Combining paleoecology with a dynamic landscape model to uncover a cryptic full-Glacial refuge in Northern Italy

Background/Question/Methods

Temperate trees persisted during the last Glacial at well-described refugia in Southern Europe.  Molecular evidence and trace pollen abundances from sedimentary archives, suggest additional refugia preserved species and genetic diversity near the large ice sheets in the Alps and Northern Europe.  However, verifying the existence and location of such cryptic refugia is difficult because populations were probably small and fragmented, and continuous sedimentary archives near the large ice sheets are rare.  The Euganian Hills in northern Italy are an ecologically-diverse area that may have served as a Glacial refuge very close to the Alps.  The hills have a buffered microclimate that is warmer than the surrounding Po Plain, especially during cold air outbreaks.  We combined paleoecological records spanning the last 33,000 years from Lago della Costa, a small lake, with simulations from the LandClim dynamic vegetation model to evaluate the Euganian Hills as a Glacial refuge.  We documented local and extra-local tree populations with macrofossil and pollen records, and developed a chironomid-inferred temperature reconstruction from the same sediments.  We simulated vegetation using two climatic scenarios developed by combining the Glacial temperature reconstruction with climate station data from Este in the Euganian Hills, and Campagnana nearby in the Po Plain. 

Results/Conclusions

Macrofossil and pollen data provide definitive evidence that cold-tolerant trees such as Larix decidua, Pinus sylvestris, and Betula spp. persisted very close to Lago della Costa during the last Glacial.  Pollen data also suggest small local populations of temperate species including Corylus avellana, Quercus spp., and Carpinus betulus.  Our chironomid-inferred temperature reconstruction indicates average July temperatures ranged between 12 and 15 °C during this period.  Together, pollen and chironomid data indicate that temperate species expanded during intervals with rising temperatures.  In addition, pollen of thermophilous species (e.g. Castanea sativa, Tilia spp.) is present during the warmest periods.  LandClim simulations are remarkably consistent with pollen data.  Larix decidua, Pinus sylvestris, and Betula pendula are abundant in both the Euganian Hills and in the Po Plain simulations.  However climatic differences are large enough that simulated biomass is higher, and temperate species are consistently present only in the Euganian Hills.  Similarly, thermophilous species are simulated only in the Euganian Hills.  Paleoecology and modeling provide independent and complementary evidence that the Euganian Hills provided the northernmost glacial refuge for temperate trees in Europe.  Our results highlight the conservation importance of disjunct populations in unique climatic settings for conserving biological diversity under extreme and changing climate.