Gaps provide opportunities for migrating forest trees to establish in a broad forest transition zone

text:Background/Question/Methods Treefall gaps are important for the regeneration of canopy trees unable to recruit in the shaded understory. We investigate gap dynamics of an old-growth red pine (Pinus resinosa) forest in the Great Lakes-St. Lawrence ecozone in Northen Ontario, Canada, a transition zone between temperate and boreal forest. This ecosystem experiences occasional fires (85 year cycles) that are important for regeneration of red pines; however we propose here that other disturbances working at smaller timescales (gaps) coupled with increased rates of forest migration may have greater influence on future forest composition than currently thought. We sampled trees with DBH greater than 1 cm in 20 treefall gaps of varying size at the Wolf Lake Forest Reserve (46°51' N, 80°37' W). We recorded environmental factors of regeneration coverage (measured with a densiometer), ambient light (measured with a quantum meter), gap age (estimated by the degree of decomposition of the fallen tree) and gap size. Our measures of regeneration coverage and ambient light were collected at breast height and are therefore a consequence of the density of saplings near the gap floor. We performed a Canonical Correspondence Analysis (CCA) on basal area of trees against these four environmental factors. Results/Conclusions Our results show that common forest tree species of this ecozone (red pine, paper birch, black spruce) are characterized by lower regeneration coverage, higher levels of ambient light, older treefalls and smaller gaps; however species more indicative of the semi-deciduous forests further south (red maple, red oak, white pine, mountain ash) showed opposite relationships. We suggest that species from the southern forest are able to recruit in large, young gaps, but are eventually out-competed by species of the red pine forest. Species of the southern forest also appear to be unable to recruit in small gaps, suggesting that the gap size effect may function as an independent mechanism as gap size was not correlated with any of the three environmental factors. We therefore performed a second CCA using relative basal area to observe if the gap size effect is due to area alone. We found that gap size is insignificant, confirming the area effect postulate. Large treefall gaps in this forest may allow northward migrating trees, such as red maples and red oaks, to establish in an otherwise competitively-exclusive environment, potentially acting as a mechanism for future forest migration in this broad forest transition zone as global temperatures are predicted to rise.