Dr. Maria Leunda

Abies alba Mill. is a prominent European tree species predominantly inhabiting cool and humid montane environments. However, paleoecological evidence reveals that during the Eemian and mid-Holocene, A. alba thrived in much warmer and drier climates. This capacity is nowadays reflected in cryptic meso- and sub-Mediterranean lowland populations. To link A. alba populations across diverse climates spanning from the Mediterranean lowlands to the Alpine timberline, we genotyped 421 specimens from Italy and Switzerland at 190 single-nucleotide polymorphisms (SNPs). Population genetic structure analyses indicate that isolated meso- and sub-Mediterranean lowland populations in Tuscany and Ticino align genetically with higher elevation populations in each region, suggesting that their capacity to thrive in warmer, drier conditions cannot be attributed to plantations with planting stock originating from different climates or to the occurrence of a single warm-adapted lineage showing a disjunct distribution, unless migration between Tuscany and Ticino stands occurred. Genotype–environment associations reveal that temperature seasonality, precipitation during critical seasons, and relative humidity are important for explaining the species' genetic variation. With genotype–environment and genotype–phenotype associations, we identified candidate adaptive genes potentially linked to climatic conditions and drought response. While certain adaptive alleles may have spread from Tuscany and Southern Italy or could be explained by a shared ancestry of Ticino and Tuscan populations, local adaptation may have occurred at specific loci. These findings underscore the importance of considering the hitherto overlooked lowland Mediterranean populations of A. alba to better understand the species' climatic niche and its potential for forest conservation and management under global warming.

Mountain lakes are particularly fragile ecosystems undergoing important ecological and depositional transformations associated with ongoing global change. However, the history of anthropogenic impacts on mountain lakes and their catchments is much longer, in many cases featuring millennia of summer pastoral farming. More recently, the growing demand for raw materials and energy linked to industrialization, particularly accelerated since the 19th century CE, meant a further increase in human impact on mountain areas. The Cantabrian Range (northern Spain) constitutes a paradigmatic case of southern European mountain range experiencing intense human impact during the past millennia and particularly the past two centuries. Here, we have reconstructed the environmental dynamics of this area during the last millennium, with a particular focus on the impact of mining, based on the multidisciplinary analysis (sedimentology, biogeochemistry, magnetic susceptibility, diatoms, pollen, charcoal and dung fungal spores) of sediment cores from Lago de La Cueva (43°03'N, 6°06'W, 1550 m a.s.l.). Fire induced deforestation during the 15th century CE increased erosion during the Little Ice Age. The onset of iron mining in the catchment 200 years ago significantly impacted the lake, increasing sedimentation rates and mining waste containing hematite and potentially toxic elements. Diatoms showed that lake regulation since the early 20th century CE severely altered the natural hydrological regime introducing rapid seasonal lake-level oscillations and increasing lakeshore erosion, water turbidity and nutrient loads. The recent environmental restoration, finished in 2006, involved the re-deposition of mine tailings. Although mining wastewater still reaches the lake, restoration effectively reduced erosion and nutrient loads. This study illustrates the complex interactions between human activities (grazing, mining, hydropower) and climate change in shaping mountain landscapes through time. Our findings highlight the usefulness of Paleolimnology to quantitatively assess the effectiveness of lake restoration programs.

Alpine ecosystems are particularly affected by climate change due to their high sensitivity to temperature variations. Understanding the response of vegetation to rapid temperature increase and human impact is necessary to produce accurate and reliable predictions of future mountain vegetation and mitigate climate change impacts. Our multiproxy study reconstructs climate, fire, and vegetation dynamics over the last 14,200 cal. BP at Lai da Vons (1991 m a.s.l.), a subalpine lake situated in the treeline ecotone in Eastern Switzerland. Our results based on geochemical analysis (XRF), pollen and charcoal analysis, and high-resolution macrofossil analysis reveal that early afforestation started already at the onset of the Holocene, dominated by Pinus cembra and Larix decidua. Larch and stone pine formed closed forests at the site at 10,300 cal. BP. From 9500 cal. BP onwards, when summer temperatures were warmer than today, Picea abies expanded simultaneously with Abies alba. In the past 6500 years, increasing human impact, culminating during two main phases of landscape opening and intensification of fire regime, led to a progressive increase of species richness. Considering future climate change, an upward shift of treeline and timberline is expected in the absence of domesticated herbivores. We advocate that low-intensity pastoralism in such a cultural landscape is essential to preserve the plant diversity of species-rich mountain meadows.

