Spatial contiguity and continuity of canopy gaps in mixed wood boreal forests: persistence, expansion, shrinkage and displacement

1. Variation in forest gap size and duration are a result of spatial contiguity and continuity of gap infilling and tree mortality over time, which influences both species recruitment and successional pathways. 2. As many gaps in boreal forests are small, their size and duration will affect the conditions influencing species recruitment. We investigate the spatial dynamics of these gaps (i.e. those which are persistent, ephemeral, expanding, displaced or disappearing) and tested whether gap spatio-temporal patterns are consistent over different temporal periods (19982003 and 20032007). 3. Forest canopy gaps were reconstructed for three plots (10, 10 and 6 ha in size) in southern boreal mixedwood forests around Lake Duparquet, north-western Quebec (Canada), using a time series of high-resolution canopy surface profiles from three light and ranging detection (lidar) system surveys during a 9-year window. High-resolution images were used to individually identify early and late successional gap makers. Dynamic changes in canopy gaps over a 9-year period were investigated by implementing concepts of random set theory within a temporal GIS framework. Mortality was higher on the gap edges than in the forest interior, and shade tolerant species were more likely to be gap makers than shade intolerant species. Edge trees that died causing the expansion of gaps were much smaller than trees creating new gaps. Although the overall gap size distribution was consistent over the 9 years studied, the proportion of the total area opening and closing varied between periods. Independent analyses of time windows show an abundance of small gaps (below 40 m2) appearing and disappearing; however, analysis of spatial contiguity shows that the majority (over 80%) of gaps of all sizes were displaced and/or expanded. 4. Synthesis. Contrary to the previous perception that small gaps are ephemeral, which would favour the recruitment of late successional species, our findings indicate that gap displacement and expansion may be a mechanism explaining the maintenance of favourable conditions for the recruitment of shade intolerant individuals, which has been previously observed in high-latitude old-growth boreal forests.