The influence of red alder patches on light, litterfall, and soil nutrients in adjacent conifer stands

To evaluate the distance over which red alder patches influence adjacent conifer stands, we measured light transmission and nutrient contents of soil and litterfall along transects crossing the boundary between alder and conifer stands at three sites (10–15, 20–25, and >= 40 years old) in coastal British Columbia. Light levels were higher in the understory of alder stands than in adjacent conifer stands. In simulated openings, light levels rapidly increased with distance from the alder edge, reaching 60% of full sunlight at south-facing edges, 5 m from northfacing edges, and 2–3 m from east- and west-facing edges. Soil N, NH4-N, NO3-N, and mineralizable N remained elevated for about 10 m from the alder boundary. Nitrogen contents of Douglas-fir seedlings grown in soil from the alder stand were elevated and correlated to soil N concentrations. Nutrient inputs in alder litterfall were positively related to concentrations of total C; total, extractable, and mineralizable N in soils; and the N, P, and B concentrations of seedlings. Alder litterfall drift extended 8–18 m into adjacent conifer stands. The optimal arrangement for alder–conifer mixtures would be alder patches or strips at least 10 m wide and about 20 m apart oriented north to south.