QUALITY INDEXES FOR BALSAM FIR SITES IN T HE ECOLOGICAL HUMID BALSAM FIR WHITE BIRCH SUBDOMAIN

Balsam fir (Abies balsamea (L) Mill.) site indices (height at 50 years at stump height) were estimated for the principal ecological types of the boreal and humid balsam fir-white birch climatic subdomain in cen tral Quebec. Ecological types consisted of units homogeneous as to the ir geomorphological deposit, their drainage class and vegetation dynam ic. Mean site index varied from 15.9 m for rich mesic seepage sites to 9.4 m for very thin and poorly drained sites. To the three productivi ty classes generally used for boreal balsam fir stands in Quebec (Lint eau 1955), our results demonstrated the need to recognize a fourth cla ss to distinguish operational groups. Rich mesic seepage sites are cle arly more productive than other mesic sites with which they were group ed in the past. The proposed classes are: 1) I+ (SI > 15 m) comprising rich seepage sites; 2) class I-, comprising mesic sites (SI from 13.5 to 15 m); 3) class II (SI from 10.5 to 13.4 m) which comprises sites that possess moderate growth limitations including imperfectly drained sites, thin (< 50 cm) mesic sites and coarse textured dry sites; 4) a nd class m (SI < 10.5 m) comprising poorly drained sites. Growth studi es of four ecological types based on permanent plots of similar densit ies, show the same relative order in productivity as the one establish ed with their site indices. Mean annual growth of these somewhat under stocked plots were 2.9, 2.0 and 1.5 m(3)/ha/year for classes I+, I-and II respectively. The principal ecological variables that influenced f ertility of these boreal sites were drainage class and the presence of seepage. Soil depth and, to a point, soil texture were not very signi ficative variables in this humid climatic zone. It must also be emphas ized that all site indices of ecological types showed high variability .