Beyond establishment: growth trajectories and evolution of chlorosis in planted white and black spruce

Successful regeneration of forest stands following clearcutting is essential for maintaining ecosystem services, particularly where natural regeneration is limited. Natural regeneration of white spruce ( Picea glauca (Moench) Voss) rarely meets reforestation standards in eastern Canada due to irregular seed production and adverse environmental conditions for seedling establishment. Moreover, white spruce can suffer from chlorosis a few years after planting. In 2008, we established a fully randomized block split-plot experiment across three boreal sites of Quebec, Canada, to study white spruce establishment through enrichment, with black spruce ( Picea mariana (Mill.) B.S.P.) included as a reference species. Seedlings were planted on three microsite types: organic horizons, mineral soil, and mechanically prepared microsites created by motor-manual site preparation. Data were collected in 2008, 2009, 2010, 2018, and 2021 to capture early establishment and mid-term responses in tree growth and health. We compared 14-year growth, survival, and chlorosis development. Our findings showed that mechanical site preparation did not result in significant differences in height or diameter growth for either species across years. However, white spruce showed a significant height increment when planted in mineral soil compared to the other treatments, with significant differences appearing in the 11th growing season. We observed a significant increase in chlorosis severity in white spruce between the 11th and 14th growing seasons compared to black spruce. Structural equation modelling revealed a negative association between chlorosis in white spruce and nutrient concentrations, suggesting that foliar nutrition may be associated with chlorosis, alongside other environmental and site-related factors.