Question: To what extent do small-scale disturbances in the forest canopy, created by natural disturbance agents, affect stand development? Doubts exist as to whether small canopy openings have any real effect on the understory tree recruitment, especially in boreal forests.

Location: Conifer and mixed stands in the Gaspesian region in eastern Québec. The main natural disturbance agents are recurring outbreaks of Choristoneura fumiferana (eastern spruce budworm) and winds.

Methods: Linear transects in 27 sites were used to describe the gap (< 0.1 ha) regime parameters, including gap fraction, gap size and change in disturbance severity through time. Three stand types were distinguished, based on a gradient of abundance of tree host species for the eastern spruce budworm. The impact of gaps was evaluated on the basis of changes in the number, the period of recruitment, and the composition of tree saplings present within gap areas. Changes were measured along the gap size gradient, and according to the pattern of recent budworm epidemics.

Results: The gap fraction is highly variable (18%–64%) and is on average relatively high (42%). Gap sizes have a positively skewed distribution. In most cases the growth rate among gap filling saplings increased sufficiently to date disturbance events. The composition and the structure of understory trees were affected by gap formation. The number of shade-intolerant tree species did increase during or following periods of particularly severe canopy disturbances. However, the establishment or survival of shade intolerant species was not restricted to larger gaps or more intensely disturbed periods.

Conclusions: In sub-boreal forests of Eastern Canada, small scale disturbances in the tree canopy influence stand regeneration dynamics, but not to the extent that parameters such as sapling composition and recruitment patterns depend on gap regime characteristics.

Nomenclature: Farrar (1995).