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International audience; Lake Bottom oxbow is a partially-filled oxbow lake within an alluvium filled sandstone canyon reach of the Dolores River in eastern Utah, USA. Two adjacent sediment cores were obtained from the Lake Bottom oxbow to better understand the depositional and environmental history of the site and region. Depositional ages were determined using radiocarbon dating (14C) of pollen extracts, optically stimulated luminescence (OSL) of sand, concentration of 137Cs and 210Pb radioisotopes, and the first occurrence of pollen from human-introduced crops (Zea mays) and trees and shrubs (Tamarix) found in the oxbow sediments. Age control from OSL, short-lived radioisotopes, and pollen are stratigraphically consistent and fit the geomorphic character of the partially-filled oxbow, which suggests it is a late Holocene feature. In contrast, the 14C results appear to be contaminated by old carbon and produced age overestimates of 3000–12,000 years. Our findings suggest that multiple dating methods should be employed when applying radiocarbon dating to settings that are susceptible to contamination by recycled old carbon, especially when dating pollen extracts that are difficult to determine organic purity. The OSL results suggest initial oxbow formation by 860 ± 270 yr (±2σ) and then rapidly infilled with fluvial sediment. The OSL age control indicates the oxbow formed during the Medieval Climate Anomaly, a time of enhanced summer moisture and active flooding in the Dolores River watershed. Similar low-elevation records in the arid Colorado Plateau are rare, making this site an important contribution to the regional paleoenvironmental record.

The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

Highlights: • 18 southern European charcoal records document 16,000 years of fire regime changes. • Since the Neolithic, land uses have artificially maintained high fire frequencies. • Human-induced decreases in forest cover led to a reduction in the biomass burned. • Human-modified landscapes affected ecological processes more meaningly than expected. Abstract: Variability in fire regime at the continental scale has primarily been attributed to climate change, often overshadowing the widely potential impact of human activities. However, human ignition modifies the rhythm of fire episodes occurrence (fire frequency), whereas land use alters vegetation composition and fuel load, and thus the amount of biomass burned. It is unclear, however, whether and how humans have exercised a significant influence over fire regimes at continental and millennial scales. Based on sedimentary charcoal records, we use new alternative estimate of fire frequency and biomass burned for the last 16000 years (here after 16 ky) that we evaluate with outputs from climate, vegetation, land use and population models. We find that pronounced regional-scale land use changes in southern Europe at the beginning of the Neolithic (8–6 ky), during the Bronze Age (5–4 ky) and the medieval period (1 ky) caused a doubling of fire frequency compared to the Holocene average (the last 11.5 ky). Despite anthropogenic influences, southern European biomass burned decreased from 7 ky, which is in line both with changes in orbital parameters leading climate cooling and also reductions in biomass availability because of land use. Our study underscores the role of elevation-dependent parameters, and particularly biomass and land management, as major drivers of fire regime variability. Results attest a determinant anthropogenic driving-force on fire regime and a decrease in fire-carbon emissions since 7 ky in Southern Europe.

1. A palaeolimnological study, covering the last c. 12,000 years, was conducted in a small subalpine lake located in the Alps to study climate change impacts on car- bon flows through food webs in small lakes. We used analysis of sedimentary pigments and carbon stable isotopic composition of chironomid remains (d13 C HC) to reconstruct past dynamics of phytoplankton community and carbon sources sustaining benthic consumers. 2. Chironomid biomass was sustained by a combination of allochthonous, auto- chthonous and CH 4 -derived organic matters, and their relative contributions were correlated to changes in temperature. Relatively high terrestrial contribu- tions to chironomid biomass were observed during period of the Holocene when in-lake production was low. Relatively high incorporation of CH 4 -derived carbon to chironomid biomass was found during anoxic events co-occurring with the Holocene thermal maximum. 3. Results were then compared with those collected in a small boreal lake in Esto- nia. We tested the hypothesis that responses in carbon flows through benthic food web to past climate change would be similar between these lakes. We found a negative correlation between d13 CHC values of both lakes and inferred air temperature, suggesting that temperature was the major driver to different food sources being incorporated into chironomid biomass. 4. Our study demonstrated that air temperature was the principal driver of the energy flows through benthic food web in the studied small lakes. We conjec- tured that carbon cycling in food webs of small lakes might be strongly sensitive to climate change. Financial support was provided by institutional research grants IUT1- 8 and IUT21-2 from Estonian Science Agency and by Estonian University of Life Sciences, project no. M160101, as well as from NEXTDATA project and the Park Paneveggio-Pale di S. Martino, Italy. The authors express their sincere thanks to W. Goedkoop for his help with the English language. We are grateful to Christian Hos- sann (INRA Nancy, Champenoux) for assistance in the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). The authors gratefully acknowl- edge the two anonymous reviewers and Alan Steinman for their con- structive comments on this manuscript. Financial support was provided by institutional research grants IUT1- 8 and IUT21-2 from Estonian Science Agency and by Estonian University of Life Sciences, project no. M160101, as well as from NEXTDATA project and the Park Paneveggio-Pale di S. Martino, Italy. The authors express their sincere thanks to W. Goedkoop for his help with the English language. We are grateful to Christian Hos- sann (INRA Nancy, Champenoux) for assistance in the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). The authors gratefully acknowl- edge the two anonymous reviewers and Alan Steinman for their con- structive comments on this manuscript.

