Flowering, Shoot Production, and Vole Bark Herbivory in a Boreal Willow

Different intensities of vole bark herbivory were simulated on the boreal willow Salix myrsinifolia-phylicifolia in field experiments, and plant responses were followed during 4 yr. Intermediate damage (every second stem barked) resulted the 1st yr in a fourfold increase in the production of basal shoots per individual, and in the following years, a twofold increase in the number of catkins per intact stem compared with control willows. However, 1 yr after damage, the average number of seeds per individual was 21% lower than in control willows and seed germination was reduced 40%. Severe damage (all stems barked) caused a nearly 10-fold increase in the production of basal shoots the 1st yr and a reduction and delay of sexual reproduction. Mortality of damaged stems was high (100%), while genet mortality was < 5% in the group in which all stems were barked. Flowering in female willows was reduced more than in male willows, and males produced catkins on younger basal shoots than females. We suggest that in this system, where the principal herbivore fluctuates between years, compensatory reactions by damaged plants might be of particular benefit, as the period of growth following damage is more or less free from herbivores.