Reconstruction of full glacial environments and summer temperatures from Lago della Costa, a refugial site in Northern Italy

Abstract

Vegetation and climate during the last ice age and the Last Glacial Maximum (LGM, ~ 23,000-19,000 cal BP) were considerably different than during the current interglacial (Holocene). Cold climatic conditions and growing ice-sheets during the last glaciation radically reduced forest extent in Europe to a restricted number of so-called "refugia", mostly located in the southern part of the continent. On the basis of paleobotanical analyses the Euganian Hills (Colli Euganei) in northeastern Italy have previously been proposed as one of the northernmost refugia of temperate trees (e.g. deciduous Quercus, Tilia, Ulmus, Fraxinus excelsior, Acer, Abies alba, Fagus sylvatica, Carpinus and Castanea) in Europe. In this study we provide the first quantitative, vegetation independent summer air temperature reconstruction for Northern Italy spanning the time ~ 31,000-17,000 cal yr BP, which covers the coldest periods of the last glacial, including the LGM and Heinrich stadials 1 to 3. Chironomids preserved in a lake sediment core from Lago della Costa (7m a.s.l.), a small lake at the southeastern edge of the Euganean Hills, allowed quantitative reconstruction of Full and Late Glacial summer air temperatures using a combined Swiss-Norwegian temperature inference model based on chironomid assemblages from 274 lakes. Chironomid and pollen evidence from Lago della Costa derives from finely stratified autochthonous organic gyttja sediments, which excludes major sediment mixing or reworking. After reconstructing paleotemperatures, we address the question whether climate conditions were warm enough to permit the local survival of temperate tree species during the LGM and whether local expansions and pollen-inferred contractions of temperate tree taxa coincided with chironomid-inferred climatic changes. Our results suggest that chironomids at Lago della Costa have responded to major climatic fluctuations such as temperature decreases during the LGM and Heinrich stadials. The vegetation of the Euganean Hills shows responses to these climatic oscillations although the effects of temperature changes were probably also strongly influenced by changes in humidity. Reconstructed July air temperatures at Lago della Costa 3 never fell below 10-13 °C (error range of reconstruction ~ ±1.5-1.6 °C), which is considerably above the limit considered necessary for forest growth (8-10°C). Instead rather mild climatic conditions prevailed ~ 31,000-17,000 cal yr BP with average summer temperatures between ~ 12 and 16 °C, which most likely allowed survival of temperate tree taxa in the warmest (and moistest) microhabitats of the Euganean Hills during the LGM. Only assuming local survival it is possible to explain the repeated expansions and collapses of temperate trees at Lago della Costa which faithfully accompanied the climatic oscillations.

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

“…Given the climatic conditions of the last glacial, potential refugia for temperate trees near the alpine ice shield would require temperatures that permit a sufficient growing season, the absence of permafrost or periglacial activity, adequate soil moisture, and topographic sheltering from strong winds (Birks & Willis, ). Our data, when combined with palaeoecological and palaeoclimatic evidence (Kaltenrieder et al., ; Samartin et al., ) support the conclusion that the Euganean Hills provided such conditions. Given that the Po Plain today retains a rather continental climate, strong microclimatic gradients may have also occurred during the LGM in the Euganean Hills.…”

“…400 km farther south (Antonioli, Bard, Potter, Silenzi, & Improta, ). The combined ostracode and chironomid records from Lago della Costa (Kaltenrieder et al., ; Samartin et al., ) indicate cold and dry LGM conditions around the study area. While advection of moist air masses may have affected the southwestern Alps and pre‐Alps (Luetscher et al., ), palaeoecological evidence suggests a very dry continental LGM climate in the extended Po Plain (Kaltenrieder et al., ; Ravazzi, Orombelli, & Tanzi, ).…”

“…Samartin et al. () concluded that mean summer temperatures never fell below 12–15°C at low to intermediate altitudes within the Euganean Hills (i.e. 0.5–2.5°C warmer than the Po Plain) during the LGM, which may have allowed Quercus petraea , Ulmus and Fraxinus excelsior to survive the harshest LGM conditions.…”

“…Our results revealed sub‐adiabatic temperature gradients of up to +0.5–1°C/100 m, which results in mean monthly temperatures being 2–4°C higher at elevated sites within the hills relative to the Po Plain during extreme winter high‐pressure anomalies, which we assume to be an analogue for LGM conditions. When comparing these values with minimum mean summer temperature requirements of different trees (Samartin et al., : table 2), it becomes evident that in addition to the aforementioned species, scattered stands of Carpinus betulus (hornbeam) and F. sylvatica could also have persisted the LGM in the Euganean Hills. Minimum summer temperatures required by warm‐loving sub‐Mediterranean Fraxinus ornus (ash), Q. pubescens (downy oak), and C. sativa lie at the edge of the possible temperature spectrum (17–18°C), which makes their presence during the LGM less likely, but not impossible, and supports the pollen‐inferred interpretations of Paganelli and Miola () on LGM survival of Castanea in sheltered areas of Northern Italy.…”

“…Here, we test the hypothesis that the special microclimate of the Euganean Hills can account for regionally distinct modern plant communities, and enabled a unique LGM refugium just south of the Alps and their glaciated forelands (Kaltenrieder, Procacci, Vanniere, & Tinner, ; Kaltenrieder et al., ; Paganelli & Miola, ; Samartin et al., ). We investigated the topoclimate of the Euganean Hills at fine spatial and temporal scales by recording half‐hourly near‐surface air temperatures over a period of 11 months (November 2015–September 2016) with 46 temperature sensors distributed along the entire elevational and aspect gradients of the hills.…”