Effects of opening size and microsite on performance of planted tree seedlings in high-elevation Engelmann spruce–subalpine fir forests managed as mountain caribou habitat in British Columbia

Abstract

Old-growth, high-elevation spruce–fir forests of the Quesnel Highland in south-central British Columbia provide winter habitat for threatened mountain caribou (Rangifer tarandus caribou (Gmelin)) and contain valuable wood resources. A group selection silvicultural system may protect caribou habitat and allow some timber extraction, if combined with timely regeneration and an extended rotation length. Sporadic seed production can limit natural regeneration and planting may be required to ensure successful regeneration after harvest. However, the opening sizes and planting microsites required for adequate tree growth and survival in these high-elevation sites are unknown. We examined, for 15years following planting, how growth and survival of planted subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry ex. Engelm.), and lodgepole pine (Pinus contorta Dougl. Ex Loud.) varied with harvest opening size (0.03, 0.13, 1.0ha), planting microsite (raised, protected, rotten wood, scarified, and a control) and elevation.Survival and growth was less in the higher elevation sites and generally increased with opening size, with the increase greater in the lower elevation sites. Survival varied from 12% for lodgepole pine in small openings at higher elevation to 82% for fir in large openings at lower elevation. Overall, fir had the greatest survival. Fir growth increased the most with opening size at higher elevation; pine growth increased the most with opening size at lower elevation. Survival was generally greater on raised, protected, or rotten wood microsites. Growth varied less across microsites than did survival and also varied less across microsites than across opening sizes or elevations. Increased seedling growth and survival for all species was associated with treatments that had earlier snowmelt and higher soil temperatures. Two cycle budworm (Chroistoneura biennis Freeman) decreased seedling growth in years of heavy infestation. In higher elevation sites, pine had the greatest average height, but relatively low survival and vigor. The poor survival of pine, its lower long-term projected growth rates, and a crown structure which does not facilitate lichen accumulation and its accessibility to caribou, make pine unsuitable for regenerating these forests managed for mountain caribou. Using the group selection silvicultural system on appropriate sites with openings greater than 0.13ha and with optimal planting microsites should allow for some extraction of timber while maintaining the old-growth attributes needed for caribou habitat in these subalpine forests.