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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.

Human activity has fundamentally altered wildfire on Earth, creating serious consequences for human health, global biodiversity, and climate change. However, it remains difficult to predict fire interactions with land use, management, and climate change, representing a serious knowledge gap and vulnerability. We used expert assessment to combine opinions about past and future fire regimes from 98 wildfire researchers. We asked for quantitative and qualitative assessments of the frequency, type, and implications of fire regime change from the beginning of the Holocene through the year 2300. Respondents indicated that direct human activity was already influencing wildfires locally since at least ~12,000 years BP, though natural climate variability remained the dominant driver of fire regime until around 5000 years BP. Responses showed a ten-fold increase in the rate of wildfire regime change during the last 250 years compared with the rest of the Holocene, corresponding first with the intensification and extensification of land use and later with anthropogenic climate change. Looking to the future, fire regimes were predicted to intensify, with increases in fire frequency, severity, and/or size in all biomes except grassland ecosystems. Fire regime showed quite different climate sensitivities across biomes, but the likelihood of fire regime change increased with higher greenhouse gas emission scenarios for all biomes. Biodiversity, carbon storage, and other ecosystem services were predicted to decrease for most biomes under higher emission scenarios. We present recommendations for adaptation and mitigation under emerging fire regimes, concluding that management options are seriously constrained under higher emission scenarios.Competing Interest StatementThe authors have declared no competing interest. Introduction Methods and materials Results and discussion Timing and type of future fire regime change Consequences of fire regime change Our capacity to prevent, control, or adapt to future fire regimes Charting a course in a world of uncertainty Expert assessment application in fire management

Many theoretical models predict when genetic evolution and phenotypic plasticity allow adaptation to changing environmental conditions. These models generally assume stabilizing selection around some optimal phenotype. We however often ignore how optimal phenotypes change with the environment, which limit our understanding of the adaptive value of phenotypic plasticity. Here, we propose an approach based on our knowledge of the causal relationships between climate, adaptive traits, and fitness to further these questions. This approach relies on a sensitivity analysis of the process-based model Phenofit, which mathematically formalizes these causal relationships, to predict fitness landscapes and optimal budburst dates along elevation gradients in three major European tree species. Variation in the overall shape of the fitness landscape and resulting directional selection gradients were found to be mainly driven by temperature variation. The optimal budburst date was delayed with elevation, while the range of dates allowing high fitness narrowed and the maximal fitness at the optimum decreased. We also found that the plasticity of the budburst date should allow tracking the spatial variation in the optimal date, but with variable mismatch depending on the species, ranging from negligible mismatch in fir, moderate in beech, to large in oak. Phenotypic plasticity would therefore be more adaptive in fir and beech than in oak. In all species, we predicted stronger directional selection for earlier budburst date at higher elevation. The weak selection on budburst date in fir should result in the evolution of negligible genetic divergence, while beech and oak would evolve counter-gradient variation, where genetic and environmental effects are in opposite directions. Our study suggests that theoretical models should consider how whole fitness landscapes change with the environment. The approach introduced here has the potential to be developed for other traits and species to explore how populations will adapt to climate change.

Diversity of plant communities in semi-natural grasslands is strongly dependent on management practices. To characterize community diversity and agricultural practices, we need easy-to-use indicators. The diagnostic species are used to characterize and differentiate vegetation types, but are they useful to assess the various facets of community diversity and the agricultural practices applied to managed grasslands? Based on the fidelity analysis of 311 phytosociological releves, considered as references for mesic grassland classification in the Franche-Comte region (eastern France), we identified eight diagnostic species of mesic pastures (Cynosurion cristati). We then compared the number of diagnostic species observed in 45 mountain pastures with taxonomic, phylogenetic and multi-trait functional diversity indices, as well as with agronomic variables describing stocking rate and fertilization. To account for the role of diagnostic species identity, we compared Spearman rank correlations obtained for these diagnostic species with correlations calculated for 1000 random samples of eight species from the regional species pool of mesic grasslands. The number of diagnostic species in the grassland plot was significantly positively correlated with most taxonomic, phylogenetic and functional diversity metrics. However, only the positive correlations with Simpson taxonomic diversity and Rao functional diversity were significantly different from those obtained from random samples. Moreover, the number of diagnostic species was significantly negatively correlated with stocking rate and fertilization intensity, although only correlations with industrial fertilizer input were significantly different from results obtained from random samples. These results show that the number of diagnostic species observed in a pasture may be a good indicator of a high taxonomic and functional diversity and of a low fertilization. Thereby, a special attention should be given to diagnostic species to assess the status of grassland biodiversity, including functional aspects.

