching can be reduced also by using compound fertilizers containing aluminium and iron. In wood ash fertilization the risk of phosphorus leaching is minor.

Publication: Ahti, E., Kaunisto, S., Moilanen, M. and Murtovaara, I. (Eds.) 2005. Suosta metsäksi (From mire to forest). Ecologically and economically sustainable forestry on drained peatlands. Final report of the research programme. Finnish Forest Research Institute, Research Papers, No. 947. In Finnish only.

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