FOLIAGE QUALITY CHANGES DURING CANOPY DEVELOPMENT OF SOME NORTHERN HARDWOOD TREES

The ephemerality of high quality foliage in spring may act as a defense for trees against early season folivores, but only if the duration of high quality is so short that it is difficult for insects to synchronize their eclosion with the period of high quality foliage that follows budbreak. The rate of change in foliage quality on a day to day basis through the spring was determined for 9 species of hardwood trees in 2-3 years. Measurement of physical and chemical parameters and a bioassay with gypsy moth larvae both showed decreasing quality during the three to five weeks of canopy development in all species. Rates of decline differed among species but the patterns were similar from year to year on a degree-day scale. Growth rates of larvae raised through the first stadium on foliage of differing ages reflected these changes in foliage acceptability. Increasing toughness and declining nitrogen and water contents of leaves were correlated with changes in acceptability to larvae but explained only a small part of the variation in acceptability. The host-seeking period of gypsy moth larvae overlapped with the availability of highly acceptable foliage of the most preferred host species. Less preferred species had more rapid declines in foliage acceptability, and hence narrower overlaps with the host-seeking period, which may provide defense against use by this generalist forest pest.