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 exhi bited stunted growth and chlorotic foliage. The potential inhibitory effect of salal on hemlock was tested by germinating and measuring the primary gr owth of seedlings in the presence of salal litter and different concentrati ons of the litter leachate. Effects of above ground and belowground competi tion of salal and its presumed growth inhibitory effects on western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western red cedar (Thuja plicata Don n ex D, Don) were tested with pot experiments under greenhouse conditions. Seed germination and primary growth of hemlock seedlings were not significa ntly affected by water leachates of salal leaf or litter, nor were they aff ected 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. Comb ined aboveground and belowground competition from salal had the strongest n egative effects on both hemlock and cedar, causing significant reductions i n stem height, basal diameter, and biomass. Belowground competition from sa lal affected the conifers more than aboveground competition. The results su ggest that the growth inhibitory effect of salal on conifers is primarily d ue to competition for nutrients rather than allelopathic effects of litter.