Shoot damage effects on regeneration of maples (Acer) across an understorey-gap microenvironmental gradient

1 We measured whole-plant survival, frequency of leader (mainstem terminal bud) damage among survivors and growth responses of intact and damaged juve niles of three Acer species over 3 years along a microenvironmental gradien t represented by the understorey and two sizes of artificially created cano py gaps in central Massachusetts, United States of America. 2 For all species combined, survival decreased while the frequency of leade r damage among survivors increased across the gradient of microsite exposur e. Acer rubrum L. (red maple) showed the highest survival (65-93%) but also very high leader damage (80-97%). Acer pensylvanicum L. (striped maple) sh owed fairly high survival (81-93%) in all but the most exposed microsites ( 24-36%) and had the lowest leader damage overall (17-44%). Acer saccharum M arsh. (sugar maple) was intermediate for both survival (25-86%) and leader damage (55-96%). 3 Growth differed significantly among sites and species. Both intact and da maged plants showed greater growth in gaps than in understorey, particularl y in large gaps. For most growth variables in most microsites, A. pensylvan icum : A. rubrum > A. saccharum when plants were intact, but A. rubrum grea ter than or equal to A. pensylvanicum > A. saccharum when damaged. Species differences in growth varied among sites, with large gaps producing more pr onounced effects than small gaps and understorey for both intact and damage d plants. 4 Growth recovery was inversely related to leader damage frequency among sp ecies, and thus at least partially offset the effects of damage on net grow th across the populations. 5 The microsite- and species-specific responses to leader damage may influe nce gap regeneration and forest composition. A. pensylvanicum may be favour ed by its regrowth advantage over A. rubrum and A. saccharum in the underst orey, where damage is likely to occur during prolonged pre-gap periods. In contrast, A. rubrum shows a decisive advantage over its congeners in regrow th in the centres of large gaps, where the probability of a juvenile tree c apturing canopy gap space is highest.