Changes in tree cover influence many aspects of the earth system. Recent regional changes in tree cover, as documented by remote-sensed observations, are insufficient in capturing the response to large climate changes or differentiating the impacts of human activities from natural drivers. Pollen records provide an opportunity to examine the causes of changes in tree cover in response to large climate changes in the past and during periods when human influence was less important than today. Here, we reconstruct changes in tree cover in Europe through the Holocene using fossil pollen records, using the modelled relationship between observed modern tree cover and modern pollen samples. At a pan-European scale, tree cover is low at the beginning of the Holocene but increases rapidly during the early Holocene and is maximal at ca. 6500 cal. BP, after which tree cover declines to present-day levels. The rapidity of the post-glacial increase in tree cover and the timing and length of maximum tree cover varies regionally, reflecting differences in climate trajectories during the early and mid-Holocene. The nature of the subsequent reduction in tree cover also varies, which may be due to differences in climate but may also reflect different degrees of human influence. The reconstructed patterns of change in tree cover are similar to those shown in previous reconstructions. Our approach is relatively simple and only requires readily available data; it could therefore be applied to reconstruct tree cover globally.

Human activities have a major impact on fire regimes. Human activities that cause landscape fragmentation, such as creating roads and other infrastructure or converting areas to agriculture, tend to restrict, rather than promote, fire. The human influence is complex, however, and the impact of fragmentation on the fire regime depends on climate and vegetation conditions. Climate-induced changes in vegetation and fuel loads also affect the natural fire regime in ways independent of human influence. Disentangling the controls of fire regimes is challenging because of the multiple interactions between climate, vegetation, people and fire, and the different timescales over which they operate. We explore these relationships, drawing on statistical and modelling analyses of palaeoenvironmental, historical and recent observations at regional to global scales. We show how these relationships have changed through time and how they vary spatially as a function of environmental and biotic gradients. Specifically, we show that climate and climate-driven changes in vegetation have been the most important drivers of changing fire regimes at least until the Industrial Revolution. Statistical and modelling analyses show no discernible impact of hunter-gatherer communities, and even the time-transgressive introduction of agriculture during the Neolithic had no impact on fire regimes at a regional scale. The post-industrial expansion of agriculture was an important influence on fires, but since the late 19th century, the overwhelming influence of humans has been to reduce fire through progressive landscape fragmentation rather than through influencing ignitions. Model projections suggest that the reduction of fire through fragmentation will be outweighed by climatically driven increases by the end of the 21st century.This article is part of the theme issue 'Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks'.

Across plant communities worldwide, fire regimes reflect a combination of climatic factors and plant characteristics. To shed new light on the complex relationships between plant characteristics and fire regimes, we developed a new conceptual mechanistic model that includes plant competition, stochastic fires, and fire-vegetation feedback. Considering a single standing plant functional type, we observed that highly flammable and slowly colonizing plants can persist only when they have a strong fire response, while fast colonizing and less flammable plants can display a larger range of fire responses. At the community level, the fire response of the strongest competitor determines the existence of alternative ecological states (i.e., different plant communities) under the same environmental conditions. Specifically, when the strongest competitor had a very strong fire response, such as in Mediterranean forests, only one ecological state could be achieved. Conversely, when the strongest competitor was poorly fire adapted, alternative ecological states emerged-for example, between tropical humid savannas and forests or between different types of boreal forests. These findings underline the importance of including the plant fire response when modeling fire ecosystems, for example, to predict the vegetation response to invasive species or to climate change.

The relative importance of climate change and human activities in influencing regional fire regimes during the Holocene is still a matter of debate. The introduction of agriculture during the Neolithic provides an opportunity to examine the impact of human activities on fire regimes. Here, we examine changes in fire regimes across Iberia between 10,000 and 3500 cal. BP, reconstructed using sedimentary charcoal records. We compare the regional fire history with estimates of changes in population size, reconstructed based on summed probability distributions of radiocarbon dates on archaeological material. We also compare the fire records and population reconstructions with the timing of the onset of agriculture across the region as indicated by archaeological data. For Iberia as a whole, there are two intervals of rapid population increase centred on ca. 7400 and ca. 5400 cal. BP. Periods of rapid population growth, either for the region as a whole or more locally, do not closely align with changes in charcoal accumulation. Charcoal accumulation had already begun to increase ca. 400 years prior to the onset of the Neolithic and continued to increase for ca. 750 years afterwards, indicating that changes in fire are not directly associated with the introduction of agriculture. Similarly, there is no direct relationship between changes in charcoal accumulation and later intervals of rapid population growth. There is also no significant relationship between population size and charcoal accumulation across the period of analysis. Our analyses show that the introduction of agriculture and subsequent increases in population are not directly linked with changes in fire regimes in Iberia and support the idea that changes in fire are largely driven by other factors such as climate.

