Investigations on the birdseye grain abnormality in sugar maple (Acer saccharum Marsh.) have lately focused on possible associations with environmental conditions. One theory attributes its formation to elevated stand density, suggesting that increased competition for resources triggers birdseye production. This hypothesis has received only mixed support in related research. Recent microanatomical examination of birdseye sugar maple found localized aggregations of bark fibers press into the cambium, thereby damaging the cambial initials and, consequently, altering wood formation. No proximate cause for this phellogenetic origin was given, but elevated levels of the plant hormone ethylene acting upon the cork cambium were suspected. Birdseye formation differs from the physiological response seen in Karelian birch, and may result from xylem growth suppression due to poor nutrient supply following physiological changes in tissue structure. Both birdseye maple and Karelian birch are examples of environmentally triggered metabolic disorders with promise for silviculturally-based propagation.