Dieback of the cushion plant Silene acaulis at its southern limit of distribution in the Apennines

Here, we quantified the dieback of Silene acaulis subsp. bryoides along a 500 m elevational transect from 1900 to 2400 m a.s.l. in the Sibillini Group (Apennines). We also investigated the role of the 2022 summer heat wave that affected the Apennines as a putative cause. The intensity of the dieback was not uniformly distributed across the elevational gradient, but was widespread at 1900 and 2000 m a.s.l. and decreased with increasing elevation. Specifically, 40.7% and 38.4% of plants were affected by dieback at 1900 and 2000 m a.s.l., respectively, but less than 1% at elevations above 2300 m a.s.l. Regarding climate, the maximum air temperature in 2022 was 3.7, 4.5, and 3.4 degrees C above the respective long-term average in May, June, and July. A similar trend was observed for the minimum temperature. The monthly maximum (21.5 degrees C) and minimum (14.6 degrees C) temperatures in July were the highest ever recorded since the monitoring station was commissioned in 2005. In terms of daily maximum temperatures, three consecutive days with temperatures above 25 degrees C were recorded in July 2022, an upper limit that has only been reached on four days in the last 17 years. Regarding precipitation, 180 mm of precipitation fell during the May-August growing season compared to the long-term average of 255 mm, a decrease of 30%. More importantly, only 15.2 mm of rain fell in July 2022, compared to a long-term average of 54.3 mm. The sudden dieback of Silene highlights the vulnerability of high elevation vegetation to climate change. Moreover, the loss of Silene cushions is alarming for the survival of the species, which is long lived but very slow growing.