Thinning effect on light regime and growth of underplanted red oak and black cherry in post-agricultural forests of south-eastern Canada

Over the last century, north-eastern North America has seen the gradual abandonment of much agricultural land that had become unsuitable for modern practices. This shift in land-use has released large areas that are now dominated by early-successional forests with limited mid- and late-successional species regeneration. Enrichment underplanting, with the partial preservation of the existing vegetation to serve as a shelterwood, has been proposed to restore the production of valuable timber. The present study reports mid-term (8-12 year) results from an enrichment planting experiment located at two sites in the St-Lawrence River Valley, eastern Canada. The objective was to identify the required frequency of low-intensity thinnings necessary to secure an optimal density of promising stems of black cherry (Prunus serotina Ehrh.) and red oak (Quercus rubra L.). Partial thinning treatments were applied during the 3rd and 7th growing seasons and compared to a single thinning applied at the 7th year. Each thinning treatment increased available light to 20-30 % in the understory (measured the following year at 2 m from ground level). The height of red oaks released twice was almost double that of red oaks thinned only once. The effect on black cherry was less dramatic and significant only at one of the two sites. A higher intensity or an earlier application of thinning regimes may be required for black cherry. The early thinning allowed a larger number of planted trees to react positively to the second thinning. The practice of double thinning over a period of 12 years allowed for the production of a significant number of competitively successful trees for the landowner.