Critical biomass harvesting indicator for whole-tree extraction does not reflect the sensitivity of Swedish forest soils

Stefan Löfgren,Johan Stendahl,Erik Karltun

doi:10.1016/j.ecolind.2021.108310

Abstract

There is a growing societal demand to increase the use of forest biomass for substitution of fossil fuels. The production of this biomass must be sustainable and an indicator for critical biomass harvesting (CBH) has been suggested in order to sustain forest soil fertility and mitigate soil acidification at whole-tree harvesting. The CBH indicator is based on an acidity mass balance approach in line with the critical load of acid deposition (CL) concept. Countries like Sweden, the Netherlands and the state of Quebec, Canada apply such mass balance approaches for developing forest biomass harvesting guidelines. The implementation of this type of policy instrument may restrict the use of harvest residues for bioenergy and thereby the substitution of fossil fuels. It may as well affect the forestry sector revenue negatively. To maintain credibility for enforced limitations, it is important that the risk assessment and suggested policy implications are based on solid scientific methods and assumptions. The mass balance approach have been criticized for being too uncertain and not sufficiently validated for being used to guide ecosystem management. In this paper we use published Swedish data on soils, acid deposition, forest production and information from international scientific literature to critically examine the CBH indicator. We conclude that the CBH indicator 1) does not account for all relevant processes 2) it exaggerates the sensitivity and correlates poorly to actual forest soil acid-base status and edaphic conditions and 3) data availability does not allow the indicator to be calculated at a high enough spatial resolution for advice on management for forest owners. The concerns for the mass-balance approach and CBH indicator are discussed in an international perspective.

Highlights

The “Critical load” concept (Nilsson and Grennfelt, 1988) is used for policy negotiations worldwide within the Convention on Long-range Transboundary Air Pollution (CLRTAP) for diminishing the emissions of sulphur and nitrogen (Grennfelt et al, 2020)
The indicator is based on acidity mass balances and a “Critical biomass harvesting” (CBH) level, which corresponds to the harvesting level when acid neutralizing capacity (ANC) in soil solution equals zero (Akselsson and Belyazid, 2018, see below)
The estimated proportion of Norway spruce stands at county level exceeding the critical biomass harvesting at WTH and without ash re turn varies between 1 and 62% (Swedish Forest Swedish Forest Agency, 2021, Table 1), with the largest shares (>30%) in southeast Sweden

Summary

Introduction

The “Critical load” concept (Nilsson and Grennfelt, 1988) is used for policy negotiations worldwide within the Convention on Long-range Transboundary Air Pollution (CLRTAP) for diminishing the emissions of sulphur and nitrogen (Grennfelt et al, 2020). Soil fertility and potential acidification based on mass-balances have been used as indicators for sustainable forest production and biomass extraction (van Breemen et al, 1983; Ranger and Turpault, 1999; Thiffault et al, 2011; Achat et al, 2015; Nilsson, 1988) as well as for improving biomass harvesting guidelines (de Vries et al, 2021; Titus et al, 2021; MFFP, 2020). These nutrient mass balances have been criticized for being too uncertain and insufficiently validated for

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Conclusion