Growth and survival of black spruce trees after commercial thinning: influence of root grafting and tree competition

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Trees are traditionally considered as individual entities competing with each other for resources. We know, however, that they are not distinct individuals because they may share substances or directly affect each other's growth through the frequent presence of root grafts. Silvicultural practices such as thinning are designed to increase merchantable yield and profitability of forest stands by reducing the number of stems and concentrating resources (light, water and nutrients) on remaining crop trees. In many cases, however, tree growth stagnates or decreases after thinning (labeled as «thinning shock») and stands often show high mortality rates. We have shown for jack pine that root grafting could explain growth decreases trees after thinning because residual trees continue to support the root and stump biomass of removed trees. The main goal of this project is to determine the ecological significance of root grafting in pure and mixed black spruce stands. Specifically, we want to 1) understand the role of root grafting in the response of black spruce stands to thinning in terms of growth, mortality post treatment and windthrow susceptibility. Secondly, we want to 2) examine if root grafting offers a competitive advantage to trees against trees of another or the same species using pure and mixed black spruce-jack pine stands and 3) develop a model, that uses above-ground traits to predict belowground root grafting dynamics. Thinned and unthinned stands will be hydraulically excavated to expose roots systems, and root, stems and root grafts will be retrospectively analyzed using dendrochronology techniques.