What lies beneath? - Using the National Forest Inventory tree core archive to assess environmental changes in Swedish boreal forests

Sep 30, 2026

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Boreal forests represent a major global carbon (C) sink, yet the persistence of their enhanced growth under environmental change—such as rising atmospheric carbon dioxide (CO2)—remains uncertain. This thesis investigates long-term shifts in boreal forest functioning by integrating stemwood traits, stable-isotope chronologies, and structural properties from the Swedish National Forest Inventory’s tree-core archive spanning the mid-20th century to today. Results show a widespread decline in wood stable nitrogen isotopes (δ15N), signaling reduced nitrogen (N) availability, explained primarily by rising CO2 rather than N deposition or temperature variability. Evidence for Progressive Nitrogen Limitation (PNL) emerges from contrasts between N2-fixing and non-fixing genera and from altered soil N cycling processes, while wood chemistry, density, and nutrient concentrations vary across time and space.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages112

Reading Time4 h 42 min

AccessFree to download

Contents

List of publications

List of figures

Abbreviations

1. Introduction

1.1 Boreal Forests

What lies beneath? Using the National Forest Inventory tree core archive to assess environmental changes in Swedish boreal forests Kelley R. Bassett Faculty of Forest Sciences Department of Forest Ecology and Management Umeå DOCTORAL THESIS Umeå 2026 Acta Universitatis Agriculturae Sueciae 2026:30 Cover: Swedish forest. Photo: Martin Fryklund ISSN 1652-6880 ISBN (print version) 978-91-8124-247-8 ISBN (electronic version) 978-91-8124-277-5 [https://doi.org/10.54612/a.7i9rgdiapn](https://doi.org/10.54612/a.7i9rgdiapn) © 2026 Kelley R. Bassett, [https://orcid.org/0009-0003-3151-0740](https://orcid.org/0009-0003-3151-0740) Swedish University of Agricultural Sciences, Department of Forest Ecology and Management, Umeå, Sweden The summary chapter is licensed under CC BY 4.0. To view a copy of this license, visit [https://creativecommons.org/licenses/by/4.0/. Other](https://creativecommons.org/licenses/by/4.0/. Other) licenses or copyright may apply to illustrations and attached articles. Print: SLU Grafisk service, Uppsala 2026 What lies beneath? Using the National Forest Inventory tree core archive to assess environmental changes in Swedish boreal forests Abstract Boreal forests represent a major global carbon (C) sink, yet the persistence of their enhanced growth in response to environmental change, e.g., rising atmospheric carbon dioxide (CO2), remains uncertain. In this thesis, I investigate long-term changes in boreal forest functioning using stemwood traits, stable-isotope chronologies, and structural properties derived from the unique tree-core archive of the Swedish National Forest Inventory, spanning the mid-20th century to the present. A central result is the widespread and consistent decline in wood stable nitrogen isotopes (δ15N), indicating decreased nitrogen (N) availability in boreal forests. This trend is observed across large gradients in atmospheric N deposition and is best explained by rising CO2 rather than N deposition or temperature variability. Comparisons between N₂-fixing and non-fixing tree genera further demonstrate that these declines are driven by changes in soil N cycling processes, providing strong evidence for Progressive Nitrogen Limitation (PNL) as the driving mechanism. In parallel, stemwood chemistry and structure show significant variability over time and space. Wood N concentrations were stable, while C concentrations and C:N ratios declined. Wood density and C concentration also varied with species, latitude, elevation, and tree size, challenging the assumption of constant traits used in forest C accounting. Methodologically, this work highlights the limitations of conventional dendrochronological approaches for mobile elements such as N, which I have shown require improved sampling strategies. Overall, my findings indicate that boreal forests are undergoing increasing N limitation under elevated CO2, which may constrain their future capacity to sequester C. Keywords: boreal forests, progressive nitrogen limitation (PNL), wood δ 15N, carbon dioxide (CO2), dendroecology; isotope natural abundance, wood density, carbon and nitrogen concentration Vad döljer sig undan ytan? Användning av Riksskogstaxeringens borrspånsarkiv för att bedöma miljöförändring i Sveriges borealaskogar Sammanfattning De boreala skogarna utgör en viktig global kolsänka (C), men det är osäkert om deras ökade tillväxt som svar på miljöförändringar, till exempel stigande koldioxidhalter (CO2) i atmosfären, kommer att bestå. I denna avhandling undersöker jag långsiktiga förändringar i boreala skogars funktion med hjälp av stamvedsegenskaper. Kronologier, som sträcker sig från mitten av 1900-talet till idag, baserade på stabila isotoper och strukturella egenskaper har skapats med hjälp av det unika arkivet med borrkärnor från den svenska Riksskogstaxeringen. Ett centralt resultat är den utbredda och konsekventa minskningen av stabila kväveisotoper (δ15N) i stamveden, vilket indikerar

FAQs

What does the thesis aim to determine about boreal forests under environmental change?

It examines whether enhanced boreal forest growth in response to environmental change persists, focusing on long-term changes in forest functioning.

Which key finding relates to nitrogen availability in boreal forests?

Wood stable nitrogen isotopes (δ15N) show a widespread, consistent decline, indicating decreased nitrogen (N) availability.

What mechanism does the thesis support for the observed nitrogen limitation?

Comparisons between N2-fixing and non-fixing tree genera and evidence on soil N cycling processes support Progressive Nitrogen Limitation (PNL), explained best by rising CO2 rather than nitrogen deposition or temperature variability.

Sep 30, 2026

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