Variable Budbreak and Insect Folivory of Gambel Oak (Quercus gambelii: Fagaceae)

Budbreak of individual ramets of clonal Gambel oak (Quercus gambelii: Fagaceae) was advanced experimentally to test the effects of altered timing of budbreak and hence, leaf age, on patterns of folivory. Early-season folivory by leaf-chewing insects was significantly greater on younger leaves of control ramets than older leaves of experimental ramets. However, differences in cumulative folivory between control and experimental ramets disappeared by the end of the growing season. Colonization by three species of late-season leafminers was also unaffected by altered budbreak, indicating the impact of altered budbreak on folivores is concentrated early in the growing season. Analyses of nutritional and allelochemical content of experimental and control ramets correspond to the patterns of folivory; leaves of experimental ramets were significantly higher in gallotannin content than leaves of control ramets, but these differences also waned by the middle of the growing season. Our results suggest that variable budbreak can alter degree of folivory on Gambel oak, but only early in the growing season. The evolutionary implications of, and constraints on, variation in budbreak in woody plants are discussed. Many folivorous insect species that feed on deciduous or semi-evergreen trees prefer and perform better on younger leaves than old ones (e.g., Feeny, 1970; Rockwood, 1974; Schweitzer, 1979; Cates, 1980; Rauscher, 1981; Kraft and Denno, 1982; Coley, 1980; Niemela and Haukioja, 1982; Niemela et al., 1982; Fowler and Lawton, 1984). As leaves age, they change in surface area, water content, toughness, thickness, chemical composition and nutritional quality (Feeny, 1970; Crawley, 1983; Raupp and Denno, 1983; Coley, 1980; Faeth, 1985a). Therefore, the synchrony between budbreak of host trees and folivore emergence from overwintering stages is critical to folivore success. Insects that emerge too early may starve (Futuyma and Wasserman, 1980; Hunter, 1990), while insects that emerge too late may encounter a suboptimal food source (Embree, 1965; Feeny, 1970, 1976). Variation in timing of budbreak often occurs within populations of trees. For example, pedunculate oak (Quercus robur) trees vary in timing of budbreak by as much as two weeks within a given locality (Feeny, 1970). Because timing of budbreak is at least partly determined by genetic factors (Kramer and Kozlowski, 1979), variation in time of budbreak has been considered a plant defense against folivores, similar to other variable plant characteristics such as allelochemistry, nutrition, and morphology (Haukioja et al., 1985; Tuomi et al., 1989). Despite the potential of variable budbreak in altering folivore abundances on trees, relatively few studies of herbivore-plant interactions have focused on this plant trait. Those studies that have, have produced conflicting results. Feeny (1976) concluded that early or late flushing oaks harbor fewer insects than the population in general. Defoliated branches of mountain birch (Betula pubescens ssp. tortuosa) break bud later in the following year, and Tuomi et al. (1989) hypothesized that these branches should have less herbivory than branches that flush leaves earlier. Similarly, late breaking holly oaks (Quercus ilex) had less herbivory from the folivore Tortrix viridana than early breaking trees (Du Merle, 1988). In contrast, experimentally retarded, younger foliage of Betula pubescens had more herbivory than older, control foliage (Fowler and Lawton, 1984). Also, tropical plants with late, asynchronous leaf flush typically have higher rates of herbivory (Rockwood, 1974; Lieberman and Lieberman, 1984; Aide, 1988). We tested the influence of variable budbreak on folivory by advancing the timing of budbreak experimentally within clones of Gambel's oak (Quercus gambelii Nutt.). Since spring folivores generally prefer and perform better on younger leaves than older leaves, presumably because of E THWESTERN TURALIST 38(1):1-8 RCH The Southwestern Naturalist TABLE 1-Budbreak phenology of experimental (E) and control (C) ramets.