Effects of vernal freeze damage on trembling aspen growth and colonization by Chaitophorus stevensis

Background/Question/Methods: Climate change is predicted to lead to warmer springtime weather punctuated by intermittent hard freezes in northern temperate and montane regions. These climate conditions are expected to accelerate springtime leaf flush of perennial woody species, exposing the new growth to damaging frosts. Trembling aspen is a widespread early successional tree species in North America, is highly genetically diverse, and is a host for many species of herbivorous insects. We investigated the combined influence of vernal freeze damage and plant genotype on the growth of aspen and subsequent colonization by the green aphid, Chaitophorus stevensis. We employed a 2 × 6 factorial design with two temperature treatments (freeze-damage and no freeze damage) and 6 aspen genotypes (representing a diversity of chemo-phenotypes). Half of the 3-year-old saplings of each genotype were exposed to subfreezing temperatures (-4°C) during shoot elongation, while the remaining trees were chilled to +3°C. Following cold exposure, we monitored tree mortality, growth, and colonization by aphids. 

Results/Conclusions: Acute effects of vernal freeze damage to aspen included severe defoliation, shoot dieback, and increased mortality of some genotypes. Surviving trees produced new lateral buds within two weeks, and had fully-expanded leaves within 3-4 weeks post-treatment. At 72 days post-treatment, the damaged trees were on average half the size (d²×h) and mass as the undamaged control trees, and had smaller, thinner leaves. Colonization of aspen by C. stevensis varied by plant genotype. At 48 days post-treatment (mid-summer), aphid colonization favored plants without vernal freeze damage, but at 71 days post-treatment (late summer), colonization favored trees with vernal freeze damage. Aphid reproduction was also faster in late summer on the trees with prior freeze damage. We hypothesize that the differences in aphid colonization and reproduction on trees with and without vernal freeze damage was a consequence of altered leaf age and chemistry. These results suggest that more frequent damaging vernal freezes will negatively impact the establishment and growth of aspen, and may shift aspen population genetic structure and alter aspen-insect interactions.