Effects of red alder and paper birch competition on juvenile growth of three conifer species in southwestern British Columbia

We examined the effects of competition from red alder (Alnus rubra Bong.) and paper birch (Betula papyrifera Marsh.) on the growth of western redcedar (Thuja plicata Donn), western hemlock (Tsuga heterophylla Sarg.), and Douglas-fir (Pseudotsuga menziesii Franco) at a site near Maple Ridge, British Columbia, Canada. At this site, the three coniferous species and two broadleaf species had been planted in 1999 as part of a long-term experiment examining effects of broadleaf density on conifer growth. Red alder and paper birch were planted alone and as a 50:50 mixture at four densities (0, 277, 556, and 1150stemsha⁻¹). Red alder density was positively correlated with total available nitrogen in soil measured using ion-exchange membranes whereas density of paper birch was not. For Douglas-fir only, foliar N concentration was positively related to red alder density. Relationships between stem volume increment and competition indices varied between the three conifer species: (1) For Douglas-fir the highest correlations were obtained using a distance-independent competition index based on broadleaf crown surface area and diffuse transmittance (DIFN) measured using LAI-2000 plant canopy analyzers had the lowest R ² of the competition measures tested; (2) for western redcedar the highest correlations were with DIFN and the second best competition measures were distance-dependent indices; (3) for western hemlock broadleaf competition did not have significant effects on growth. Height of each species at the time of measurement and crown surface area appear to be more important than shade tolerance in affecting conifer growth. At this site, significant reductions in stem volume (30%) of Douglas-fir and western redcedar occurred when red alder density exceeded 500stemsha⁻¹ while paper birch was not detrimental to western redcedar growth until densities exceeded 1000stemsha⁻¹ and birch density did not significantly affect Douglas-fir. The influence of red alder on soil N and foliar N of Douglas-fir may be leading to a weaker relationship between growth and DIFN for Douglas-fir compared to that for western redcedar.