Intensified forestry can change surface water quality

“The ambition to use forestry as a climate mitigation measure should take into consideration the potential effects of forestry management methods on soil and surface water quality, in particular in acid-sensitive areas such as southern Norway.” This is the main message of a new paper titled “Intensified forestry as a climate mitigation measure alters surface water quality in low intensity managed forests”.

Forest fertilisation and increased removal of biomass have been proposed as climate change mitigation measures to increase carbon sequestration. But these and other forest practices may have undesirable effects on surface water quality. In naturally acid-sensitive areas such as much of Fennoscandia, a concern is acidification due to acid deposition in combination with forest practices that increase the removal of base cations and leaching of nitrate. The authors applied the biogeochemical model MAGIC (Model of Acidification In Catchments) to a coniferous-forested catchment at Birkenes, southernmost Norway. They simulated the effects of forest fertilisation and harvest on soil and streamwater. The model was used to simulate fertilisation and clearcutting of the mature forest by either conventional stem-only harvest or whole-tree harvest.

The modelling results showed that intensified forestry may cause substantial effects on surface water quality in acid-sensitive areas. The stem-only harvest had a more pronounced effect on nitrogen leaching than the whole-tree harvest, but the nitrate peak diminished a few years after the clearcut. Due to poorly buffered soils at the study site, the nitrate leaching resulted in a brief, but significant decline in acid neutralising capacity (ANC) and pH. The greater biomass removal with the whole-tree harvest resulted in a long-term depletion of soil base cations and a setback in the positive trend in stream ANC by several decades.

Nitrogen fertilisation was simulated ten years before the harvest. The modelling results revealed that the fertilization was not followed by an immediate nitrogen pulse in surface water, but gave elevated nitrogen leaching after clear-cut with both harvest methods. It should be noted that the Birkenes study represents an extreme case where 100% clear-cut of a catchment was modelled.

Valinia, S., Kaste, Ø. and Wright, R.F. 2021 Intensified forestry as a climate mitigation measure alters surface water quality in low intensity managed forests, Scandinavian Journal of Forest Research, 36:1, 15-31, DOI: 10.1080/02827581.2020.1854339