GROWTH OF UNDERSTORY PICEA-GLAUCA, CALAMAGROSTIS-CANADENSIS, AND EPILOBIUM-ANGUSTIFOLIUM IN RELATION TO OVERSTORY LIGHT TRANSMISSION

The potential for use of a partial canopy for controlling growth of Calamagrostis canadensis (Michx.) Beauv., and Epilobium angustifolium L. among regenerating Picea glauca (Moench) Voss saplings was assessed in the understory of 24 established stands in the P. glauca - Viburnum - Rubus pubescens association of the lower boreal cordilleran ecoregion of Alberta. Stand overstories were dominated by Populus tremuloides Michx., P. glauca, or were a mixture of these two species The composition, basal area, and light transmission of the overstory of each stand were measured. Hardwood-dominated overstories transmitted between 14 and 40% of incoming light while P. glauca canopies transmitted between 5 and 11% of light. Cover and height of C. canadensis and E. angustifolium decreased with decreasing light transmission; at 40% light, both species were greatly reduced compared with open-grown conditions and both were virtually eliminated from stands with less than 10% light. The annual height increment of P. glauca saplings increased from 5 cm at 10% light to 25 cm at 40% light; the latter increment was approximately equal to growth in 100% light conditions. The number of buds, the diameter of the current leader, and the height to diameter ratio of the tree also increased with light transmittance.