Temporal variability of aging error and its potential effects on black spruce site productivity estimations

Abstract

As a black spruce (Picea mariana (Mill.) BSP) tree expands its adventitious rooting system, its root collar is amalgamated into the stump. Total tree age determined by ring counts at ground level or 1m height on the stem is thus underestimated. This age underestimation would increase with stand age because of the continuous growth of adventitious roots and the sinking of the stem under its own weight. Tree age underestimation could lead to productivity overestimation. The goal of this study was to evaluate the effect of stand age on aging error of trees and productivity estimation. Three trees within each of fifteen non-paludified and naturally regenerated black spruce stands aged 37–204years were excavated to harvest the stump and locate the root collar. Stump sections were cross-dated down to the root collar to obtain trees’ total biological age which was then compared to a ring count done at 1m height. Height and volume growth curves obtained by stem analysis were compared with those derived from Pothier and Savard’s (1998) models. Age difference between total biological age and age at 1m (aging error) ranged 9–58years and significantly increased with stand age. Site indices (SI; height of trees at 50years of age) were significantly over-estimated by using age at 1m, and the overestimation significantly increased with stand age and aging error. The use of age at 1m lead to poor modeling of height- and volume-growth trajectories, as early height and volume-growth obtained through stem analysis was slower than predicted and stand senescence also occurred later. Due to their period of horizontal growth, seedlings don’t accumulate any height during the first years. Additionally, early tree growth of our oldest trees seems to have been slower than early growth of our younger trees, probably because of less favorable growing conditions. Despite large differences between volume and height growth trajectories, predicted volumes from theoretical SI calculations were not significantly different from observed values using stem analysis, at the time of sampling. Predictive models should nevertheless be adjusted by using total biological age or time since the last fire, to consider the first years of slow growth and to obtain more accurate productivity estimations.