Influence of social status on crown geometry and volume increment in regular and irregular black spruce stands

A model of bole volume increment based on crown dimensions and tree social status is analyzed for its ability to predict bole volume increment independently of stand structure. Data were collected in two boreal black spruce (Picea mariana (Mill.) BSP) stands in Quebec. One stand is even-aged and of seed origin, and the other stand is uneven-aged and of layer origin. A varying parameter approach is taken to show that the crown profile of black spruce depends on competition. Formal expressions for crown surface area and volume are developed from the crown profile and are used to derive a potential growth function for bole volume. Three social status indices are considered to characterize competition experienced by a subject tree: a distance-dependent competition index relating differences of height between the tree and its neighbors to the distance separating them, and two distance-independent indices, one based on tree height relative to dominant stand height and one based on height growth relative to potential height growth of a competition-free tree. These indices are combined with the potential growth function to successfully model bole volume increment. Both stands have an equal weight in the unexplained residual variance. Furthermore, all three social status indices perform equally well at predicting bole volume increment. The use of a potential growth function with crown parameters is further discussed.