Height growth pattern of white spruce in natural subregions in Alberta, Canada

Based on the provincial stem analysis data of 1018 felled dominant and codo minant trees, height growth patterns of white spruce were compared among th e five major natural subregions (CMX - central mixedwood, DMX - dry mixedwo od, WMX - wetland mixedwood, UFH - upper foothills, and LFH - lower foothil ls) in Alberta. The comparison used the ratio of height at 70 and 30 years of breast height age (Z ratio) as a quantitative measure of height growth p attern (i.e., the response variable), site index (height at breast height a ge of 50 years) as the covariate, and natural subregion as the factor. Resu lts indicated that (1) the height growth pattern in WMX natural subregion w as significantly different from other natural subregions and (2) no signifi cant differences in height growth pattern were found among other natural su bregions. Two reference-age invariant polymorphic height and site index cur ves were developed: one for the WMX natural subregion and the other for the rest of the natural subregions. Comparisons between the two curves and the previously developed provincial curve indicated that, for the same site in dex, trees in WMX subregion grow much slower on good sites (site index > 15 m) after site index age. When the provincial height and site index curve w as applied to WMX natural subregion, large errors (up to 25%) in gross volu me estimation were found. However, volume estimation errors were very small (<2.5%) when the provincial curve was applied to the other four natural su bregions. It is recommended that the natural subregion-based curves should be used for predicting white spruce site index or height at any age in WMX natural subregion. (C) 2000 Elsevier Science B.V. All rights reserved.