Fig. 1. Tree height (a) and relative growth rate (RGR) (b) of black spruce trees on non-scarified and scarified plots, at two boreal sites in Québec. Trees were planted as containerized 2-year-old seedlings in 1999. Two way mixed model ANOVA revealed a significant (P < 0.001) effect of scarification on tree height and RGR across both sites (N = 250 trees per treatment across sites). Vertical lines denote standard errors of the means (n = 90 in Abitibi; n = 160 in Côte-Nord).

Fig. 2. Forest floor gravimetric moisture content (a) and nitrogen mineralization rate per total N (b) in non-scarified and scarified plots at two boreal sites in Québec. Each bar represents mean values based on 3 bulk samples × 5 plots × 4 sampling dates (n = 60). Two way mixed model ANOVA revealed a significant (P < 0.001) main effect of treatment on forest floor moisture content (Fig. 2a). In the case of N mineralization (Fig. 2b), a significant site × treatment interaction required us test the effects of scarification within each site; consequently, statistically different (P = 0.037) means were found in Côte-Nord (different lower-case letters), but not in Abitibi (ns = not significant). Vertical lines denote standard errors of the means.

Table 1. Mean gas exchange and foliar nitrogen measurements of one-year-old black spruce needles in scarified and non-scarified plots in two sites of Québec, 18 years after treatment. Trees were planted as containerized 2-year old seedlings in 1999. The reported treatment means are based on 2 trees × 5 plots × 4 sampling dates (n = 40). The reported P values result from two-way mixed model analyses of variance testing the effects of treatments and sites, as well as their interaction. Values in parentheses denote one standard error of the mean. |

| Abitibi | Côte-Nord | Significant findings |

Non-scarified | Scarified | Non-scarified | Scarified |

*P*_{n} | 9.58 (0.90) | 8.92 (0.76) | 9.15 (0.98) | 6.75 (0.92) | Non-scarified > Scarified; (P = 0.087) |

*A* | 0.033 (0.003) | 0.032 (0.003) | 0.025 (0.002) | 0.022 (0.002) | Abitibi > Côte-Nord; (P = 0.027) |

*E* | 4.39 (0.61) | 3.00 (0.29) | 4.52 (0.43) | 3.84 (0.40) | Non-scarified > Scarified; (P < 0.003) |

Foliar N | 7.13 (0.23) | 6.83 (0.22) | 6.90 (0.21) | 7.50 (0.18) | Site x Treatment; (P = 0.087) |

Fig. 3. Instantaneous water use efficiency (a) and photosynthetic nitrogen use efficiency (b) of black spruce trees on non-scarified and scarified plots, at two boreal sites in Québec. Trees were planted as containerized 2-year-old seedlings in 1999. Each bar represents mean values based on 2 trees × 5 plots × 4 sampling dates (n = 40). Two way mixed model ANOVA revealed significant main effects of treatments (P < 0.001) and sites (P < 0.001) on water use efficiency (Fig. 3a), and a significant effect of sites (P = 0.011) on photosynthetic nitrogen use efficiency (Fig. 3b).Vertical lines denote standard errors of the means.

Fig. 4. Results of linear regression analysis testing the relationship between δ^{18}O and δ^{13}C isotopic values of one-year-old needles in non-scarified (closed symbols) and scarified (open symbols) plots, at two boreal sites in Québec. Needles were collected from two black spruce trees per plot, in September 2016 (n = 10). Trees were planted as containerized 2-year-old seedlings in 1999. We found a significant positive regression slope (solid line) on scarified plots only, on both the Abitibi (P = 0.021, R^{2} = 0.51) and the Côte-Nord (P = 0.024, R^{2} = 0.49) sites.