Over the last years, perennial ice deposits located within caves have awakened interest as places to study microbial communities since they represent unique cryospheric archives of climate change. Since the beginning of the twentieth century, the temperature has gradually increased, and it is estimated that by the end of this century the increase in average temperature could be around 4.0°C. In this context of global warming the ice deposits of the Pyrenean caves are undergoing a significant regression. Among this type of caves, that on the Cotiella Massif in the Southern Pyrenees is one of the southernmost studied in Europe. These types of caves house microbial communities which have so far been barely explored, and therefore their study is necessary. In this work, the microbial communities of the Pyrenean ice cave A294 were identified using metabarcoding techniques. In addition, research work was carried out to analyze how the age and composition of the ice affect the composition of the bacterial and microeukaryotic populations. Finally, the in vivo effect of climate change on the cellular machinery that allow microorganisms to live with increasing temperatures has been studied using proteomic techniques.

Predicting the effects of ongoing climate warming on vegetation requires a long-term perspective of past ecosystem dynamics. We therefore analysed the sedimentary record from the mire Gola di Lago (985 m a.s.l.) in southern Switzerland, to better understand the vegetation response to past rapid climate change. We present a high-resolution pollen and plant macrofossil study from the Lateglacial to the Early Holocene (13 400–10 400 cal. a BP), a climatic transition that represents a close analogue to current global warming. The vegetation responses during this time match previously analysed palaeoecological sites south of the Alps. At the end of the Bølling-Allerød interstadial, the site was surrounded by open larch forest. The beginning of the Younger Dryas was characterized by the local presence and rapid expansion of Pinus cembra, whereas the second part was dominated by Pinus sylvestris, Betula and Larix decidua. These vegetation dynamics agree well with independent climate data indicating a cold and dry start and a subsequent shift to slightly warmer climate. The following rapid temperature increase at the beginning of the Holocene resulted in an increase in forest density and the establishment of novel temperate vegetation assemblages, without major changes in species diversity. Noteworthy, during the Younger Dryas–Early Holocene transition, long-lived cold-adapted tree line species such as P. cembra and L. decidua persisted over several centuries, while open boreal forests dominated by P. sylvestris and Betula expanded, indicating high resilience to climatic changes. The results of Gola di Lago suggest that climate warming led to a significant change in vegetation communities through species range shifts. Our study also indicates that some species may potentially be able to locally persist under current global warming, forming novel vegetation assemblages with newly establishing species.

Aim: Pinewood decline and scrubland expansion are major features of Late Holocene vegetation history in the Cantabrian Range. However, the drivers of this remarkable vegetation shift remain to be investigated. Here, we aim at disentangling the role of past land use and climate in shaping the high-elevation Cantabrian landscape during the past two millennia.
Location: Cantabrian Range (northern Iberia).
Taxa: Pinus sylvestris, Betula, Ericaceae, Juniperus, Poaceae.
Methods: We conducted high-resolution multiproxy palaeoecological analyses (pollen, plant macrofossils, microscopic charcoal and dung fungi) on lake sediments from Lago del Ausente to reconstruct vegetation, fire occurrence and grazing through time. The chronology is based on ¹⁴C (terrestrial plant macrofossils) and ²¹⁰Pb dating, and Bayesian age-depth modelling (‘rbacon’). We carried out cross-correlation analysis to quantify vegetation responses to fire.
Results: Between 250 and 900 CE, the vegetation above 1700 m a.s.l. consisted of subalpine scrubland and scattered P. sylvestris trees/stands. Pinewoods with Betula were widespread at slightly lower elevation. This vegetation was resilient to moderate fire disturbance associated with limited pastoral activities. In contrast, enhanced fire occurrence alongside heavier pastoralism led to the demise of pinewoods and their replacement with Betula stands, subalpine scrublands, and meadows between 900 and 1100 CE. Later, the subalpine scrubland-birch tree line did not respond to Little Ice Age cooling. However, further intensification of transhumant herding between 1300 and 1860 CE (‘La Mesta') triggered birch decline and the establishment of the modern treeless landscape.
Main conclusions: The extant high-elevation vegetation of the Cantabrian Range is largely the legacy of intensive land use starting more than one millennium ago. Recurrent and severe fires to promote pasturelands led to the regional extirpation of the previously widespread Pinus sylvestris. Future management should aim at preserving the valuable cultural open landscape of mountain scrubland and meadows and also at restoring patches of ancient pine-birch woodlands.

The rapid development of ancient DNA (aDNA) analysis in the last decades has induced a paradigm shift in ecology and evolution. Driven by a combination of breakthroughs in DNA isolation techniques, high-throughput sequencing and bioinformatics, ancient genome-scale data for a rapidly growing variety of taxa is now available, allowing researchers to directly observe demographic and evolutionary processes over time. However, the vast majority of palaeogenomic studies still focuses on human or animal remains. In this article, we make the case for a vast untapped resource of ancient plant material that is ideally suited for palaeogenomic analyses: Plant remains such as needles, leaves, wood, seeds or fruits that are deposited in natural archives, such as lake sediments, permafrost or even ice caves. Such plant remains are commonly found in large numbers and in stratigraphic sequence through time and have so far been used primarily to reconstruct past local species presences and abundances. However, they are also unique repositories of genetic information with the potential to revolutionize the fields of ecology and evolution by directly studying microevolutionary processes over time. Here, we give an overview of the current state-of-the-art, address important challenges, and highlight new research avenues to inspire future research.

