PhD defence: Carl-Fredrik Johannesson

Carl-Fredrik Johannesson defends his thesis:

Forest management effects on soil carbon storage and soil CO₂ and CH₄ fluxes in high latitude forests

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Supervisors:
Associate Professor Klaus Steenberg Larsen, IGN
Senior Research Scientist Jenni Nordén, NINA, Norway
Senior Research Scientist Hanna M. Silvennoinen, NINA, Norway

Assessment Committee:
Professor Mari Pihlatie, University of Helsinki, Finland Professor
Tobias Rütting, University of Gothenburg, Sweden
Associate Professor Jesper Riis Christiansen (chair), IGN

Abstract (shortened):
Forests are important storages and sinks of carbon (C) and high latitude forests specifically store the majority of total ecosystem C in soils, and still accumulate C. Thus, when aiming to preserve or enhance the forest C sink in boreal or northern temperature forests, it is vital that soils are not neglected. The forests of the Nordics and Canada are to a large extent managed, but whether soil C storage and accumulation is affected positively or negatively depends on the specific management practice. Forest harvesting is one of the management practices that has the largest impact on soil C storage and accumulation, but application of biochar and nitrogen fertilizer have also been found to affect the soil C cycle. In Paper I of this thesis the effect of clear-cut harvesting on soil C storage is investigated based on a chronosequence approach using National Forest Soil Inventory data from Sweden, Denmark, Norway, Finland and Canada. 

Soil CO₂ and CH₄ fluxes were site dependent, with lower CO₂ efflux at the clear-cut, mature-unthinned and mature-thinned sites than at the thinned and old-unmanaged sites. Possible explanations for these patterns include C input limitations at the clear-cut site and high soil moisture levels limiting oxygen availability at the mature-unthinned and mature-thinned sites. Net soil CH₄ consumption was highest at the clear-cut site and lowest at the mature-thinned site which we hypothesize is due to differences in soil moisture levels which regulate O₂ availability and diffusion of atmospheric CH₄ into the soil. Application of nitrogen  fertilizer led to decreased soil CO₂ efflux only at the mature-unthinned site, and while it led to a decreased mean net soil CH₄ consumption at all sites, this effect was associated with large uncertainties. The effect of biochar was site dependent,  resulting in decreased soil CO₂ efflux at the clear-cut and thinned sites and positive effect sizes at the mature-unthinned and mature-thinned sites. The mean net soil CH₄ consumption increased in response to biochar addition at the mature-unthinned and mature-thinned sites and decreased at the clear-cut and thinned sites, yet these effects were associated with large  uncertainties. The largest reductions in the net soil CO₂ and CH₄ budgets in response to biochar were achieved at the sites believed to be the driest, and likely due to exacerbated soil moisture limitations. Such increased moisture limitations may have negative effects also on tree growth. Thus, trade-offs between the effects on soil greenhouse gas budgets and C sequestration by trees need to be carefully considered before conclusions on the climate change mitigation potential of biochar can be drawn. Our results suggest that application of nitrogen fertilizer and biochar has limited potential for reducing soil CO₂ and CH₄ budgets in boreal forests dominated by Norway spruce (Picea abies).

In short, this thesis provides evidence of the magnitude and persistence of soil C stock declines after clear-cut harvesting, it provides guidance on how to improve the accuracy of greenhouse gas flux rate estimates and shows that the effects of biochar and nitrogen fertilizer on soil CO₂ and CH₄ fluxes depend on the site-specific soil environment.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk before the defence. After the defence the thesis will become available from the Royal Danish Libary at kb@kb.dk.