COS 255-3 - Changes in understory vegetation 12 years after commercial thinning and gap creation in naturally regenerated stands and white spruce plantations

Background/Question/Methods

Commercial thinning has increased in popularity over the last few decades, with new methods being introduced to better attain ecosystem-based management objectives. The understory, which fulfills numerous ecosystem functions, can be altered by these sylvicultural practices. Under certain circumstances, commercial thinning and gaps could help in the restauration of the complex structure of naturally regenerated and planted stands. However, the response of the understory to these treatments is still not well known. Our objective was to evaluate the effect of these treatments on the understory community in naturally regenerated and white spruce (Picea glauca) planted stands in Eastern Québec. Understory communities were surveyed before thinning and gap creation, and 1, 2 and 12 years after treatment. The experimental design was composed of experimental units that were commercially thinned (none, thinning from below and thinning by the release of 50 or 100 crop trees per hectare) combined with 3 types of gaps (none, 100m² and 500m²) where we compared the understory composition, functional assemblage and structure.

Results/Conclusions

Our results showed differences in specific diversity, functional richness and vertical structure between naturally regenerated stands and plantations. Gaps had the greatest effect on the 3 biodiversity characteristics. Large gaps (500m²) caused the most change on the understory communities. When compared to small gaps (100m²), large gaps favoured intolerant, ruderal species as well as the development of dense shrub and herbaceous layers. The effect of large gaps was greater in planted stands than in naturally regenerated ones. Commercial thinning, on the other hand, did not affect the understory communities. Our results suggest that commercial thinning had limited effect on understory plant diversity but floristic changes persist up to 12 years after small to moderate gap creation, particularly in plantations.