Altered leaf morphology, leaf resource dilution and defense chemistry induction in frost-defoliated aspen (Populus tremuloides).

In May 2007, a widespread frost event defoliated much of Utah's high elevation aspen. About 5 weeks later, the frost-defoliated aspen produced a second leaf flush. The objective of this study was to characterize changes in leaf morphology and function in re-flush leaves following frost defoliation. Leaf size and thickness, photosynthesis, carbohydrate and nutrient status, and defense chemistry (phenolic glycosides and condensed tannins) were measured in first and second flush leaves. The second flush leaves produced two different morphological responses depending on frost damage severity. Severe frost damage was characterized by patchy canopy re-flushing with leaves that were on average four times larger than the first flush leaves. Moderate frost damage produced full canopy flushes with second flush leaves that were typically smaller than the first flush leaves. The second flush leaves tended to be thicker, and had significantly lower nutrient and sucrose concentrations, but had equal or higher rates of photosynthesis. These leaves showed a general pattern of defense chemistry induction with phenolic glycosides and condensed tannins increasing two- to threefold. Some of the changes in leaf morphology and defense chemistry observed in second flush leaves in 2007 persisted in leaves produced in the following year. We hypothesize that defense chemistry induction following abiotic defoliation serves as insurance against secondary defoliation events by herbivores that may further deplete nutrient and carbohydrate leaf resources below threshold points that are critical for physiological function. Resource dilution and allocation to secondary defense may place constraints on growth capacity.