Adapted forest fertilisation: more good than harm?

Around 60 000 hectare of forest land are fertilised each year. It gives an increased growth of about 900 000 cubic metres, which amounts to almost one per cent of the yearly harvest in Sweden. It is primarily forests owned by big forest companies which are fertilised. The fertilised forest area in Sweden may increase, but hardly more than double.

‘All forests are not worth fertilising. On fertile land you do not see any effect of fertilisation. Neither should you fertilise land with thick peat cover nor forests not harvest-ready,’ says Annika Nordin, professor of SLU and director of the interdisciplinary research programme Future Forests and expert on forest fertilisation.

More grasses in fertilised forests

Boreal coniferous forest ecosystems are naturally nitrogen-poor. Therefore, nitrogen fertilisation affects most plants. The potential to accelerate its growth differs between different plants when the supply of nitrogen increases. It results in an altered competition between species. Fast-growing grasses and herbs are favoured at the expense of more slowly growing plant species. You can tell immediately. But trees have a countermove.

As trees assimilate added nutrients and expand their heads, the available light is reduced for vegetation on the ground and the expansion of grasses declines. Ten years after fertilisation you can seldom see any effect on the ground vegetation. But following harvest of a fertilised forest, the vegetation in the clearing will consist of more grasses and herbs than that of a clearing of a non-fertilised forest.

Fertilisation contributes somewhat to nitrogen added to the Baltic Sea

Forest fertilisation carries an increased risk of nitrogen leakage from forest soils to groundwater and surface water. But provided that young or old forests are not fertilised, and that fertilisation is only carried out on relatively nutrient-poor areas of inflowing groundwater, the risk of nitrogen-leaching is low, results from Future Forests show. The risk is slightly elevated after thinning and final felling or perturbations such as storm damage and widespread infestation of pests and pathogens on trees. Nitrogen leaching from forest soils to groundwater can be transported via rivers and lakes to the surrounding sea.

‘Fertilisation of forests in Sweden is estimated to add another 600 to 700 tonnes of nitrogen to the Baltic Sea. The natural leaching of nitrogen from Swedish forest soils is 40 000 tonnes per year,’ says Annika Nordin.

Increased growth climate-beneficial

Fertilisation of forests means emissions of greenhouse gases. In the manufacturing of commercial fertilisers, fossil natural gas is used to convert nitrogen gas in air into first ammonia and then the fertiliser substance ammonium nitrate. In addition, fertilisers are dispensed by helicopter or tractor, which further increases the emissions of greenhouse gases.

‘At the same time, increased forest growth benefits the climate. Timber reserves increase so that it is possible to harvest more timber, which can replace energy-intensive materials like steel and concrete. More wood, tops and branches, are also harvested which can replace the fossil fuel coal, in for example municipal power plants,’ says Annika Nordin.

Climate benefits of forest fertilisation have been estimated to exceed emissions more than 20 times. For every kilo of carbon dioxide emitted in the manufacturing and spreading of a fertiliser, 20 kilos of carbon dioxide are stored through increased forest growth after fertilisation.