Phenotypic plasticity of the phenology of seven European tree species in relation to climatic warming

To evaluate the potential responses of individual trees to climatic warming, phenological observations of clones of Larix decidua (Mill.), Betula pubescens (Ehrh.), Tilia cor-data (Mill.), Populus canescens (Ait.), Quercus robur (L.), Fagus sylvatica (L.) and Picea abies (L.) relocated over a large latitudinal range in Europe were analysed. The magnitude of the response of the clone was compared to that of genetically different trees of the same species in part of the latitudinal range, which were assumed to have adapted to their local climates. It was found that the responses of the date of leaf unfolding and the date of leaf fall in the clones to temperature were similar in magnitude to those in the genetically different trees. This demonstrates that trees possess considerable plasticity and are able to respond phenotypically to a major change in their local climate. For the clones of Larix decidua and Quercus robur the duration of the growing season may decrease with increasing temperature, because leaf fall is advanced more than leaf unfolding. In Betula pubescens and Populus canescens, leaf unfolding and leaf fall are advanced equally, whereas in Tilia cordata and Fagus sylvatica the date of leaf fall seems to be unaltered but the date of leaf unfolding advances with increasing temperature. These differences in the duration of the growing season at increased temperature may alter the competitive balance between the species. Descriptive dynamic models showed that most of the variance in the date of leaf unfolding can be accounted for by temperature. However, a generally applicable model of leaf fall based on temperature and/or photoperiod could not improved the null model, i.e. the mean date of leaf fall, because of variability in other environmental factors. The lowest temperatures around the dates of leaf unfolding and leaf fall differed among the clones. The hypothesis that the survival of the clones is curtailed by spring frosts was supported. Thus, these lowest temper-tures around leaf unfolding may represent thresholds below which the species cannot survive. It is argued that these thresholds may be a particularly sensitive means to evaluate the impact of climatic warming on the geographical distribution of tree species.