Dynamics and morphology of giant circular patterns of low tree density in black spruce stands in northern Quebec

Giant circular patterns of low tree density in black spruce (Picea mariana) stands were investigated in the Abitibi region of Quebec. We used dendroch ronological techniques to test the hypotheses that ring patterns of low tre e density are caused either by radial changes in spruce mortality or produc tivity. Seven circles were sampled. We found no gradient in the age of spru ce along circle radii suggesting that rings of low tree density do not expa nd radially, that is, they are not spatially dynamic entities. The results indicate, however, that spruce trees were less dense and productive within the rings due to excessive moisture in the soil. Measurements of surface el evation, thickness of the organic layer and elevation of the mineral substr ate across the circles revealed that a depression in the mineral soil benea th the rings traps the surface water and this area of poor drainage seems t o prevent the establishment of black spruce within the rings. The origin of the ring-shaped depressions was attributed to geological or geomorphologic al causes.