(1) Background: Frequent fire, climate variability, and human activities collectively influence savanna ecosystems. The relative role of these three factors likely varies on interannual, decadal, and centennial timescales. Here, we tested if Euro-American activities uncoupled drought and fire frequencies relative to previous centuries in a temperate savanna site. (2) Methods: We combined records of fire frequency from tree ring fire scars and sediment charcoal abundance, and a record of fuel type based on charcoal particle morphometry to reconstruct centennial scale shifts in fire frequency and fuel sources in a savanna ecosystem. We also tested the climate influence on fire occurrence with an independently derived tree-ring reconstruction of drought. We contextualized these data with historical records of human activity. (3) Results: Tree fire scars revealed eight fire events from 1822&ndash, 1924 CE, followed by localized suppression. Charcoal signals highlight 13 fire episodes from 1696&ndash, 2001. Fire&ndash, climate coupling was not clearly evident both before and after Euro American settlement The dominant fuel source shifted from herbaceous to woody fuel during the early-mid 20th century. (4) Conclusions: Euro-American settlement and landscape fragmentation disrupted the pre-settlement fire regime (fire frequency and fuel sources). Our results highlight the potential for improved insight by synthesizing interpretation of multiple paleofire proxies, especially in fire regimes with mixed fuel sources.

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.

Fire is a key Earth system process, with 80% of annual fire activity taking place in grassland areas. However, past fire regimes in grassland systems have been difficult to quantify due to challenges in interpreting the charcoal signal in depositional environments. To improve reconstructions of grassland fire regimes, it is essential to assess two key traits: (1) charcoal count, and (2) charcoal shape. In this study, we quantified the number of charcoal pieces in 51 sediment samples of ponds in the Great Plains and tested its relevance as a proxy for the fire regime by examining 13 potential factors influencing charcoal count, including various fire regime components (e.g. the fire frequency, the area burned, and the fire season), vegetation cover and pollen assemblages, and climate variables. We also quantified the width to length (W:L) ratio of charcoal particles, to assess its utility as a proxy of fuel types in grassland environments by direct comparison with vegetation cover and pollen assemblages. Our first conclusion is that charcoal particles produced by grassland fires are smaller than those produced by forest fires. Thus, a mesh size of 120μm as used in forested environments is too large for grassland ecosystems. We recommend counting all charcoal particles over 60μm in grasslands and mixed grass-forest environments to increase the number of samples with useful data. Second, a W:L ratio of 0.5 or smaller appears to be an indicator for fuel types, when vegetation surrounding the site is before composed of at least 40% grassland vegetation. Third, the area burned within 1060m of the depositional environments explained both the count and the area of charcoal particles. Therefore, changes in charcoal count or charcoal area through time indicate a change in area burned. The fire regimes of grassland systems, including both human and climatic influences on fire behavior, can be characterized by long-term charcoal records.

Citation: Berangere Leys and Philip E Higuera and Kendra K McLauchlan and Paul V Dunnette. (2016). Wildfires and geochemical change in a subalpine forest over the past six millennia. Environmental Research Letters, 11(12), 125003.

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.

International audience; Forest fires are controlled by local (vegetation, aspect, elevation, land uses) and regional (climate) factors. The role of these factors remains poorly assessed on centennial to millennial time scales. Here, we investigate fire and vegetation history in five subalpine sites over the past 8000 years to determine when, how, and why there were differences in fire regimes. We focus on time and aspect separately and account for vegetation composition. There are no significant differences in the fire return intervals among sites (~290 years between two fires). The rise of fire frequency from its lowest values (1.5 fires.1000 yr−1) at 8000–7000 years before present (yrBP) to its highest at 4000-3000 yrBP (5 fires-1000 yr−1) is well explained by the high temperatures and low precipitation of the Holocene, both leading to better conditions for fire ignition and spread. From 8000 yrBP, climate is likely the main factor driving the fire frequency on the north-facing sites, which contained mixed Larix decidua-Pinus cembra vegetation; whereas south-facing sites, dominated by the flammable P. cembra and broadleaved trees, appear to be driven by local features, including land use. A shift in the main drivers occurs at 4000 yrBP, when local processes start to control burning activity at all sites. Forest clearance for land use might have directly increased fire frequency, but has also helped to suppress fires through fuel limitation. During the last four millennia, land use likely altered the natural climate-vegetation-fire system mainly by converting forests to grassland by pastoral practices, thus leading to less fire and making the fire-vegetation-climate relationships more complex.

Vegetation composition and fire frequency are tightly linked in North American grasslands and have varied considerably throughout the Holocene in response to different drivers. Yet, detailed records of both long-term changes in grassland vegetation composition and diversity, coupled with fire history, are still relatively sparse. In this study, we examine a sediment core from Fox Lake, Minnesota, using pollen, charcoal, magnetic susceptibility, organic carbon (%C), and silica (%Si) records with the aim of understanding grassland structure and function during the Holocene, particularly in the context of vegetation composition and diversity, erosion, and fire activity. Nonarboreal pollen comprises between 37% and 86% of the assemblage throughout the record with the largest percentages occurring during the mid-Holocene (~8000–4000 yr BP). The pollen record also suggests that at 8200 yr BP, there was an abrupt shift from oak-elm woodland to a more open landscape of grassland or savanna, which remained throughout the mid-Holocene. Additionally, the pollen data suggest that vegetation composition exhibited little change in diversity through time despite recurring fire. Charcoal concentrations varied from 30 to nearly 1200 particles cm−3, indicating changes in relative amount of biomass burned, but the morphotypes of charcoal pieces indicate that woody fuels persisted during the mid-Holocene despite the apparent grassland-dominated landscape. Magnetic susceptibility in the sediment ranges from −0.9 to 22.4 (×10−5 SI) throughout the record, with the biggest increase occurring as the vegetation shifted from woodland to grassland entering the mid-Holocene. Organic carbon ranges from 4.6% to 20.0% and exhibits a slow but steady increase after the 8200 yr BP event. Silica decreases slightly but remains generally high between 20.4% and 22.5%.

