The positive effects of early and repeated fertilisation of young Norway spruce forests are highlighted in an extensive study from Linnaeus University and the Swedish University of Agricultural Sciences. Meanwhile, a new doctoral thesis shows that the negative consequences are short-lived. These results lead the researchers to call for a change in the view of forest fertilisation in Sweden and Northern Europe. Previous assumptions that fertilisation in younger forests is unprofitable have now been disproved.
Spruces are growing unusually fast in a forest area north of Ljungby in the Småland region of Sweden. This is one of ten sites in the country where researchers from Linnaeus University and the Swedish University of Agricultural Sciences have studied the effects of fertilisation on young spruce stands. The results indicate that fertilisation leads to a greater increase in stem volume than previously believed.
"Our study shows that fertilisation in young spruce forests results in more than a doubling of the stem volume compared with trees that have not been fertilised. Similar studies exist in Europe and North America that show good results, but not as large an increase in growth as we have achieved", explains Carl Svensson, doctoral student at the Department of Forestry and Wood Technology.
Good for the economy and the climate
Fertilisation involves the addition of nutrients such as nitrogen, phosphorus, and potassium to forest soils. The purpose is to increase the growth and development of the trees, which raises the economic value of the forest.
Adding nitrogen once late in the forest's growth is an established method for increasing the growth rate of coniferous forests in Northern Europe, but fertilising earlier and more frequently is not as common. In Sweden, only 0.15 percent of the forest stands were fertilised in 2021, and an even smaller percentage, 0.04 percent, last year.
"We have drawn some important conclusions. Early nutrient supply results in a greater growth increase than when fertilising older forests. Adding nutrients every other year gives the same result as doing it annually, which reduces the need for fertiliser as well as the costs. Even nutrient supply every third year results in a significant growth increase. We also noted that the effect is the same regardless of whether nutrients are spread manually in experiments or mechanically on a practical scale", says Martin Bader, professor at the Department of Forestry and Wood Technology.
Recently, the positive climate effects of fertilisation have also been highlighted. As trees grow, they sequestrate carbon dioxide from the atmosphere through photosynthesis. Each cubic metre of Norway spruce forest sequestrates nearly one ton of carbon dioxide, making the forest valuable as a carbon sink.
"Early and repeated fertilisation of forests in Northern Europe can contribute to meeting an increased demand for biomass while helping to mitigate the effects of climate change. It is probably the most effective way to increase the climate benefit in forests per unit area", says Carl Svensson.
At the same time, forest soil fertilisation is controversial in Sweden. Nutrient addition leads to temporarily increased emissions of greenhouse gases from the soil and risks contributing to eutrophication or acidification of the surroundings if not absorbed by the trees. It is important to consider the forest's conditions, the researchers behind the new study emphasise.
"We urge forest managers and owners to refrain from adding nutrients blindly, without first checking the forest's actual nutrient needs. We have previously shown that there are contexts where fertilisation does not have positive effects. Before deciding to fertilise, an analysis of the leaves' nutrient content and an assessment of the forest's current nutrient status should be made. It is also important to consider the soil's previous use and its characteristics such as soil type, water availability, and natural productivity", explains Johan Bergh, professor of forest management.
The new results show that it is possible to fertilise young forests without risking significant nitrogen leakage, meaning the leaching of nutrients that can lead to eutrophication of lakes and watercourses.
"We think these results can contribute to a reassessment of the view on forest fertilisation in Swedish forestry. Today, fertilisation is not recommended in Götaland, while it occurs to some extent in Svealand and Norrland. But all advice concerns fertilisation at the end of the rotation period", Carl Svensson continues.
Short-lived negative effects
Further support for the positive effects of forest fertilisation is provided in a new doctoral thesis by Charlotta Håkansson at the Department of Forestry and Wood Technology. She has studied the flows of greenhouse gases in the stand outside Ljungby, where 60 hectares of mixed forest with spruce and birch were planted after the storm Gudrun. Parts of the stand were fertilised on several occasions, and the thesis shows that nutrient addition has had a positive impact on the forest as a carbon sink.
The first and second years after fertilising with 150 kilograms of nitrogen per hectare, the flows of carbon dioxide from the soil increased, but after the third year, no such effect was seen, even though the same amount of fertiliser was added again.
"It turned out that fertilisation of young forests has relatively small and short-lived effects on the flows of greenhouse gases such as carbon dioxide, methane, and nitrous oxide from forest soil, which in the long term could make it a viable management practice to increase the sequestration of carbon dioxide from the atmosphere", says Charlotta Håkansson.
Becomes a carbon sink earlier
At the same time, the growth effect was much more long-lasting. The fertilised young spruces grew on average 56 percent more, which corresponds to capturing 8.7 tons more carbon per hectare each year. Just eight years after planting, the forest became a carbon sink. The birches also responded positively to early fertilisation, but the effect was not as clear.
"The earlier a new stand transitions from a carbon dioxide source to a carbon sink, the better. By adding fertiliser, we can achieve this transition sooner", Charlotta Håkansson concludes.
The study "Early and repeated nutrient additions support far greater stemwood production in Norway spruce than traditional late-rotation fertilization" by Carl Svensson, Martin Bader, and Johan Bergh at Linnaeus University and four researchers from the Swedish University of Agricultural Sciences (SLU). The study is published in the journal Forest Ecology and Management.
Charlotta Håkansson's doctoral thesis "Greenhouse gas fluxes and carbon sequestration in young Norway spruce stands: the effects of fertilization". Learn more about Håkansson’s doctoral project on the project page.
The research is part of the Forestry and Wood research group.