In northern temperate regions, climate change is anticipated to lead to warmer average springtime weather punctuated by hard frosts. Warmer early-season conditions can accelerate springtime leaf flush of perennial hardwood species, increasing the probability of exposing new growth to damaging freezes. Trembling aspen is a genetically diverse, widespread, early successional tree species in North America that is a food source for many mammalian and insect herbivores. We employed a 2 (damage) × 6 (genotype) factorial design to investigate the independent and interactive effects of imposed vernal freeze damage and plant genotype on aspen growth and defense across successive growing seasons. In spring, 2014, half of the 3-year-old experimental saplings were exposed to subfreezing temperatures (-4°C), while the other half were chilled to +3°C. Trees were allowed to recover and enter dormancy during the remainder of the growing season. In the following year (2015), we assessed date of budbreak, tree size (d2 × h), leaf size, and foliar chemical defenses (salicinoids and condensed tannins) with known activity against mammalian and insect herbivores.
Results/Conclusions
On average, one year after exposure to a damaging spring frost, trees broke bud 2 days earlier, were 36% smaller, and had fully-expanded leaves that were 38% smaller compared with undamaged control trees. Total foliar salicinoid (salicin, tremuloidin, salicortin, and tremulacin) concentrations varied from 8 to 20% dry mass by genotype, and were on average 17% higher in trees with prior freeze damage. Freeze damage also caused a shift in the relative abundances of these four salicinoids. Foliar condensed tannin concentrations ranged from 4 to 12% among genotypes, and averaged 25% lower in trees with prior freeze damage. The capacity of vernal freeze damage to accelerate budbreak, restrict tree growth, and alter phytochemical profiles in subsequent growing seasons could potentially render trees more vulnerable to subsequent vernal frost damage and modify interactions with mammalian and insect herbivores. Overall, this study suggests that vernal freeze damage can have lasting effects on aspen, through both direct effects on phenology and growth, and indirect effects of altered chemical profiles on plant-herbivore interactions.