Influence of altitude on phenology of selected plant species in the Alpine region (1971-2000)

We present a quantitative and qualitative analysis of the influence of altitude on plant phenology over an area, delimited by 46 to 49°N latitude and 5 to 15°E longitude, that includes major parts of the Alpine region for the period 1971-2000. Our attention is focused on flowering of widely distributed plants in Europe, including some important allergenic species. We calculated the dependence of phenological mean dates on altitude for different phenophases as a linear regres- sion. Results show a statistically significant linear dependence (p < 0.0001) for each phenophase, with regression coefficients that range from 0.92 ± 0.21 d (100 m) -1 altitude for beginning of flower- ing of Norway spruce to 4.56 ± 0.21 d (100 m) -1 for beginning of flowering of common hazel. Pheno- logical temporal trends and thermal temporal trends show a less strong dependence on altitude, predominantly non-significant. The most significant regression coefficients are negative for pheno- logical trends (beginning of flowering of common alder, -0.065 ± 0.028 d yr -1 (100 m) -1 , p = 0.0187, and Norway spruce, -0.049 ± 0.020 d yr -1 (100 m) -1 , p = 0.0167) and positive for temperature trends (February, 0.0017°C yr -1 (100 m) -1 , p < 0.0001). The significant linear dependence of February tem- perature trends on altitude induces an inverse significant linear dependence for phenological trends of flowering of common alder (occurring in March). The prevalence of negative mean pheno- logical trends suggests a stronger advance of flowering phases at higher altitude. Finally, a regional analysis suggests a tendency towards a stronger altitudinal response in the northern than in the southern Alps.