Weakening of climatic constraints with global warming and its consequences for evergreen broad-leaved species

Meteorological stations located in the lower areas of southern Switzerland have recorded a period with distinct milder winter conditions since 1970 as compared with the first half of the 20th century. The twofold set of climatic parameters, the absolute values and frequency of minimum temperatures as well as the length of the growing season have shifted towards warmer conditions in the last thirty years. In this paper, consequences of the lengthened growing season to 11 months are discussed. The detected climatic change supposedly favours species with evergreen broad-leaved growth form. With the analysis of 170 resurveyed relevés the hypothesis of whether the group of evergreen broad-leaved species have succeeded in profiting from this weakening of climatic constraints was verified. Conspicuous changes have been observed not only in terms of the abundance and frequency of indigenous evergreen broad-leaved species, but also with a number of exotic species sharing equal characteristics and having succeeded in colonizing forest areas and establishing stands in the shrub layer. It is suggested that in areas with a minimum temperature above ?10 °C and sufficient water supply throughout the year, evergreen broad-leaved species become increasingly competitive as soon as the growing season (days without frost) lengthens to about 300–320 days.