Microsite controls on tree seedling establishment in conifer forest canopy gaps

Tree seedling establishment and growth were studied in experimental canopygaps to assess the effect of heterogeneity of regeneration microsites within and among gapsin mature conifer forests. Seedlings were studied for two years in closed-canopy areas andin 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 ofcentral Oregon and southern Washington. USA. We examined the relative importance ofsubstrate 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 (Pseudotsugamenziesii), and western hemlock (Tsuga heterophylla). Seedlings originating from seedsown on experimental microsites were compared with seedlings regenerating naturally ona range of microsites.
Seedling establishment was greater on decayed wood than on forest floor or mineralsoil in closed-canopy areas, particularly for Tsuga. Differences in establishment amongsubstrates declined with increasing gap size, although establishment on the forest floortended to be low in portions of large gaps exposed to direct solar radiation. In exposedareas, shade from woody debris and shade cloth facilitated establishment for all species,while shade from understory vegetation aided the establishment of Pseudotsuga but notthat 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 tendedto be greatest on forest floor and lowest on decayed wood.
Establishment and growth of the different species was related to differences in seedmass and tolerance of environmental extremes. Heterogeneity at the seedling scale (2 m) associated with gap sizeand within-gap position. Thus the gap partitioning hypothesis, which emphasizes gap sizeand within-gap position, may be too simple to account for the role of microsite heterogeneityand species' regeneration traits on the development of mature forests.
Key words: Abies amabilis; canopy gaps; disturbance; heterogeneity; microsites: old-growth for-est; Pacific Northwest; Pseudotsuga menziesii; seedling establishment; succession; Tsuga heterophylla.