In Alpine regions, speleothem development on karst systems largely occurs during warm interglacial or interstadial phases due to their limited growth during cold stages. Still, recent attention has been given to the role of clastic sediments in caves, less dependent on temperature conditions. Yet, only a small number of caves worldwide preserve both speleothems and detrital deposits. Here we present an outstanding record of fine-grain laminated sediments and carbonate speleothems from the Granito cave (South Central Pyrenees, Spain) associated with seasonal to annual hydroclimatic pulses through the Last Glacial Cycle (LGC). Analysis of cave clastic facies together with new absolute dates on glacial deposits along the valley has provided new insights into the karst-glacial interactions in relation with long-term landscape evolution, with support of geomorphological, sedimentological, mineralogical, palynological, geochemical and geochronological data (U/Th series, OSL and IRSL). The Granito cave was formed before 153.6±1.2 ky (Marine Isotopic Stage, MIS 6), as indicated by the oldest stalagmites dated in the cave, although speleothem formation occurred preferentially during MIS 5 and the Holocene interglacial stages. A 60 m thick clastic deposit was IRSL dated from 71.8±5.6 ky to after 26.5±2.5 ky corresponding with the maximum ice extent (MIE) during the LGC and subsequent glacial stabilization at 30.3±1.7 – 36.2±2.2 ky. The sediment infill was produced by water ponded in the cave coeval with the presence of glacial ice blocking the cave entrance. The cave clastic sequence includes channel, slackwater and backswamp facies, interpreted to result from pulses of sediment-laden water flow under vadose and phreatic flow regimes. The cave sediment's mineralogical composition points to an exogenous source associated with glacial till sediments overlying the hillslope above the cave and along the Ara River valley. The palynological results obtained from the cave sediments show the dominance of an open landscape composed of steppe vegetation indicating cold glacial climatic conditions. The opening of the cave occurred before 14.9±1.5 ky, the stabilization age of the innermost Ara glacier moraine, giving rise to scouring and cut-and-fill sequences within the detrital infill. Speleothem growth during the Holocene indicates the return to warm climatic conditions with the development of a soil and vegetation cover above the cave. This study shows how a combined interpretation of clastic and speleothem lithofacies is critical for elucidating landscape evolution and surface-groundwater palaeohydrological changes in northern Iberia during the LGC.

Glaciers and their microbiomes are exceptional witnesses of the environmental conditions from remote times. Climate change is threatening mountain glaciers, and especially those found in southern Europe, such as the Monte Perdido Glacier (northern Spain, Central Pyrenees). This study focuses on the reconstruction of the history of microbial communities over time. The microorganisms that inhabit the Monte Perdido Glacier were identified using high-throughput sequencing, and the microbial communities were compared along an altitudinal transect covering most of the preserved ice sequence in the glacier. The results showed that the glacial ice age gradient did shape the diversity of microbial populations, which presented large differences throughout the last 2000 years. Variations in microbial community diversity were influenced by glacial conditions over time (nutrient concentration, chemical composition, and ice age). Some groups were exclusively identified in the oldest samples as the bacterial phyla Fusobacteria and Calditrichaeota, or the eukaryotic class Rhodophyceae. Among groups only found in modern samples, the green sulfur bacteria (phylum Chlorobi) stood out, as well as the bacterial phylum Gemmatimonadetes and the eukaryotic class Tubulinea. A patent impact of human contamination was also observed on the glacier microbiome. The oldest samples, corresponding to the Roman Empire times, were influenced by the beginning of mining exploitation in the Pyrenean area, with the presence of metal-tolerant microorganisms. The most recent samples comprise 600-year-old ancient ice in which current communities are living.

Mountain glaciers have generally experienced an accelerated retreat over the last 3 decades as a rapid response to current global warming. However, the response to previous warm periods in the Holocene is not well-described for glaciers of the southern Europe mountain ranges, such as the Pyrenees. The situation during the Medieval Climate Anomaly (900-1300 CE) is particularly relevant since it is not certain whether the southern European glaciers just experienced significant ice loss or whether they actually disappeared. We present here the first chronological study of a glacier located in the Central Pyrenees (NE Spain), Monte Perdido Glacier (MPG), carried out by different radiochronological techniques and a comparison with geochemical proxies from neighbouring palaeoclimate records. The chronological model evidences that the glacier persisted during the Roman period and the Medieval Climate Anomaly. The apparent absence of ice in the past ∼ 600 years suggests that any ice accumulated during the Little Ice Age has since ablated. This interpretation is supported by measured concentrations of anthropogenic metals, including Zn, Se, Cd, Hg and Pb, which have concentrations well below those typical of industrial-age ice measured at other glaciers in Europe. This study strengthens the general understanding that warming of the past few decades has been exceptional for the past 2 millennia.