Canadian Forest Service Publications
Effect of harvest gap formation and thinning on soil nitrogen cycling at the boreal-temperate interface. 2017. Coulombe, D.; Sirois, L.; Paré, D. Can. J. For. Res. 47: 308-318.
Issued by: Laurentian Forestry Centre
Catalog ID: 37518
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In mixedwood forest, different types of commercial thinning that generate different gap sizes are being tested as alternatives to clearcutting to create forest stands with an irregular structure that would emulate the pre-industrial forests. The main goal of this study was to investigate the soil nitrogen (N) dynamics in response to two partial harvesting treatments, used alone or in combination: commercial thinning creating tree-size canopy gaps and harvest gaps creating 0.05 ha gaps. In a 30 year old balsam fir (Abies balsamea (L.) Mill.) stand, both treatments, alone or in combination, as well as unharvested controls were compared on replicated 0.75 ha plots. The most noticeable changes following treatments were observed in gaps, while commercial thinning did not significantly influence any of the parameters assessed. In gaps, increases in N mineralization rates and mineral N concentrations and proportions (NO3−-N and NH4+-N) relative to dissolved organic N were observed. Our results suggest that these changes are caused by the increase in soil temperature and water content. In these forests, the response threshold of the N cycling regarding the size of the intervention would therefore be located between the removal of one or a few trees (one to three stems, 6–12m2) and a gap of 500m2. Other studies conducted in different climate and forest types have shown that this threshold could be of equivalent or of a smaller size. These findings will contribute to optimizing our management strategies regarding partial cuts or small-scale clearcuts.
Plain Language Summary
The purpose of this study was to compare the impact of two types of treatment in 30-year-old fir forests on soil nitrogen dynamics. These silvicultural treatments were the partial cutting of 35% of the trees and the creation of a 500 m2 gap.
Changes in soil nitrogen dynamics relative to those of forests that had not been cut were only perceptible in gaps. Partial cutting did not have a significant impact on these dynamics. Changes in nitrogen dynamics affect water quality and determine plant composition, among other things. Thanks to this study, the researchers were able to determine from which size of opening changes in nitrogen dynamics begin to occur in partially cut stands. In gaps, these changes seem to be attributable to an increase in temperatures and a rise in soil moisture levels.