Effects of stress and rapid growth on fluctuating asymmetry and insect damage in birch leaves

We studied how fluctuating asymmetry (FA), which is commonly used as an non-specific stress indicator, is influenced by abiotic stress along a moisture gradient and by rapid growth, i.e. the production of larger than normal leaves after simulated browsing, in white birch (Betula pubescens) growing in a mire. Variation of abiotic stress (plots near vs far from open, wet boundary of tree stand) was obvious as interpreted from smaller leaf size and lower stature of trees growing closest to the wettest, open center of the mire. However. we did not find increased leaf FA near the wettest area, possibly because of the high background level of stress in the mire habitat or the presence of flood-tolerant genotypes. Simulated winter browsing increased leaf size in the following growing season, and the strength of the response increased with the severity of the browsing treatment. Likewise, larger leaves of browsed ramets had higher developmental instability, as measured by size-corrected FA. Moreover, the effects of simulated winter browsing on FA were proportionally greater than the size effects in expanding leaves. Simulated winter browsing also increased insect damage incurred by leaves. Our results demonstrate that, in addition to previously shown effects of stress, excess leaf growth can also increase developmental instability and FA. We also demonstrated a higher rate of developmental errors in the later than early stages of leaf expansion.