The pace and degree of nutrient limitation are among the most critical uncertainties in predicting terrestrial ecosystem responses to global change. In the northeastern United States, forest growth has recently declined along with decreased soil calcium (Ca) availability, suggesting that acid rain has depleted soil Ca to the point where it may be a limiting nutrient. However, it is unknown whether the past 60 y of changes in Ca availability are strictly anthropogenic or partly a natural consequence of long-term ecosystem development. Here, we report a high-resolution millennial-scale record of Ca and 16 other elements from the sediments of Mirror Lake, a 15-ha lake in the White Mountains of New Hampshire surrounded by northern hardwood forest. We found that sedimentary Ca concentrations had been declining steadily for 900 y before regional Euro-American settlement. This Ca decline was not a result of serial episodic disturbances but instead the gradual weathering of soils and soil Ca availability. As Ca availability was declining, nitrogen availability concurrently was increasing. These data indicate that nutrient availability on base-poor, parent materials is sensitive to acidifying processes on millennial timescales. Forest harvesting and acid rain in the postsettlement period mobilized significant amounts of Ca from watershed soils, but these effects were exacerbated by the long-term pattern. Shifting nutrient limitation can potentially occur within 10,000 y of ecosystem development, which alters our assessments of the speed and trajectory of nutrient limitation in forests, and could require reformulation of global models of forest productivity.

Fire is one of the most prevalent disturbances in the Earth system, and its past characteristics can be reconstructed using charcoal particles preserved in depositional environments. Although researchers know that fires produce charcoal particles, interpretation of the quantity or composition of charcoal particles in terms of fire source remains poorly understood. In this study, we used a unique four-year dataset of charcoal deposited in traps from a native tallgrass prairie in mid-North America to test which environmental factors were linked to charcoal measurements on three spatial scales. We investigated small and large charcoal particles commonly used as a proxy of fire activity at different spatial scales, and charcoal morphotypes representing different types of fuel. We found that small (125–250 μm) and large (250 μm–1 mm) particles of charcoal are well-correlated (Spearman correlation = 0.88) and likely reflect the same spatial scale of fire activity in a system with both herbaceous and woody fuels. There was no significant relationship between charcoal pieces and fire parameters

International audience

Summary 1. This study investigated the fire-vegetation relationship by reconstructing the long-term fire and vegetation dynamics around a small lake in the Mediterranean montane belt on Corsica Island. The vegetation is characterised by forests dominated by Pinus nigra ssp. laricio, an endemic subspecies that is currently threatened. Populations of this taxon are geographically restricted, and their ranges are decreasing, possibly because of disturbance, that is, fire and logging. 2. Here, we examine the role of fire in vegetation dynamics and its effect on diversity since the lateglacial at a montane site using sedimentary plant macrofossils and charcoal to reconstruct local vegetation and fire frequency, respectively. 3. Macrofossils of P. laricio provide evidence for its growth around the Lac de Creno since 13 200 cal. years BP, which is consistent with its natural origin in Corsica. The onset of the Holocene (~11 700 cal. years BP) was marked by a rapid shift in the treeline, the establishment of P. laricio-dominated woodlands with low species turnover, and a long-term increase in taxa richness as a result of successive expansions of broadleaved deciduous trees. In spite of fire-return intervals (FRIs) fluctuating between 30 and 490 years during the Holocene, P. laricio-dominated woodlands persisted for several millennia, and fires likely played an ecological role in the functioning of these woodlands. The 12 000 year record of mean FRI (80 years between fires; i.e. frequency of 12.5 fires 1000 year 1 ) can be used to define a baseline for the management and conservation of P. laricio montane forests. Our findings demonstrate that P. laricio has a long history, a natural origin in Corsica, and that Corsican pine forests have survived to a wide range of burning conditions over the last 13 200 years. 4. Synthesis: We present multimillennial fire and vegetation histories of a Mediterranean montane site. Pinus nigra ssp. laricio has been present near the study site since approximately 13 200 cal. years BP, demonstrating that this pine was present on the island prior to the arrival of pre-historic humans. The long-term records also show that P. laricio woodlands were mixed with deciduous broadleaf trees (Betula sp. and Fagus sylvatica), and other needleleaf trees (Abies alba), at least, and were not influenced by changes in fire frequency. We conclude that (i) fire is a natural component of the ecosystem and (ii) fires likely played an important ecological role in the functioning of the Corsican black pine woodland ecosystem.