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 Ca lamagrostis canadensis (Michx.) Beauv., and Epilobium angustifolium L. among regenerating Picea glauca (Moench) Voss saplings was assessed i n the understory of 24 established stands in the P. glauca - Viburnum - Rubus pubescens association of the lower boreal cordilleran ecoregio n of Alberta. Stand overstories were dominated by Populus tremuloides Michx., P. glauca, or were a mixture of these two species The composit ion, 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. angustif olium 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. T he annual height increment of P. glauca saplings increased from 5 cm a t 10% light to 25 cm at 40% light; the latter increment was approximat ely equal to growth in 100% light conditions. The number of buds, the diameter of the current leader, and the height to diameter ratio of th e tree also increased with light transmittance.