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Doctoral thesis, 2015

Stand development effects on N₂-fixation and seedling performance in northern boreal forests

Stuiver, Babs

Abstract

For large parts of the Swedish boreal forests, stand development starts with clear-cutting. Still, much is unknown about how stand development affects ecosystem processes and how these processes are driven by changes in vegetation and soil properties that occur during stand development. In this thesis, the processes of feather moss associated N₂-fixation and tree seedling regeneration are examined along a post clear-cut chronosequence which consisted of 32 stands; eight in each of four development stages with mean ages of 4, 16, 34, and 123 years. Stand development had statistically significant effects on the total amount of N fixed at the stand level. The N₂-fixation rates initially increased after clear-cutting and peaked in the 16-year old stands at rates of 1.0 kg N ha-1 yr-1, after which the rates declined and remained at levels similar to those found in young clear-cut stands even when stands had reached maturity. This pattern was associated with high moss biomass and positive effects of local conditions on N fixed per kg moss mass in the 16-year old stands. The cumulative contribution of N₂-fixation to the total accrual of N per ha in the aboveground vegetation and humus layer during one stand rotation was about 9%. Stand development had negative effects on the growth of Pinus sylvestris seedlings as a consequence of changes in the canopy. In contrast, the effect of stand development on Picea abies seedling growth was mainly driven by soil properties, and seedlings performed best when grown in soils originating from the 34-year old stands. Seedling mortality decreased with stand age for both P. sylvestris and P. abies seedlings, which was mainly caused by a positive effect of the canopy. The effect of several moss species on P. sylvestris seedling growth was further explored in a greenhouse mesocosm experiment. Seedling growth was most negatively affected by Hylocomium splendens and in pots with high moss biomass, likely driven by differences in moss depth related to moss species and moss biomass. This thesis shows that stand development has strong effects on N₂-fixation rates and the performance of P. sylvestris and P. abies seedlings in production forests. However, these effects and their drivers do not necessarily respond linearly to stand development and can differ between moss and tree species with potential long-term consequences for the functioning of boreal forests.

Keywords

bryophytes; chronosequence; clear-cut; forest regeneration; forest nitrogen balance; forest succession; logging; Norway spruce; Scots pine; understory vegetation

Published in

Acta Universitatis Agriculturae Sueciae
2015, number: 2015:108
ISBN: 978-91-576-8414-1, eISBN: 978-91-576-8415-8
Publisher: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences

Authors' information

Stuiver, Babs
Swedish University of Agricultural Sciences, Department of Forest Ecology and Management

UKÄ Subject classification

Forest Science
Ecology

URI (permanent link to this page)

https://res.slu.se/id/publ/68588