Covariation of fluctuating asymmetry, herbivory and chemistry during birch leaf expansion.

Fluctuating asymmetry (FA) is used to describe developmental instability in bilateral structures. In trees, high FA of leaves has been assumed to indicate the level of environmental or genetic stress, and for herbivores leaves from such trees have been shown to be in some cases (though not invariably) of higher quality compared to trees with symmetrical leaves. We demonstrated that FA of birch leaves correlated positively with growth rate of leaves, and with the amount of leaf biomass consumed by larvae of the geometrid Epirrita autumnata. Since asymmetry per se cannot define leaf quality for a herbivore, we determined the biochemical compounds which covary with the degree of foliage FA, in order to elucidate relationships between leaf FA, chemistry and herbivory. High foliar FA was characteristic of birches with high initial concentrations, and rapid seasonal decline in the concentrations of gallic acid and hydrolysable tannins, and with rapid seasonal changes in the concentrations of flavonoid-glycosides and sugars. In contrast, leaf FA was not related to concentrations of proanthocyanidins, protein-bound amino acids or soluble phenylalanine, the precursor of proanthocyanidins and proteins with aromatic amino acids. The positive correlation between leaf FA and consumption by E. autumnata was presumably related to the previously demonstrated compensatory consumption of E. autumnata to high concentrations of foliar gallotannins. Furthermore, sugars are well-known feeding stimulants. We propose that the variable results in studies correlating leaf FA and herbivory may stem from variable chemical associations of FA in different plants and of species-specific effects of compounds on insects.