Microclimatic gradients provide evidence for a glacial refugium for temperate trees in a sheltered hilly landscape of Northern Italy

Aim Refugia play a key role in conserving biodiversity during periods of unfavourable and highly variable regional climate. However, refugial populations are often small and fragmented, which makes their identification difficult. In this study, we investigate whether an area of complex topography in the southern foreland of the Alps could have provided a suitable microclimate to serve as a glacial refugium for temperate trees during the last glacial. Location Methods The Euganean Hills in Northern Italy (Veneto). We assessed the current microclimatic variability in the ecologically diverse region on a fine scale by recording half-hourly near-surface temperatures over a period of 11 months. After comparing our measurements with today's vegetation distribution, broad-scale synoptic patterns, and topoclimatic factors, we estimated refugial suitability based on extreme temperatures, climatic stability, and difference from the regional average. Results Main conclusions Present-day temperature gradients within the Euganean Hills are large enough to explain the presence of the temperate tree species Fagus sylvatica as well as Mediterranean Quercus ilex, two species that are absent elsewhere in the adjacent Po Plain. During winter, anticyclonic weather patterns resulted in strong atmospheric inversions, with temperatures increasing by +1 degrees C/100 m in the hills relative to the surrounding Po Plain. Our high-resolution climate data support multi-proxy palaeoecological records identifying the Euganean Hills as a refugium for temperate trees. Temperature anomalies of 2-4 degrees C relative to the surrounding Po Plain are sufficient to allow several temperate tree species to survive the Last Glacial Maximum (LGM) in the Euganean Hills under a harsh continental climate. Specifically, elevations >200 m a.s.l. in the central parts of the hills may have provided suitable conditions for local LGM refugia. Regions of complex topography such as the Euganean Hills conserved past and present biodiversity and provide high-priority areas for conservation under future climate warming.