Late frosts effects on Mediterranean beech forests: insights from tree rings

Beech is one of the most important European broadleaved trees. It is a mesic species tolerant to very cold winters, but sensitive to late spring frost and to summer drought. Phenological observations show that beech unfolding is occurring up to 3.5 days earlier per decade, increasing sensitivity to spring frost events. We aimed to assess i) how frequent and spatially extended are severe late-frost events at some Mediterranean beech forests during the last decades; ii) how their effects are recorded in tree-ring width series. Upon literature and satellite image analysis we selected a few sites that experienced recent frost events in the Central and Southern Apennines, Eastern Italian Pre-Alps and central Spanish Pyrenees. At each site, we sampled 60 dominant beech trees within 3 plots located along an altitudinal gradient to detect different frost damage intensity. We extracted cores for dendrochronological analyses searching for specific markers to identify late frost damages. Our first results, compared with other beech chronologies, show that in central and southern Apennines two documented late frost events (in 1957 and 2016) caused tree-ring different growth responses along the elevation gradient. In some sites frost caused tree-ring reduction only at mid and high altitudes whereas in others high altitudes trees were not affected. This can be explained with a thermal inversion or with a delay of the leaf unfolding at higher altitudes. Finally, beech forests sensitivity to late frosts seems locally controlled by stand altitude and aspect that modulate tree phenology, leaf damage intensity and xylogenesis.