Pre-commercial thinning enhances competitive traits of boreal ericaceous shrubs and reduces soil fertility

Abstract

Boreal ericaceous shrubs, such as Kalmia angustifolia, have evolved various traits allowing them to interfere with the growth of conifer trees. These include the ability to spread through rhizomatous growth and to produce high amounts of tannins that interfere with soil N cycling. We examined how pre-commercial thinning (PCT) of young black spruce (Picea mariana) stands, a common silvicultural practice, increases the aboveground and belowground spread of various ericaceous shrubs, the chemical quality of leaves and litter, as well as soil nitrogen cycling. We compared these effects in relatively cool-moist and relatively warm-dry climates. Within the warm-dry climate, we also compared the effects of PCT on fertile clay and on nutrient-poor till deposits. The total cover of ericaceous shrubs was higher in PCT than in control plots in all three site types. On nutrient poor till deposits (in both climates), the current-year rhizome length and specific rhizome mass of K. angustifolia were higher in PCT than in control plots. Soil mineralizable NH4+-N was lower under patches of K. angustifolia than under patches of feather moss (Pleurozium schreberi). Across all 24 plots, canopy openness had a positive relationship with ericaceous shrub cover, current-year rhizome length, foliar tannins and phenolic concentrations. Although PCT might increase tree growth in the short-term, its positive effects on ericaceous shrubs increases the risk of conifer seedling growth check when the next clearcut harvest occurs, especially in cool moist climates and on nutrient poor soil deposits.