Effect of a major canopy disturbance on the coexistence of Acer saccharum and Fagus grandifolia in the understorey of an old-growth forest

1 In forest communities, species coexistence can be favoured by disturbance-related variations in light regime coupled with rank reversal in species performance. The objective of this study was to determine if a major canopy disturbance, resulting from an ice storm, would favour the coexistence of sugar maple ( Acer saccharum ) and beech ( Fagus grandifolia ) in an old-growth forest located near the northern limit of the species’ range. 2 The growth, density and frequency of occurrence of understorey stems were evaluated 7 years after the ice storm and compared with pre-disturbance values to determine if sugar maple, a slightly less shade-tolerant species, would be favoured over beech. 3 Although height and radial growth increased threeto five-fold, and > 70% of the 5–10 cm d.b.h. stems showed a release, sugar maple did not benefit more from the opening of the canopy than beech. 4 The inability of sugar maple to outgrow beech might be related to the high proportion of beech root sprouts at our site, suggesting that the potential for the disturbancemediated mechanism to slow the competitive exclusion of maple might decrease as the importance of vegetative reproduction increases in beech. 5 Prior to the disturbance, sugar maple was more abundant and occurred in a larger proportion of the plots among small size classes, while beech dominated in the larger size classes. Although some increases in the density of both species were detected after the disturbance, the general trends of relative abundance and distribution of the species were not modified significantly. 6 Our results do not indicate that a single canopy disturbance such as the major 1998 ice storm significantly favoured sugar maple over beech. When a disturbance opens the canopy, competition for light in gaps may tend to maintain or reinforce a pre-existing hierarchy instead of reversing it, especially when interspecific differences in postdisturbance growth are small or absent, as in this study. 7 This study shows that the dynamics of sugar maple – beech communities are more complex than previously envisaged and that revised models should incorporate additional factors (e.g. vegetative reproduction) that might also play an important role in regulating community dynamics. Key-words : advance regeneration, disturbance-mediated coexistence, gap dynamics, ice storm, root sprouts, vegetative reproduction Journal of Ecology (2007) 95 , 458–467 doi: 10.1111/j.1365-2745.2007.01219.x