During whole-tree harvesting, there is a risk that the soil will become impoverished of nutrients. To prevent this, nutrients are returned to the harvested site as wood ash. This action also reduces the risk of soil acidification. Nevertheless, the ash does not result in any increase of tree growth if it is added on mineral soils, which is where the measure is most commonly used in Sweden. Due to the lack of growth response, the interest from landowners is small, which is why ash amendment today is not done enough to compensate for the need. On the other hand, if the ash is added on organic soils, several studies show that an obvious gain in tree growth can be achieved. This is because the ash contains all the nutrients that a tree needs, except for nitrogen. On organic soils, the lack of nitrogen is usually not a problem, in contrast to mineral soils.
In Finland, ash is spread on organic soils specifically for this purpose; to increase forest growth. But it is possible to make this activity a commercial reality also in Sweden. However, this is presently not the case since the investigations of environmental risks of ash fertilization have not been considered sufficient. Except for increasing forest growth, ash fertilization would contribute to the natural circulation and thereby reduce the large amount of unused landfill ash.
This study consists of two parts. In part I, which is a literature study, the main results from the science of ash fertilization are presented to illustrate the benefits and disadvantages of the method, as well as the need for further knowledge.
In part II, the purpose was to determine the area and geographical placing of sites that were suitable for ash fertilization and owned by the forest company Sveaskog. For the selection of suitable sites, certain stand criteria were formed. They were based on peat depth, drainage, vegetation type, stand age, basal area and height. The stand data used was collected from the Swedish National Forest Inventory (SNFI). Also, the net present value (NPV) of increased tree growth was calculated for stands of different ages and with varying expected growth responses. Both ash spreading with tractor and helicopter were investigated. The NPV was calculated with and without law restrictions (3 tons of ash/ha during the whole rotation period and 5 tons of ash/ha every 30 years, respectively). These two scenarios where then compared.
In the literature study it was made clear that ash fertilization on organic soils increase both the pH of the soil, the mineralization of nitrogen as well as the available amounts of calcium, magnesium and potassium. Also, there was no doubt that there is an obvious gain in tree growth if enough ash is added (>2ton/ha). Several studies indicate that ash fertilization does not increase the risk of heavy metal and nutrient leaching to surface and ground waters to a considerable rate.
Sveaskog is the owner of about 52 000 ha of fertile peat lands. Approximately 13 600 ha of these could be used for ash fertilization. More than 90 % of the selected areas where found in the northern and southern regions of the country (Norrland and Götaland, respectively).
It was shown that ash fertilization should be a profitable method for Sveaskog. At a starting stand age of 20 years and an expected growth increase of 3 m3/ha/year, the NPV was approximately 19 900 SEK/ha if restrictions were disregarded and about 11 700 SEK/ha if restrictions were applied, both cases considering a discount rate of 3 %, a rotation period of 100 years and ash spreading with tractor.
Generally, the greater the growth response and the lower the stand starting age, the higher was the NPV, except for the stand age 80 years. The NPV for fertilization disre...
