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2. Wood Energy and Environment |
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Printable PDF version of section 2.4
Possible environmental effects of energy production:
Page and images by Markus Huhtinen 12.10.2006 |
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2.4 Nutrient loss from Forests 2.4.1 Biomass distribution The balance of forest ecosystem is disturbed when timber is harvested. It is clear that when the foliage and/or trunks are removed, the disturbance becomes more serious. The nutrient losses on the long term are evident, and the fertility of the forest soil may decrease as bigger share of the biomass is constantly removed in the loggings. The share of foliage biomass is 15-40 % of the total above ground biomass depending on the tree species. In average, the recovery rate of the logging residues in mature spruce stands has varied from 65 to 75 % of potential logging residue mass and from 20 to 30 % of the merchantable wood. There is a big difference between fresh "green" and dried "brown" biomass. When the needles have shedded, the weight of the residue can be 20-30 % less than initially, but there are hardly any differences in energy contents (Alakangas et al 1999). The nutrients of needles (foliage) then naturally stay within the forest ecosystem. The amount of trunkwood is estimated to be 20-25 % of the total biomass of the forest depending naturally on tree species and site factors. In young Finnish pine stands reaching the first thinning stage 60 % of the nitrogen (N), phosphor (P) and potassium (K) above the ground level are located in the crown mass (Hakkila and Fredriksson 1996). 2.4.2 Nutrient balance of forests The cycle of nutrients between soil and vegetation is an essential part of the forest ecosystem. A part of the nutrients taken up by trees returns back to the soil with forest litter, but a larger part is accumulated in the biomass. This part is released only when when the biomass starts to decompose after the death of the tree (Savolainen&Berggren 2000). The circulation is not totally closed: leaching causes nutrient loss, but disintegration of minerals and fallout from the atmosphere increase the amount of nutrients available (Figure 2.4.1).
Figure 2.4.1 Nutrient cycle in the forest (Simplified from Mälkönen 1974). The nutrient losses cause slow but long-lasting effects. For example, the complete after-cutting release of nitrogen and phosphorus from the branches can take more than ten years. Several research studies have been carried on in Scandinavia about the growth response of the stands to the whole-tree removal in early thinnings. In general, the results show little decrease in growth during first five years, but researchers expect the impact to increase over time. According to Finnish studies the loss of growth has been more significant (10%) is spruce stands than in pine stands. However, in the thinnings of mainly pine-dominated poor soils the crown mass should not be removed at all. If all the branches and treetops are removed in final cuttings, all the nutrients they consist are also removed from the ecosystem. However, harvesting logging residues at the final felling stands decreases the leaching of minerals during the first few years after cutting. The leaching of nitrogen, calcioum and magnesium from the whole-tree harvesting stands has been 67-78 % of the leaching from the stands where only stem wood has been harvested (Alakangas et al. 1999). The eutrophication of the ground cover vegetation has also decreased. This has positive effect on the forest regeneration. 2.4.3 Ash recycling The balance between the mineral substances such as calcium (Ca), potassium (K) and magnesium (Mg) is significant for the long-term production capacity of the forest land. Minerals are mainly brought to the forest by decomposition of biomass, and are withdrawn by harvesting the biomass (Ström 1994). If treetops, branches and stemwood are systematically withdrawn from the forest in fellings, there is a great risk that the nutrient balance becomes negative. If there is no compensation for the loss of minerals, the long-term production capacity of the ecosystem may deteriorate. The decomposition of biomass also compensates the natural acidification of the forest land which occurs during the growth of tree crop. An ecologically sustainable solution would be recycling the ash formed in the wood combustion. Theoretically all nutrients (excluding nitrogen) lost during the accrual of the biomass would be returned to the stand in question. There are, however, some problems and restrictions in the practice. Ash is basic and can be used used in the same way as lime to decrease the negative effects of detoxification. The lime effect of the tonne of ash corresponds to 150-450 kg of calcium (Egnell et al. 1998). Ash should be hardened, agglomerated and it should have a slow leaching time in order to be spread in the forest. Leaching rate of half of the tree generation rotation cycle would be ideal (Egnell et al. 1998). This is achieved by various methods of granulating. The dose of ash should be monitored carefully. In Swedish conditions no more than 3 tonnes of ash per hectare should be spread in the forest during a rotation time if tops and branches are removed. 2 tonnes should compensate the nutrient loss of stems and bark (Egnell et al. 1998). No negative ecological effects of recycling stabilised ash in moderate amounts (1-3 tonnes/hectare) have been found in a research project which has continued several years (Nohrstedt 1996).
2.1 Greenhouse Effect and Wood Fuel
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The share of branches and foliage biomass is high especially in the young stands (M. Huhtinen).
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