Bassett Gundale, Kelley
- Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
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 δ15N; carbon dioxide (CO2); dendroecology; isotope natural abundance; wood density; carbon and nitrogen concentration
Published in
Acta Universitatis Agriculturae Sueciae
2026, number: 2026:30
Publisher: Swedish University of Agricultural Sciences
SLU Authors
UKÄ Subject classification
Forest Science
Publication identifier
- DOI: https://doi.org/10.54612/a.7i9rgdiapn
- ISBN: 978-91-8124-247-8
- eISBN: 978-91-8124-277-5
Permanent link to this page (URI)
https://res.slu.se/id/publ/146443