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

Abstract

Based on the provincial stem analysis data of 1018 felled dominant and codominant trees, height growth patterns of white spruce were compared among the five major natural subregions (CMX — central mixedwood, DMX — dry mixedwood, WMX — wetland mixedwood, UFH — upper foothills, and LFH — lower foothills) 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 pattern (i.e., the response variable), site index (height at breast height age of 50 years) as the covariate, and natural subregion as the factor. Results indicated that (1) the height growth pattern in WMX natural subregion was significantly different from other natural subregions and (2) no significant differences in height growth pattern were found among other natural subregions. Two reference-age invariant polymorphic height and site index curves 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 index, 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 was applied to WMX natural subregion, large errors (up to 25%) in gross volume estimation were found. However, volume estimation errors were very small (<2.5%) when the provincial curve was applied to the other four natural subregions. 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.