Effect of the extreme summer heat waves on isolated populations of two orophitic plants in the north Apennines (Italy)

One of the lesser known effects of global climate change is the occurrence of heat waves. Climatic models predict that heat waves will become more intense, longer lasting and/or more frequent, as a consequence of the increased inter-annual variability and increased average values of summer temperatures. Plants are damaged by heat waves through direct effects of extreme temperatures influencing plant physiology and through indirect effects, like drought and exposure to high ozone concentration. This study investigates the flowering abundance and biomass production of two orophytic species, Alopecurus alpinus Vill. and Vicia cusnae Foggi et Ricceri following the heat wave that occurred in the summer of 2003 and analyses the effects of summer temperatures during the period 1999-2004 on the species reproductive performance. In 2003, we observed a significant decrease in the number of flowering stems and flowers per flowering stem for both species. Flower production reached its lowest value in correspondence to the heat wave in 2003 and Redundancy Analysis showed that flower production was related to the mean June temperature. Flower production was more sensitive than vegetative growth, which was maintained. This suggests that changes in reproductive strategies, e.g. changes in the ratio between sexual and clonal reproduction, may occur by as an effect of extreme weather events. Such changes may be of great importance when the population consists of a small number of flowering individuals, as is the case for A. alpinus and V. cusnae in the study area. As a consequence, although the plants generally responded positively to gradual warming, we found that, during the monitoring period 1999-2004, extreme temperatures had a negative effect on A. alpinus and V. cusnae.