Altered leaf morphology and defense chemistry induction in frost damaged trembling aspen

Background/Question/Methods

Frost damage in northern Arizona aspen populations in 1999, is hypothesized to have been an inciting factor that contributed to recent aspen dieback in Arizona. In May of 2007, widespread frost damage defoliated much of Utah’s high elevation aspen. Approximately five weeks later the frost damaged aspens produced a second leaf flush. The objective of this study was to characterize changes in leaf morphology and function in second flush leaves to better understand the implications of frost damage in aspen. Leaf size and thickness, carbohydrate and nutrient status, and defense chemistry (phenolic glycosides) were measured in first and second flush leaves.

Results/Conclusions

Second flush leaves produced two different morphological responses depending on frost damage severity. Severe frost damage was characterized by patchy canopy leaf flushing with leaves that were on average four times larger than first flush leaves. Less severe frost damage produced full canopy flushes with second flush leaves that were 50% smaller than first flush leaves. Second flush leaves tended to be thicker, and had significantly lower nutrient (~30%) and carbohydrates concentrations. In contrast, phenolic glycoside content was significantly higher (2-3 fold) in second flush leaves. We hypothesize that defense chemistry induction following abiotic defoliation serves as insurance against a second defoliation event by herbivores that may further deplete nutrient and carbohydrate leaf resources below critical thresholds. Resource dilution and allocation to secondary defense may however, place constraints on growth capacity.