Using ash to fertilize peatland forests is among the most effective ways of improving forest carbon sinks and storages, says Senior Specialist Antti Leinonen from the Finnish Ministry of Agriculture and Forestry. What would be the effect of growth increase on Finland’s carbon sinks?
Using ash to fertilize peatland forests is one of the most rapid methods of improving forest growth, says Senior Specialist Antti Leinonen from the Finnish Ministry of Agriculture and Forestry.
According to Leinonen, improving the growth of peatland forests and especially the maintenance of soil health is of importance because these forests are so extensive. According to National Resources Institute Finland (Luke), the share of peatland forests of the total growing stock in Finnish forests is about 23 percent. In terms of tree growth and fellings, the share of peatland forests is about 20 percent.
The forestry expert organization Tapio considers that doubling the current number of hectares fertilized could increase annual growth by one million cubic metres at best. Fertilized forests sequester more carbon than the emissions caused by the manufacturing and use of fertilizers.
Calculations indicate that increasing tree growth by one million cubic metres would sequester about 750,000 metric tons of carbon dioxide. The carbon footprint of the average Finn is 10,300 kilograms, so the extra growth would correspond to the carbon footprint of about 73,000 Finns. This is about 1.3 percent of Finland’s population.
Forest fertilization has been advocated by, among others, Ilkka Hämälä, President and CEO of Metsä Group, in an interview with forest.fi.
Effect visible for decades
The ash fertilization of peatlands is called remedial fertilization. According to Antti Leinonen, ash fertilization accelerates tree growth for decades, which means the best stands to fertilize are young ones.
Ash increases the potassium and phosphorus content of soil. The lack of these is what normally constrains the growth of peatland forests.
’Remedial fertilization maintains and speeds up the growth of peatland forests,’ says Leinonen.
In an interview with the Finnish Broadcasting Company (Yle), Markku Remes, Senior Forest Management Specialist at the Finnish Forest Centre, said that after ash fertilization, tree growth will increase by up to 50 percent in a couple of decades.
’Using ash can bring up to three cubic metres of extra growth per hectare and year,’ Remes told Yle.
Nitrogen fertilization on mineral soils will also increase carbon sinks. Specialist in Silviculture Varpu Kuutti at Tapio says that as an example, the nitrogen fertilization of one hectare will improve carbon sequestering by an amount corresponding to the carbon footprint of one average Finn.
In addition to improving carbon sinks, forests are also fertilized to achieve stouter logwood.
Fertilizing young stands is profitable
According to Antti Leinonen, the best type of forest to fertilize with ash is a young stand on drained peatland. Ash is not normally used on undrained peatland.
’In addition to the availability of soil nutrients, tree growth is affected by the state of drainage. If trees are to grow, oxygen must be available to the roots at the depth of 0–35 centimetres. In other words, the drainage must also be appropriate,’ Leinonen says.
Leinonen points out that before fertilization, existing ditches should be checked and improved to achieve appropriate drainage.
’Peatland drainage is based not only on ditching, but also the evaporation through the trees. Adding ash to the soil increases the needle mass and thus improves evaporation. In fact, using ash can often help postpone the need for ditch clearing, unless they are completely overgrown,’ Leinonen says.
Healthy tree stock is the key
Ash fertilization does not pay if the site is naturally poor in nutrients and tree growth is poor.
’Sites considered for fertilization should include a sufficient number of healthy and vital trees, whose growth can be maintained and improved with ash. Normally, sufficient growing stock to make ash use profitable is found on sites comparable with Vaccinium-type drained peatlands or sites richer than that,’ says Leinonen.
Sites considered for fertilization should include a sufficient number of healthy and vital trees, whose growth can be maintained and improved with ash.
Preliminary follow-up results by Luke indicate that growth can also be improved on some sites that are poorer than Vaccinium-type drained peatlands, though the benefits from ash use may be limited by the quality and quantity of trees on the site.
Little impact on forest nature
Ash fertilization does not affect forest nature greatly. According to Antti Leinonen, the use of ash will boost surface vegetation to some extent. The current assumption is that fertilization does not cause emissions of greenhouse gases from the peat soil.
According to studies published in Sweden, ash use hasn’t been found to increase greenhouse gas emissions from the soil of peatland sites.
’According to studies published in Sweden, ash use hasn’t been found to increase greenhouse gas emissions from the soil of peatland sites. The studies also found that measurable changes in soil organisms only occurred on some sites. And in any case, these changes are reversed over time,’ Leinonen says.
Fertilizing old forests on mineral soils is useful
Fertilizing forests on mineral soils to improve growth is a different matter. According to Leinonen, boron deficiency can be seen in young stands as the emergence of competing top shoots and lack of chlorophyll in needles. Another nutrient that can be added is nitrogen, which accelerates and strengthens tree growth.
’Improved growth will continue for about ten years, as against the several decades achieved with ash on peatlands. Growth fertilization is especially profitable in maturing forests, as it will significantly improve and speed up the increasing of stoutness before regeneration felling,’ Leinonen says.
