Variation in stand density, black spruce individual growth and plant community following 20 years of drainage in post-harvest boreal peatlands

- Drainage helped post-harvest black spruce peatland sites to form a forested stand.
- Rich and thin organic soils were invaded by competitive species.
- Moderately rich sites reacted favorably to drainage.
- Poor and very poor sites reacted locally and showed minor growth increase.

Drainage can help restore hydrological conditions conducive to the growth of black spruce (Picea mariana) seedlings on logged forested peatlands. The impact of drainage was assessed by analysing the variation in stand density at stump height, black spruce seedling annual diameter growth and composition of plant communities on 20 of the oldest operationally drained sites in Quebec's James Bay region. These sites, each consisting of a drained, post-harvest area and a corresponding undrained, post-harvest control area, were classified into five ecosites based on soil characteristics and plant communities. The ecosites studied varied from those with rich mineral soils covered with a shallow organic layer to very poor, deep, organic soils. Results showed that the growth response of black spruce seedlings following drainage was strongly dependent on the ecosite. Ecosites RE37 and rich RE39 showed a substantial and steady growth response of black spruce to drainage over time. Therefore, drainage caused a significant increase in stand density and growth rate of black spruce seedlings in moderately rich ecosites. In the other ecosites studied, the peat layer was either too thick (poor RE39 and very poor RE39) or too shallow (RE26) for black spruce stand densities and individual stems growth to increase with drainage, compared to those of the control areas.