Fluctuating asymmetry as an indicator of elevation stress and distribution limits in mountain birch (Betula pubescens)

Fluctuating asymmetry (FA) has been suggested as a useful indicator of elevation stress and, hence, distribution limits in plants. However, no plant studies have been carried out to test (i) whether FA shows a gradual increase towards the alpine distribution limit and (ii) whether FA responds to elevation stress independent of other stressors which is necessary for FA to be a useful indicator in this context. To test these two hypotheses, this 2-year field study investigated the dose–response relationship between elevation stress and FA in mountain birch (Betula pubescens) under contrasting levels of insect attack in northern Norway. The results showed that FA increased linearly from sea level towards the tree line in both years independent of insect attack, which had no observable effect on FA, i.e. insect attack did not appear to disturb the FA-elevation relation. Thus, in mountain birch, FA appeared to be a reliable indicator of elevation stress. Further investigation is now needed in order to develop this hypothesis.