Trees are larger on southern slopes in late-seral conifer stands in northwestern British Columbia

Abstract

Mid-latitude forests commonly show increased productivity and ultimately produce larger trees on shaded, northern aspects compared with those on sunny, southern aspects. Little research has been conducted on this phenomenon at higher latitudes where solar radiation is less available. We examined patterns of canopy tree size in a set of 142 naturally regenerated, late-seral conifer stands in northwestern British Columbia, Canada, at latitudes of 54°N–56°N. The height, diameter, and total stem volume of three shade-tolerant conifers (western hemlock (Tsuga heterophylla (Raf.) Sarg.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), and hybrid spruce (Picea glauca × engelmannii (Moench) Voss)) and shade-intolerant lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) were analyzed. In contrast to most previous studies, mean canopy tree height, diameter, and volume were greatest on southern aspects and lowest on northern aspects for the subset of sites with pronounced slopes (≥30%). There was some variation in patterns among species, with western hemlock responding most strongly to topography, and species-specific effects of topography on height–diameter allometry. We suggest that decreased levels of radiation at northern latitudes and increased light and warmer temperatures on steeper, southern aspects promote growth. In contrast, colder, shaded, northern aspects with increased snow accumulation, delayed snowmelt, and decreased nitrogen mineralization inhibit growth. Our results, together with published data, suggest that a latitudinal shift from higher forest productivity on northern aspects to higher productivity on southern aspects typically occurs between 40°N and 50°N.