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Risk of eutrophication and acidification if forest fertilization is introduced in southern Sweden

Forest with green moss. Photo.
The forest's environmental goals are not being met today. Klas Lucander has investigated what effects increased nitrogen fertilization could have on the forest. Photo: Shutterstock

Forests are important for climate change mitigation, both as raw material for biofuels and for carbon storage. At the same time, forests are under pressure from a changing climate and more intensive forestry. A new thesis by Klas Lucander at Lund University shows the possible consequences for forests of fertilisation, and how this could lead to eutrophication and acidification instead of tree growth in southern Sweden.

Increased temperatures, long periods of drought and wide variations in precipitation. These are some of the climate changes that are expected to become more common in the future. And this will affect Swedish forests, which have a key role to play in the era of climate change, both as a source of income, as a resource for fossil-free biofuel and as a carbon sink.

Environmental targets linked to forests are not reached

Sweden has set ambitious environmental targets, several of which are linked to forests. But progress is not going in the right direction to reach them, according to Klas Lucander. He has written the thesis "Direct and indirect pressures of climate change on nutrient and carbon cycling in northern forest ecosystems: Dynamic modelling for policy support" at the Centre for Environmental and Climate Science at Lund University.

- The targets are not being met today and the future looks uncertain. Although sulphur deposition has decreased over the years, which is positive, forest recovery is taking a long time. And now climate change is the big threat to achieving the targets," he says.

In his thesis, Klas Lucander focused on three of the Swedish environmental targets:

  • Only natural acidification
  • No eutrophication
  • Limited climate impact

Risk of eutrophication and increased acidification

The studies show how increased pressure on forests can affect carbon sequestration and the cycle of key nutrients. One example is what happens with increased fertilisation of forests, which the Swedish University of Agricultural Sciences has been commissioned by the previous government to investigate. Current recommendations are that forests in southern Sweden should not be fertilised, due to historically elevated nitrogen deposition in the area.

- There is an ongoing debate about changing the recommendations to allow nitrogen fertilisation throughout the country. Based on our modelling study, we could see a negligible effect on tree growth in southern Sweden from nitrogen fertilisation, but a large effect on increased leaching of inorganic nitrogen. So based on our results, we should not expand the areas for fertilisation," says Klas Lucander.

Big differences within the country

The most common problem in Swedish forests is that growth is limited by a low supply of nitrogen in the soil. Forest ecosystems therefore generally have a large capacity to take up available nitrogen, and it is only when the forest is harvested that organic nitrogen is leached into the environment.

However, there are large differences across the country. Northern Sweden has had low nitrogen deposition, while south-western Sweden has experienced elevated nitrogen deposition in recent decades, with signs that forests are close to nitrogen saturation. Several forest areas show greatly increased leaching of nitrogen in growing forests, according to Klas Lucander. This might risk leading to acidification and eutrophication of lakes, seas, and other water bodies, and to a deterioration of groundwater quality.

The history of the land is important

Another conclusion of the thesis is that the historical use of forest land is of great importance. It influences soil composition and thus nitrogen dynamics and the risk of nitrogen leaching. Neighbouring geographical areas, which have had similar historical nitrogen deposition and the same climate, may have different conditions depending on whether the land has been used for forestry for a long time or whether it has previously been pastureland, for example.

Klas Lucander has used the dynamic forest ecosystem model ForSAFE for the calculations in his research.

- Such a model can be useful for forestry planning, so that environmental objectives can be achieved in the future. And the next step for the model could be to further develop the process descriptions to be able to say something more about how nitrogen fertilisation affects carbon sequestration in the soil," says Klas Lucander.

It is worth mentioning that models always have their limitations. For example, the influence of understory vegetation is not included in this study. Understory vegetation takes up some of the nitrogen that would otherwise be leached out.

Read the thesis

The thesis can be downloaded at the following link:

Direct and indirect pressures of climate change on nutrient and carbon cycling in northern forest ecosystems: Dynamic modelling for policy support