SYSTEMATIC WITHIN-TREE VARIATION IN MOUNTAIN BIRCH LEAF QUALITY FOR AGEOMETRID, EPIRRITA-AUTUMNATA

1. We studied within-tree variation in leaf quality of the mountain bi rch, Betula pubescens ssp. tortuosa, for larvae of the autumnal moth, Epirrita autumnata. 2. The purpose of the study was to determine the p ossible occurrence of systematic differences in larval growth on short shoot leaves (i.e. leaves of the same age): among leaves facing in di fferent compass directions, between leaves of lower and upper branches , among leaves on different positions within a branch and among leaves of different sizes within a short shoot. We also measured larval grow th between short shoot and long shoot leaves (i.e. between leaves of d ifferent age). 3. The larvae grew best on leaves on the north side of trees and most poorly on south side leaves, the east and west sides be ing intermediate. Leaves from the upper branches supported larval grow th better than leaves from the lower ones. The larvae grew better on t he smallest leaf of each short shoot and were able to utilize it more efficiently than the two larger leaves. Short shoot leaves from the ba sal and middle parts of the upper branches of the trees were of better quality for the larvae than short shoot leaves from the tip part of t he branches, The larval growth rate did not differ between short shoot and long shoot leaves. In general, within-tree variation in the larva l growth rate was lower than variation among different trees. 4. Damag e to leaves can decrease leaf quality for herbivores in the same year (rapidly inducible responses) or the following year(s) (delayed induci ble responses). Our results show that systematic within-tree variation in larval growth can be as great as the effects of rapidly inducible responses and that variation among individual trees can be as great as the mean effects of delayed inducible responses.