Initial distance to Kalmia angustifolia as a predictor of planted conifer growth

Abstract

In the Canadian boreal forest, conifer plantations are mainly used to overcome poor natural regeneration following harvesting or wildfires. However, competitive interactions can impair the successful establishment of forest plantations, especially in the presence of ericaceous species, such as Kalmia angustifolia, that are good competitors for soil resources. We used data from a silvicultural trial established in northwestern Quebec (Canada) to test the hypothesis that Kalmia’s effect on seedling growth is an asymptotic nonlinear function of proximity to Kalmia at time of planting. Our main objective was to derive species-specific thresholds of Kalmia-proximity tolerance at the time of planting for boreal sites prone to Kalmia invasion following harvesting. Using nonlinear quantile regressions (90th percentile), height, diameter and growth of planted Picea mariana and Pinus banksiana seedlings over 8 years was regressed against the initial distance to the nearest Kalmia stem at time of planting. We also investigated the relation between annual growth and the distance to nearest Kalmia stem during any given year between age 2 and 8-years. Finally, we evaluated how mechanical soil scarification influenced Kalmia recovery over time by looking at its rate of spread, estimated from mean distance to planted seedlings over 8 years. Silvicultural treatments increased the intercepts and asymptotes of all variables for Pinus. However, asymptotic values for Picea could not be identified, thus precluding threshold assessment for initial distance to Kalmia at planting for this species. For Pinus planted on scarified sites, a Kalmia-free radius of 60 cm would lead to ~80 to 85 % of the 90th percentile of maximum potential height at age 8 years. Mechanical scarification created Kalmia-free microsites that were rapidly re-invaded by spread of Kalmia; planting immediately following soil preparation of these sites is therefore important for ensuring successful plantation establishment.