WHY DOES EARLY-SEASON HERBIVORY AFFECT SUBSEQUENT BUDBURST?

Abstract

Delays in budburst in the year following herbivory have previously been attributed to a defensive response to early-season herbivory or localized resource deficien- cies (resource availability hypothesis) limiting bud development. We carried out field ex- periments to test the resource availability hypothesis as well as the crown architecture hypothesis, which attributes changes in the time of budburst in the year following herbivory to changes in the proportion of different bud types. We examined predictions from these hypotheses using a defoliator, Choristoneura fumiferana; a defoliator/stem feeder, Zeira- phera canadensis; and a stem galler, Adelges abietis; on white spruce, Picea glauca. The time of budburst and final leaf size of unexploited midcrown shoots of white spruce were compared to those of adjacent exploited shoots. We also evaluated budburst in a manipulated field study, in which tree growth was reduced through root pruning and increased through fertilization. Partial defoliation by C. fumiferana and Z. canadensis did not affect budburst, but did result in smaller leaves. Almost complete defoliation of shoots by C. fumiferana and galling by A. abietis resulted in delayed budburst. Reductions in leaf size were directly related to the amount of herbivory (i.e., low or high) by C. fumiferana and Z. canadensis. Budburst of trees whose growth rate had been decreased by root pruning or increased by fertilizing occurred later and sooner, respectively, than budburst on unmanipulated trees. The time of budburst was inversely correlated with foliar nitrogen and water content and the length of unexploited shoots. These results supported the resource availability hypothesis. The in- fluence of resource availability on time of budburst was observed at the shoot, branch, and whole-tree levels. When >50% of stems were destroyed by Z. canadensis, budburst occurred earlier or at the same time in the following spring. This was due to the production of basal buds, which burst before all other buds on a shoot. The production of early-bursting basal buds more than compensated, or just compensated, for a small delay in budburst of terminal, distal-lateral, and medial-lateral buds. Thus, changes in the time of budburst in the year following herbivory were due to changes in crown architecture as well as to changes in resource availability. We predict that increased intra-tree heterogeneity in the time of bud- burst should make P. glauca more, rather than less, susceptible to two of the herbivores studied because of the presence of a highly nutritious food source, newly burst buds, for a longer period of time. We suggest that the influence of previous herbivory on the time of budburst in the next growing season is a commonly overlooked factor that may have a large influence on herbivore abundance and distribution.