The availability of fuel and climate are major factors responsible for forest fire activity over time. Here, we tested the hypothesis that forest ecosystems containing a high shrub biomass, which constitutes a fuel load, and affected by a warmer climate, which is associated with drier conditions and a longer fire season, are more prone to fire. Fire occurrence and woody vegetation histories were reconstructed for a subalpine site (Lago di Colbricon Inferiore) in the Dolomites, part of the eastern Italian Alps, for the past 13,000 years. The modern wet climate prevents fire in this area, in spite of the warm summers and an abundant biomass of dwarf pine (Pinus mugo) and three other conifer tree species (Pinus cembra, Picea abies, and Larix decidua). Past fire history reconstructed from sedimentary charcoal showed a median fire return interval of 140 years (30e735 yr fire � 1 ), with a high variability (SD � 170 years) throughout the Holocene, suggesting that the past environment was more favourable to fire than the modern one, probably due to a drier climate or to different fuel availability. The subalpine community containing P. mugo remained stable for the past 9000 years, despite the variability of the fire return interval. Interestingly, the fire frequency is higher at Lago di Colbricon than at sites in the western Alps that lack P. mugo, suggesting that this species tolerates fire disturbance. In fact, it probably favours the spread of fire due to its flammable biomass, prostrated form, and dense layering canopy, thus offsetting the influence of the wet climate. Since the 19th century, the removal of dwarf pine to promote subalpine grasslands may have suppressed fires in this region.

So far, no phytolith extraction protocols have been tested for accuracy and repeatability. Here we aim to display a phytolith extraction method combining the strengths of two widely used protocols, supplemented with silica microspheres as exogenous markers for quantifying phytolith concentrations. Phytolith concentrations were estimated for samples from two sedimentary sequences in which numerical age–depth models make it possible to calculate phytolith influxes (phytolith numbers per cm2per yr). Analysis of replicates confirmed the statistical robustness, the repeatability and the very few biases of our extraction technique for small phytoliths, since the relationship between grass silica short cells and microspheres was kept stable. Furthermore, we demonstrated that silica microspheres are robust exogenous markers for estimating phytolith concentrations. The minimum number of items (i.e., phytoliths plus silica microspheres) that must be counted to estimate phytolith concentrations and thus influxes depends on the ratio of phytoliths to microspheres (R) and is minimized when R = 1. Nevertheless, we recommend using ratios R ≤ 1 in order to avoid having the counting process become excessively time-consuming, because microspheres are easier to identify and count than phytoliths.

Fire-history reconstructions inferred from sedimentary charcoal records are based on measuring sieved charcoal fragment area, estimating fragment volume, or counting fragments. Similar fire histories are reconstructed from these three approaches for boreal lake sediment cores, using locally defined thresholds. Here, we test the same approach for a montane Mediterranean lake in which taphonomical processes might differ from boreal lakes through fragmentation of charcoal particles. The Mediterranean charcoal series are characterized by highly variable charcoal accumulation rates. Results there indicate that the three proxies do not provide comparable fire histories. The differences are attributable to charcoal fragmentation. This could be linked to fire type (crown or surface fires) or taphonomical processes, including charcoal transportation in the catchment area or in the sediment. The lack of correlation between the concentration of charcoal and of mineral matter suggests that fragmentation is not linked to erosion. Reconstructions based on charcoal area are more robust and stable than those based on fragment counts. Area-based reconstructions should therefore be used instead of the particle-counting method when fragmentation may influence the fragment abundance.

International audience; Paleofire events obtained from the statistical treatment of sedimentary charcoal records rely on a number of assumptions and user’s choices, increasing the uncertainty of reconstructio\ns. Among the assumptions made when analyzing charcoal series is the choice of a filtering method for raw Charcoal Accumulation Rate (CHARraw). As there is no ultimate CHARraw filtering method, we propose an ensemble-member approach to reconstruct fire events. We modified the commonly used procedure by including a routine replicating the analysis of a charcoal record using custom smoothing parameters. Dates of robust fire events, uncertainties in fire-return intervals and fire frequencies are derived from members’ distributions. An application of the method is used to quantify uncertainties due to data treatment in two CHARraw sequences from two different biomes, subalpine and boreal.

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