Résumé - CAFD

Ground layer composition affects tree fine root biomass and soil nutrient availability in jack pine and black spruce forests under extreme drainage conditions.

Marine Pacé, Nicole J. Fenton, David Paré, Yves Bergeron.

In the boreal forest, long-lasting canopy gaps are associated with lichens on dry sites and with Sphagnum spp. on wet sites. We hypothesize that ground layer composition plays a role in maintaining gaps through its effects on fine root biomass (Ř?2mm) and soil nutrient availability. Along gradients of canopy openness in both jack pine-lichen and black spruce-moss forests, the relationships between canopy closure, ground layer composition, tree fine root biomass and soil nutrients were analyzed and decomposed using path analysis. The effects of lichen and Sphagnum spp. removal on tree fine root biomass and soil nutrients were tested in situ. Although variations in pine fine root biomass were mainly explained by stand aboveground biomass, lichen removal locally increased fine root biomass by more than 50%, resin extractable soil potassium by 580% and base cations by 180%. While Sphagnum cover was identified as a key driver of stand aboveground biomass reduction in paludified forest sites, its removal had no short-term effects on spruce fine root biomass and soil nutrients. Our results suggest that lichens, unlike Sphagnum spp., affect tree growth via direct effects on soil nutrients. These two different patterns call for different silvicultural solutions to maintain productive stands.