Growth inhibitory effects of salal on western hemlock and western red cedar

Salal (Gaultheria shallon Pursh) regenerates vigorously after harvesting in the cedar-hemlock (CH) forests of coastal British Columbia, Canada. Eight years after harvesting, regenerating conifers on salal-dominated sites exhibited stunted growth and chlorotic foliage. The potential inhibitory effect of salal on hemlock was tested by germinating and measuring the primary growth of seedlings in the presence of salal litter and different concentrations of the litter leachate. Effects of above ground and belowground competition of salal and its presumed growth inhibitory effects on western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western red cedar (Thuja plicata Donn ex D, Don) were tested with pot experiments under greenhouse conditions. Seed germination and primary growth of hemlock seedlings were not significantly affected by water leachates of salal leaf or litter, nor were they affected when grown directly on partially decomposed salal litter, The primary root growth of hemlock seedlings was significantly increased in CH humus. Salal litter had no significant effect on hemlock and cedar seedlings. Combined aboveground and belowground competition from salal had the strongest negative effects on both hemlock and cedar, causing significant reductions in stem height, basal diameter, and biomass. Belowground competition from salal affected the conifers more than aboveground competition. The results suggest that the growth inhibitory effect of salal on conifers is primarily due to competition for nutrients rather than allelopathic effects of litter.