Effects of red alder on growth of Douglas-fir and western redcedar in southwestern British Columbia

Red alder (Alnus rubra [Bong.]), is the most common broadleaf species in the Pacific Northwest, and is widely distributed as a component of mixed stands with conifers. It rapidly overtops associated juvenile conifers and is a strong competitor. However, red alder is also a symbiotic nitrogen fixer and can contribute to improvement in soil nitrogen, as well as serving other roles such as improving biodiversity and forest health. We examined data collected in mixed red alder-conifer plantations established between 1992 and 1999 to examine the effects of red alder on the growth of Douglas-fir (Pseudotsuga menziessi (Mirb.) Franco, Fd) and western redcedar (Thuja plicata Donn., Cw). Results suggest that densities of up to 400 alder trees per hectare may be acceptable and possibly desirable on nitrogen deficient sites. The growth of Fd may be enhanced when red alder density is relatively low at older ages. We found that the competitive effects of red alder on conifer growth were consistently lower than that of the conifers (Fd and Cw) while alder had stronger effects on light availability. The relationship between volume growth and competition indices varied between the two conifer species: (1) for Fd the highest correlations were obtained using a distance-dependent competition index based on the height differences, and (2) for Cw the highest correlations were with distance-independent Lorimer's index. As also expected due to Cw's greater shade tolerance, its volume growth showed less sensitivity to the presence of red alder than Fd.