partir de placettes permanentes ayant des densit& comparables, dCmontre un ordre relatif de la productivit6 des types kologiques tout B fait semblable B l'ordre relatif de lew indice de qualit6 de station. Ces groupes de placettes, qui Ctaient sous-stockkes, prksentaient des accroissements annuels moyens de 2,9,2,0 et 1,5 m3/ha/an pour des stations de classes I+, I-et 11. Les principaux factem hlogiques qui influencent la f d t 6 de ces stations bodales sont la classe de drainage et la presence de drainage oblique. Par contre, 1'Cpassew du dCpSt et, dans une moindre mesure la textwe, semblent Stre des facteurs moins determinants dans cette zone climatique humide. I1 faut enfin noter que les coefficients de variation des indices de qualit6 de station sont importants B l'intkriew d'un m8me type Ccologique.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 central QuCbec. Ecological types consisted of units homogeneous as to their geomorphological deposit, their drainage class and vegetation dynamic. 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 productivity classes generally used for boreal balsam fir stands in Quebec (Linteau 1955), ow results demonstrated the need to recognize a fourth class to distinguish operational groups. Rich mesic seepage sites are clearly more productive than other mesic sites with which they were grouped 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 h itations including imperfectly drained sites, thin (< 50 cm) mesic sites and coarse textured dry sites; 4) and class III (SI c 10.5 m) comprising poorly drained sites. Growth studies of fow ecological types based on permanent plots of similar densities, show the same relative order in productivity as the one established with their site indices. Mean annual growth of these somewhat understocked plots were 2.9,2.0 and 1.5 m3/ha/year for classes I+, Iand I1 respectively. The principal ecological variables that influenced fertility of these boreal sites were drainage class and the presence of seepage. Soil depth and, to a point, soil textwe were not very significative variables in this humid climatic zone. It must also be emphasized that all site indices of ecological types showed high variability.
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