Relationship between ''birdseye'' sugar maple (Acer saccharum) occurrence and its environment

We tested the premise that the ''birdseye'' grain abnormality in sugar maple (Acer saccharum Marsh.) develops from local environmental conditions, with special emphasis on the role of competition in birdseye formation. Previous experience with birdseye maple frequency and the inherent differences in stand structure between old-growth and managed northern hardwoods led to stratification by stand type. Old growth contained considerably more birdseye than managed stands, but the levels in both types exceeded previously published frequencies. Unlike earlier studies, we did not fmd greater local density (and, presumably, greater competition) surrounding birdseye maples (versus non-birdeyes) in either old-growth (31.5 and 30.9 m(2).ha(-1), respectively) or managed northern hardwoods (25.0 and 23.7 m(2).ha(-1), respectively). Except for diameter, no tree variable strongly correlated with birdseye occurrence. Two plot-level variables, stand density and percent hemlock stems, strongly correlated with stand-level birdseye frequency primarily because of differences related to stand type, not because they represented causal factors. A logistic regression model was developed to estimate the likelihood of birdseye occurrence in an individual tree. Model performance varied, with non-birdseyes being predicted more accurately than birdseye maples. No evidence in this study explicitly supported a competition-birdseye linkage, but the frequency of birdseye in old growth suggests that prolonged suppression may influence birdseye formation from the extended exposure to a highly competitive environment. Management apparently results in decreased birdseye occurrence, a trend that might be ameliorated through retention of higher residual basal area and structure similar to old-growth stands.