Red alder (Alnus rubra [Bong.]) (hereafter referred to as alder) is the most common broadleaf tree species in the coastal Pacific Northwest, and occurs in pure stands and in mixed stands with coniferous species (Deal and Harrington 2006). Alder in conifer dominated stands can be beneficial because it improves soil nitrogen through fixation of atmospheric nitrogen, and it adds biodiversity. In young conifer stands, alder is considered a strong competitor due to its rapid juvenile growth, which can greatly reduce light availability and lead to competition for canopy growing space. In 1992, a long-term experimental project (EP 1121.01) was initiated to improve our understanding of both beneficial and competitive effects of alder in mixture with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western redcedar (Thuja plicata Donn.) (hereafter referred to as cedar). The objectives of this research are to (1) examine the effects of alder density on soil nitrogen and foliar nitrogen in Douglas-fir, (2) study the effects of alder density on total stand volume, (3) investigate the effects of alder density on conifer growth, and (4) provide an assessment of the effects of plot radius on the estimation of competition effects. Study results found significant variation in Douglas-fir and cedar response across sites. Soil mineralizable nitrogen was found to increase with increased alder density and, Douglas-fir stand volume was found to have benefited from 200 alder per hectare at age 15. For cedar 100 alder per hectare was found the most beneficial. This Extension Note provides a summary of recently published research (Fang 2018; Fang et al. 2019).
Fang, Chengdong, Comeau, Philip G.; Harper, George. 2019. Influence of Red Alder Density on Growth of Douglas-fir and Western Redcedar: 20-year Results. Ministry of Forests, Lands, Natural Resource Operations and Rural Development. Extension Note (FLNRORD). EN122