Growth intercepts for estimating site quality of young white spruce plantations in north central Ontario

Growth intercepts were used to estimate site index (defined as height of the trees 15 years after breast height (1.3 m) was attained) of dominant, planted white spruce (Piceaglauca (Moench) Voss) in north central Ontario. The growth intercepts were computed using four methods to select internode lengths from series of one through seven internodes, starting at each of eight heights from 0 to 3.0 m. Precision increased rapidly with more internodes and higher starting heights but quickly slowed to only marginal increases. Removing the smallest internode increased precision when growth intercepts started below 1.3 m. Removing the largest and both the smallest and largest internodes reduced precision from all starting heights. The most precise and practical growth intercepts used the average length of the first three, four, and five internodes above 2.0 m, explaining 83, 85, and 89% of the variation in site index, respectively.