Seedling bank dynamics in boreal balsam fir forests

Abstract

The seedling banks of 14 balsam fir (Abies balsamea (L.) Mill.) stands of various ages affected to different degrees by the last spruce budworm (Choristoneura fumiferana (Clem.)) outbreak were studied in the boreal zone north of Lake Saint-Jean, Quebec. The objectives of the study were to (1) characterize the age structure of fir seedling populations, (2) observe possible links between the seedling age structure and that of the arborescent stratum, and (3) evaluate the impact of the last spruce budworm outbreak on the age structure of seedlings and compare it with the mortality dates of overstory trees. Seedling age structure and mortality distribution of the overstory trees were established. The stands all originated following one of the region's outbreaks that occurred either in the late nineteenth or in the twentieth century. Seedling age structures had normal distributions centered on 20 years and, as for mortality distributions of the overstory trees, they were very similar between the stands. Few significant relationships were observed between the seedling age structures, the density of the stands, and the mortality distributions. There were relationships between the quantity of fir trees and the mortality and between the seedling ages and overstory tree ages. Two hypotheses are proposed to explain the similarity of the seedling banks: (1) the seedling bank is always being renewed by a turnover process and (2) the bank establishes and grows to a certain point corresponding generally to 20 growth rings or 20 terminal bud scars, and then it stagnates. Results from this study and field observations suggest that the seedling bank dynamics could be, at least in part, based on a strategy that enables the fir to remain under cover for long periods. This strategy would imply a momentary lack of radial and apical growth.