FACTORS LIMITING EARLY GROWTH OF WESTERN REDCEDAR, WESTERN HEMLOCK AND SITKA SPRUCE SEEDLINGS ON ERICACEOUS-DOMINATED CLEAR-CUT SITES IN COASTAL BRITISH-COLUMBIA

A 3 year field and pot study was conducted to determine the effects of several biotic and abiotic factors on the early growth of western hem lock (Tsuga heterophylla (Raf.) Sarg.), western redcedar (Thuja plicat a, Donn) and Sitka spruce (Picea sitchensis, Bong. Carr.) seedlings es tablished on 2- and 8-year-old clearcut sites previously occupied by o ld-growth western hemlock and western redcedar forests (referred to as younger and older CH, respectively), and on adjacent 2-year-old clear cut sites previously occupied by second-growth western hemlock and ama bilis fir (Amabilis amabilis (Dougl.) Forbes) forests (referred to as younger HA) in coastal British Columbia. The objective of the study wa s to determine which factors are associated with the poor growth chara cteristic of the salal (Gaultheria shallon, Pursh) dominated CH clearc ut sites. No soil moisture deficits were measured on any of the three types of clearcut sites at any time of year. The best seeding growth w as on the younger HA sites followed by the younger CH sites and then t he older CH sites. The better growth on the younger HA sites was assoc iated with a higher availability of N and P in the first 20 cm depth o f the forest floor. No differences in matric soil water potential and pH, and only small differences in soil temperature were measured betwe en the three types of clearcut sites. Complete removal of the competin g vegetation on the younger and older CH sites resulted in an increase in conifer seedling growth and in the availability of N (22-40%) and P (15-32%); however, it did not affect cellulose decomposition and mat ric soil water potential, and increased soil temperature only slightly . Both western hemlock and Sitka spruce seedlings were very responsive to differences in nutrient availability measured between types of cle arcut sites and planting treatments. In contrast, western redcedar was not responsive. All three conifer species had very high mycorrhizal c olonization on the younger CH sites, and this was not altered by the r emoval of the competing vegetation (mainly salal). These results sugge st that the nutritional stress and poor growth of conifers on salal-do minated CH clearcut sites in coastal British Columbia can be explained by: (1) inherently low forest floor nutrient availability; (2) compet ition between salal and conifer seedlings for scarce nutrients and nut rient immobilization in salal; (3) declining nutrient availability a f ew years after clearcutting and slashburning.