Effects of intensive biomass harvesting on forest soils in the Nordic countries and the UK: A meta-analysis

Nicholas Clarke,Lars Pødenphant Kiær,O Janne Kjønaas,Teresa G Bárcena,Lars Vesterdal,Inge Stupak,Leena Finér,Staffan Jacobson,Kęstutis Armolaitis,Dagnija Lazdina,Helena Marta Stefánsdóttir,Bjarni D Sigurdsson

doi:10.1016/j.foreco.2020.118877

Abstract

The use of biomass from forest harvesting residues or stumps for bioenergy has been increasing in the northern European region in the last decade. The present analysis is a regional review from Nordic and UK coniferous forests, focusing on the effects of whole-tree harvesting (WTH) or whole-tree thinning (WTT) and of WTH followed by stump removal (WTH + S) on the forest floor and mineral soil, and includes a wider array of chemistry data than other existing meta-analyses. All intensified treaments led to significant decreases of soil organic carbon (SOC) stock and total N stock in the forest floor (FF), but relative responses compared with stem-only harvesting were less consistent in the topsoil (TS) and no effects were detected in the subsoil (SS). Exchangeable P was reduced in the FF and TS both after WTT and WTH, but significant changes in exchangeable Ca, K, Mg and Zn depended on soil layer and treatment. WTH significantly lowered pH and base saturation (BS) in the FF, but without apparent changes in cation exchange capacity (CEC). The only significant WTH-effects in the SS were reductions in CEC and BS. Spruce- and pine-dominated stands had comparable negative relative responses in the FF for most elements measured except Mg and for pH. Relative responses to intensified harvesting scaled positively with growing season temperature and precipitation for most variables, most strongly in FF, less in the TS, but almost never in the SS, but were negative for P and Al. The greater reduction in FF and TS for soil organic carbon after intensive harvesting decreased with time and meta-regression models predicted an average duration of 20–30 years, while many other chemical parameters generally showed linear effects for 30–45 years after intensified harvesting. Exchangeable acidity (EA), BS and pH all showed the reversed effect with time, i.e. an initial increase and then gradual decrease over 24–45 years. The subsoil never showed a significant temporal effect. Our results generally support greater reductions in nutrient concentrations, SOC and total N in forest soil after WTH compared with SOH in northern temperate and boreal forest ecosystems.

Highlights

In the northern European region, there is considerable interest in using biomass from forest harvesting residues or stumps for bioenergy
All intensified treaments led to significant decreases of soil organic carbon (SOC) stock and total N stock in the forest floor (FF), but relative responses compared with stem-only harvesting were less consistent in the topsoil (TS) and no effects were detected in the subsoil (SS)
The only significant response in pH was found in the forest floor, where pH was significantly lower for whole-tree harvesting (WTH) compared to stem-only thinning (SOH)

Summary

Introduction

In the northern European region, there is considerable interest in using biomass from forest harvesting residues or stumps for bioenergy. As a large part of the nutrients in trees are in the foliage, twigs and branches, removing these tree com ponents will reduce nutrient supply to the soil, which might in the long term increase the risk for nutrient imbalance and reduced forest pro duction (Raulund-Rasmussen et al, 2008; Helmisaari et al, 2011; Tveite and Hanssen, 2013). This may lead to reduced future biomass carbon (C) sequestration and stocks in living forest biomass. There has been concern that stump harvesting might lead to reductions in SOC stocks, due to the missing input from stumps as well as by an enhanced decay following the soil disturbance (Persson, 2017)

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