Fine-root dynamics in mixed boreal conifer-broad-leafed forest stands at different successional stages after fire

Fine-root (diameter less than or equal to 10 mm) standing biomass, length, distribution, production, and decomposition were studied in mixed conifer - broad-leafed forest stands 48, 122, and 232 years after fire on clay soils in the southern boreal forest of Quebec. A combination of ingrowth bags, soil cores, and root litter bags was used. Forest composition changed from trembling aspen- (Populus tremuloides Michx.) dominated stands in the youngest stage to balsam fir (Abies balsamea (L.) Mill.), and then to eastern white-cedar (Thuja occidentalis L.) stands in the oldest stage. The fine- and small-root standing biomass did not differ significantly between the forest successional stages. However, the total root length was significantly more developed in the 48-year-old successional stand than in the 232-year-old stand. Within the investigated soil profile (depth = 30 cm) most roots (>80%) were found in the 5 to 8 cm thick organic layer and the top 10 cm of the mineral soil. Root biomass in the organic layer increased significantly after fire, and a simultaneous increase in thickness of the organic layer was observed. The ingrowth of roots into ingrowth bags during one and two growing seasons was more than twice as high in the youngest stand as in the oldest one. However, the differences were not statistically significant because of high variation. Fine roots of aspen decomposed significantly faster than those of balsam fu and cedar in all forest stands. The results suggest that root production, the rate of decomposition, and presumably the rate of turnover are higher in forest stands dominated by early successional broad-leafed species such as aspen than in stands composed of late successional coniferous species such as fir, spruce, and cedar. Differences in root dynamics may contribute significantly to the change in the carbon and nutrient cycling often reported with succession in the boreal forest.