Canadian Forest Service Publications
Ground layer composition may limit the positive impact of precommercial thinning on boreal stand productivity. 2017. Pacé, M.; Barrette, J.; Fenton, N.; Paré, D.; Bergeron, Y. For. Sci. 63(6):559-568.
Issued by: Laurentian Forestry Centre
Catalog ID: 38917
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In the boreal forest, ground-layer composition may modulate the effects of precommercial thinning (PCT) on stand productivity by affecting tree growth conditions. Based on data from 15 years of PCT monitoring in black spruce (Picea mariana) and jack pine (Pinus banksiana) stands, the objectives of this study were to investigate the effects of PCT on ground-layer composition and the way ground-layer composition is related to tree growth, stand productivity, and the PCT impact on stand productivity. PCT favored lichen expansion in xeric sites. The positive impact of PCT on stand productivity after 15 years was lower in sites with high year-one lichen cover, suggesting that the aboveground positive effect of PCT on growth may have been mitigated by a belowground negative feedback resulting from lichen expansion in xeric sites. Although Sphagnum spp. cover was not affected by PCT, 15-year increase in stand productivity was lower in sites with high year-one Sphagnum spp. cover. These results suggest that xeric stands with high lichen cover should not be targeted for PCT because of either null or negative effects on stand productivity. Subhydric stands with high Sphagnum spp. cover should also be avoided because of lower potential stand productivity.
Plain Language Summary
The results of this research show that stands established on soils with a lichen cover that drain rapidly and those on soils with a sphagnum cover that drain poorly should not be favoured for precommercial thinning operations due to the mixed effects of this treatment on their productivity.
This study was based on the monitoring of black spruce and jack pine stands over a period of 15 years following precommercial thinning. The goals were to study the effect of precommercial thinning on the composition of ground vegetation strata, its impact on stand productivity, and to assess how the composition of these vegetation strata influences tree growth and stand productivity.
Precommercial thinning is conducive to the spread of lichen on sites that drain rapidly. Fifteen years after the treatments, tree growth and stand productivity were less significant in sites where lichen was highly present. The expected positive effect of precommercial thinning was reduced by the spread of lichen. Although the sphagnum cover was not affected by precommercial thinning, stands were less productive in sites where sphagnum was highly present.
The results will help forest managers target sites for precommercial thinning operations more effectively, enabling them to select sites where the positive effects of the operations will be most significant.