Why does early-season herbivory affect subsequent budburst?

Delays in budburst in the year following herbivory have previously been att ributed to a defensive response to early-season herbivory or localized reso urce deficiencies (resource availability hypothesis) limiting bud developme nt. We carried out field experiments to test the resource availability hypo thesis as well as the crown architecture hypothesis, which attributes chang es in the time of budburst in the year following herbivory to changes in th e proportion of different bud types. We examined predictions from these hyp otheses using a defoliator, Choristoneura fumiferana; a defoliator/stem fee der, Zeiraphera canadensis; and a stem galler, Adelges abietis; on white sp ruce, Picea glauca. The time of budburst and final lear size of unexploited midcrown shoots of white spruce were compared to those of adjacent exploit ed shoots. We also evaluated budburst in a manipulated field study, in whic h tree growth was reduced through root pruning and increased through fertil ization. Partial defoliation by C. fumiferana and Z. canadensis did not affect budbu rst, but did result in smaller leaves. Almost complete defoliation of shoot s 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 who se growth rate had been decreased by root pruning or increased by fertilizi ng occurred later and sooner, respectively, than budburst on unmanipulated trees. The time of budburst was inversely correlated with foliar nitrogen a nd water content and the length of unexploited shoots. These results suppor ted the resource availability hypothesis. The influence of resource availab ility on time of budburst was observed at the shoot, branch, and whole-tree levels, When >50% of stems were destroyed by Z. canadensis, budburst occur red earlier or at the same time in the following spring. This was due to th e production of basal buds, which burst before all other buds on a shoot. T he production of early-bursting basal buds more than compensated, or just c ompensated, for a small delay in budburst of terminal, distal-lateral. and medial-lateral buds. Thus, changes in the time of budburst in the year foll owing herbivory were due to changes in crown architecture as well as to cha nges in resource availability. We predict that increased intra-tree heterog eneity in the time of budburst 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 ti me. We suggest that the influence of previous herbivory on the time of budb urst in the next growing season is a commonly overlooked factor that may ha ve a large influence on herbivore abundance and distribution.