The climate, the fuel and the land use: long-term regional variability of biomass burning in boreal forests

The influence of different drivers on changes in North American and European boreal forests biomass burning (BB) during the Holocene was investigated based on the following hypotheses: land use was important only in the southernmost regions, while elsewhere climate was the main driver modulated by changes in fuel type. BB was reconstructed by means of 88 sedimentary charcoal records divided into six different site clusters. A statistical approach was used to explore the relative contribution of (1) pollen-based mean July/summer temperature and mean annual precipitation reconstructions, (2) an independent model-based scenario of past land use (LU), and (3) pollen-based reconstructions of plant functional types (PFTs) on BB. Our hypotheses were tested with: (1) a west -east northern boreal sector with changing climatic conditions and a homogeneous vegetation, and (2) a north -south European boreal sector characterized by gradual variation in both climate and vegetation composition

Reconstructions of biomass burning from sediment charcoal records to improve data-model comparisons

The location, timing, spatial extent, and frequency of wildfires are changing rapidly in many parts of the world, producing substantial impacts on ecosystems, people, and potentially climate. Paleofire records based on charcoal accumulation in sediments enable modern changes in biomass burning to be considered in their long-term context. Paleofire records also provide insights into the causes and impacts of past wildfires and emissions when analyzed in conjunction with other paleoenvironmental data and with fire models. Here we present new 1000 year and 22 000 year trends and gridded biomass burning reconstructions based on the Global Charcoal Database version 3, which includes 736 charcoal records (57 more than in version 2). The new gridded reconstructions reveal the spatial patterns underlying the temporal trends in the data, allowing insights into likely controls on biomass burning at regional to global scales. In the most recent few decades, biomass burning has sharply increased in both hemispheres, but especially in the north, where charcoal fluxes are now higher than at any other time during the past 22 000 {years}. We also discuss methodological issues relevant to data-model comparisons, and identify areas for future research. Spatially gridded versions of the global dataset from GCDv3 are provided to facilitate comparison with and validation of global fire simulations

Assessing changes in global fire regimes

PAGES, Past Global Changes, is funded by the Swiss Academy of Sciences and the Chinese Academy of Sciences and supported in kind by the University of Bern, Switzerland. Financial support was provided by the U.S. National Science Foundation award numbers 1916565, EAR-2011439, and EAR-2012123. Additional support was provided by the Utah Department of Natural Resources Watershed Restoration Initiative. SSS was supported by Brigham Young University Graduate Studies. MS was supported by National Science Centre, Poland (grant no. 2018/31/B/ST10/02498 and 2021/41/B/ST10/00060). JCA was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 101026211. PF contributed within the framework of the FCT-funded project no. UIDB/04033/2020. SGAF acknowledges support from Trond Mohn Stiftelse (TMS) and University of Bergen for the startup grant ‘TMS2022STG03’. JMP participation in this research was supported by the Forest Research Centre, a research unit funded by Fundação para a Ciência e a Tecnologia I.P. (FCT), Portugal (UIDB/00239/2020). A.-LD acknowledge PAGES, PICS CNRS 06484 project, CNRS-INSU, Région Nouvelle-Aquitaine, University of Bordeaux DRI and INQUA for workshop support.Background The global human footprint has fundamentally altered wildfire regimes, creating serious consequences for human health, biodiversity, and climate. However, it remains difficult to project how long-term interactions among land use, management, and climate change will affect fire behavior, representing a key knowledge gap for sustainable management. We used expert assessment to combine opinions about past and future fire regimes from 99 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. Results Respondents indicated some direct human influence on wildfire since at least ~ 12,000 years BP, though natural climate variability remained the dominant driver of fire regime change until around 5,000 years BP, for most study regions. Responses suggested a ten-fold increase in the frequency of fire 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 frequency, severity, and size in all biomes except grassland ecosystems. Fire regimes showed different climate sensitivities across biomes, but the likelihood of fire regime change increased with higher warming 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, while recognizing that management options are constrained under higher emission scenarios. Conclusion The influence of humans on wildfire regimes has increased over the last two centuries. The perspective gained from past fires should be considered in land and fire management strategies, but novel fire behavior is likely given the unprecedented human disruption of plant communities, climate, and other factors. Future fire regimes are likely to degrade key ecosystem services, unless climate change is aggressively mitigated. Expert assessment complements empirical data and modeling, providing a broader perspective of fire science to inform decision making and future research priorities.Peer reviewe

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced

40 WORKSHOP REPORTS Multi-Scale Analyses of Fire-Climate- Vegetation Interactions on Millennial Scales

(GPWG) is to facilitate scientific research on fire activity in the Earth system through the development of a global charcoal dataset (GCD). Analysis and synthesis of sedimentary charcoal records from around the globe has enabled the identification and explanation of spatiotemporal patterns in paleofire activity, created a framework for exploring fire-climate-vegetation linkages at decadal-to- millennial time scales, and allowed evaluation of fire model simulations at regional to global scales. Science emerging from the GPWG community includes a public-access database and multi-authored publications describing observed spatiotemporal changes in fire at global and regional scale

HANNIBAL'S INVASION ROUTE: AN AGE-OLD QUESTION REVISITED WITHIN A GEOARCHAEOLOGICAL AND PALAEOBOTANICAL CONTEXT

International audienceThe point of Hannibal's departure from New Carthage in Iberia, in 218 bc, and his subsequent march along the Mediterranean coast to the Pyrénées and on to the Rhône Basin, has been reconstructed by ancient historians with considerable accuracy. The latter 400-km phase through the Alps, however, has been the subject of some controversy as to whether the Punic Army followed a southern versus a northern invasion route, or some intermediate variant. What is certain from the ancient texts is that Hannibal was trapped by Gallic tribes in a large defile—a gorge large enough to hold the entire army—along the approach to the high col of passage on to the Po River Plains of northern Italia. The entrapment involved an enfilade attack planned by an unknown Gallic commander, a military operation that nearly decimated the Punic Army. Previous arguments as to the location of the defile have hinged on inconclusive topographic, geological and geomorphic assessments. New data from palaeobotanical reconstruction of the northern approach route show the Gorges de la Bourne and the Gorge du Bréda, astride the Isère River, to have been forest covered during the invasion, which would have made the Gallic assault impossible. The existing evidence argues for a southern route, the approach through the narrow defile of the Combe de Queyras, with passage over the Col de la Traversette, as argued by Sir Gavin de Beer nearly a half century ago. Narrowing the approach route focuses on sites worth geoarchaeological exploration

Long-Term effects of climate and land-use change on larch budmoth outbreaks in the French Alps

The intensity of cyclic larch budmoth (Zeiraphera diniana Guenée; LBM) outbreaks across the European Alps has been reported to have weakened since the early 1980s. In addition to a warmer climate, changes in land-use cover over modern and historical times may have affected the LBM system. Here, we present tree-ring-based reconstructions of LBM outbreaks from a mixed subalpine larch?pine forest in the French Alps for the period 1700?2010. Temporal variation in LBM outbreak severity was mainly driven by land-use changes, including varying forest structure and species composition. Human population pressure and associated resource demands for fuel wood and construction timber not only resulted in a reduction of larch and subsequent suppression of pine, but also supported an overall grassland expansion for livestock. Superimposed on modern land abandonment and pine re-colonization is a strong warming trend, which may also contribute to the observed late 20th-century weakening of Alpine-wide cyclic LBM outbreaks. Our results suggest that a complex interplay of different factors triggered less synchronized LBM outbreaks at broader scales, with overall significantly lower intensities at local scales