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

One of the lesser known eff ects 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 eff ects of extreme temperatures infl uencing plant physiology and through indirect eff ects, like drought and exposure to high ozone concentration. Th is study investigates the fl owering 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 eff ects of summer temperatures during the period 1999 – 2004 on the species reproductive performance. In 2003, we observed a signifi cant decrease in the number of fl owering stems and fl owers per fl owering stem for both species . Flower production reached its lowest value in correspondence to the heat wave in 2003 and Redundancy Analysis showed that fl ower production was related to the mean June temperature. Flower production was more sensitive than vegetative growth, which was maintained. Th is suggests that changes in reproductive strategies, e.g. changes in the ratio between sexual and clonal reproduction, may occur by as an eff ect of extreme weather events. Such changes may be of great importance when the population consists of a small number of fl owering 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 eff ect on A. alpinus and V. cusnae .