Impact of climatic factors on the developmental stability of mountain birch growing in a contaminated area.
We examined developmental stability in mountain birch Betula pubescens subsp. czerepanovii along a strong pollution gradient in the Kola Peninsula, north-western Russia, over a period of 8 years (1993-2000). In particular, we investigated whether climatic factors or aerial pollution could increase developmental instability, which was assessed through leaf fluctuating asymmetry (FA). Contrary to our expectations, leaf FA was not correlated with either distance from the pollution source, or with the levels of nickel and sulfur dioxide pollution in the study plots, or with shoot length or leaf size. There was, however, variation in FA between years: highest values of developmental instability were recorded during the coldest summers. There was also considerable among-tree variation in FA within plots, suggesting that birch genets either responded differently to environmental stress or demonstrated a different degree of genetic stress. The significant interaction between study year and birch genet suggests that the detected variation was a consequence of different sensitivity of the trees to climatic fluctuations rather than of different sensitivity to pollution. Growth rate of birch correlated positively with developmental stability, i.e. faster leaf growth was associated with lower FA both at the scale of individual trees (presumably genetic differences) and at the scale of study years (environmental differences). The lack of detectable response to aerial pollution by mountain birch in this study may be an outcome of selection for pollution-resistant genotypes, which may have occurred due to an exceptionally high emission pressure during the past decades. On the other hand, resistance to temperature variations may not have evolved because warm and cold periods appear to occur somewhat regularly in the Kola Peninsula, and therefore variation in temperature optima between genotypes is favoured by natural selection. Our results demonstrate that FA may serve as a useful measure of climatic stress in plants.