Microsite Controls on Tree Seedling Establishment in Conifer Forest Canopy Gaps

Abstract

Tree seedling establishment and growth were studied in experimental canopy gaps to assess the effect of heterogeneity of regeneration microsites within and among gaps in mature conifer forests. Seedlings were studied for two years in closed-canopy areas and in recently created gaps ranging in size from 40 to 2000 m2 in four stands of mature (90–140 yr) and old-growth (>400 yr) Douglas-fir forest in the western Cascade Range of central Oregon and southern Washington, USA. We examined the relative importance of substrate type, shade from logs and shade cloth, density of understory vegetation, gap size, and forest age on the success of Pacific silver fir (Abies amabilis), Douglas-fir (Pseudotsuga menziesii), and western hemlock (Tsuga heterophylla). Seedlings originating from seed sown on experimental microsites were compared with seedlings regenerating naturally on a range of microsites. Seedling establishment was greater on decayed wood than on forest floor or mineral soil in closed-canopy areas, particularly for Tsuga. Differences in establishment among substrates declined with increasing gap size, although establishment on the forest floor tended to be low in portions of large gaps exposed to direct solar radiation. In exposed areas, shade from woody debris and shade cloth facilitated establishment for all species, while shade from understory vegetation aided the establishment of Pseudotsuga but not that of Tsuga. Establishment in shaded portions of gaps declined with vegetation and shade-cloth cover. Seedling size increased with gap size, decreased with level of shade, and tended to be greatest on forest floor and lowest on decayed wood. Establishment and growth of the different species was related to differences in seed mass and tolerance of environmental extremes. Heterogeneity at the seedling scale (<10 cm) often overrode larger-scale environmental gradients (>2 m) associated with gap size and within-gap position. Thus the gap partitioning hypothesis, which emphasizes gap size and within-gap position, may be too simple to account for the role of microsite heterogeneity and species’ regeneration traits on the development of mature forests.