<p>Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. The Reading Palaeofire Database (RPD) represents the most comprehensive compilation of sedimentary charcoal data currently available. It contains 1673 individual charcoal records from 1480 sites worldwide, with sufficient metadata to allow for the appropriate selection of sites to address specific questions. Most of the records have new age models, made by re-calibrating the radiocarbon ages using INTCAL2020 and Bayesian age-modelling software. In this talk we will show how these data are being used to document changing fire regimes during the Late Quaternary and to explore how fire regimes have responded to changes in climate, vegetation and human activities. We will demonstrate the progress that has been made to calibrate the charcoal records and make quantitative estimates of fire properties. We will also explore how these data can be used to evaluate and benchmark process-based fire-enabled models. Finally, we will highlight opportunities to use the palaeo-record together with models to explore fire regimes and their consequences for land-surface processes, biogeochemical cycles and climate.</p>

Climate changes have been invoked to explain recent changes in wildfire regimes in Mediterranean regions, and climate projections suggest that there will be an increase in fire weather during the 21st century. However, humans influence natural fire regimes today directly by supressing or igniting fires, and indirectly by changing fuel types and fuel structure through land use changes. Recent observations provide only a limited basis for determining the relative importance of climate and human activities for fire. The diachronous introduction of agriculture during the Neolithic provides an opportunity to examine the potential impact of human activities on fire regimes. We reconstruct fire history using sedimentary charcoal records and population change based on summed probability distributions of radiocarbon dates on archaeological material, focusing on the interval between 10,000 and 3,500 cal. BP. The archaeological radiocarbon dates are also used to map the onset of agriculture through time across the region. For Iberia as a whole, we identify two periods of rapid population growth, centred on ca. 7,400 and ca. 5,400 cal. BP. However, these periods of rapid population growth are not synchronous with changes in charcoal accumulation. Changes in charcoal accumulation are not aligned with the time-transgressive dates for the introduction of agriculture across the region; charcoal accumulation was already increasing ca. 400 years prior to the onset of agriculture and continues to increase for ca. 200 years afterwards. There is also no consistent correlative relationship between population and fire across the period of analysis. Our analyses show that there are no direct links between the introduction of agriculture or subsequent increases in population and changes to fire regimes in Iberia in the early to mid-Holocene, suggesting that changes in fire regimes were largely driven by climate changes.

Abstract. Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using INTCAL2020 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1681 records (entities) from 1477 sites worldwide. The database (DOI: 10.17864/1947.319) is available from https://researchdata.reading.ac.uk/id/eprint/319.

Abstract. Charcoal accumulated in lake, bog or other anoxic sediments through time has been used to document the geographical patterns in changes in fire regimes. Such reconstructions are useful to explore the impact of climate and vegetation changes on fire during periods when the human influence was less prevalent than today. However, charcoal records only provide semi-quantitative estimates of change in biomass burning. Here we derive quantitative estimates of burnt area from vegetation data in two stages. First, we relate the modern charcoal abundance to burnt area using a conversion factor derived from a generalized linear model of burnt area probability based on eight environmental predictors. Then, we establish the relationship between fossil pollen assemblages and burnt area using Tolerance-weighted Weighted Averaging Partial Least-Squares with sampling frequency correction (fxTWA-PLS). We test this approach using the Iberian Peninsula as a case study because it is a fire-prone region with abundant pollen and charcoal records covering the Holocene. We derive the vegetation-burnt area relationship using the 29 records that have both modern and fossil charcoal and pollen data, and then reconstruct palaeo-burnt area for the 114 records with Holocene pollen records. The pollen data predict charcoal abundances through time relatively well (R2 = 0.47) and the changes in reconstructed burnt area are synchronous with known climate changes through the Holocene. This new method opens up the possibility of reconstructing changes in fire regimes quantitatively from pollen records, which are far more numerous than charcoal records.

Invasions by non-native plant species are widely recognized as a major driver of biodiversity loss. Globally, (sub-)tropical islands form important components of biodiversity hotspots, while being particularly susceptible to invasions by plants in general and vines in particular. We studied the impact of the invasive vine A. leptopus on the diversity and structure of recipient plant communities on the northern Caribbean island St. Eustatius. We used a paired-plot design to study differences in species richness, evenness and community structure under A. leptopus-invaded and uninvaded conditions. Community structure was studied through species co-occurrence patterns. We found that in plots invaded by A. leptopus, species richness was 40–50% lower, and these plots also exhibited lower evenness. The magnitude of these negative impacts increased with increasing cover of A. leptopus. Invaded plots also showed higher degrees of homogeneity in species composition. Species co-occurrence patterns indicated that plant communities in uninvaded plots were characterized by segregation, whereas recipient plant communities in invaded plots exhibited random co-occurrence patterns. These observations suggest that invasion of A. leptopus is not only associated with reduced species richness and evenness of recipient communities in invaded sites, but also with a community disassembly process that may reduce diversity between sites. Given that A. leptopus is a successful invader of (sub-)tropical islands around the globe, these impacts on plant community structure highlight that this invasive species could be a particular conservation concern for these systems.

Anthropogenic land-use change and ecosystem management have a demonstrable impact on modern fire regimes. However, when in time this influence was first felt is still an open question. We investigate whether an anthropogenic signal can be identified in Holocene fire records from the Iberian Peninsula, a region with abundant palaeoenvironmental and archaeological data. We analyse sedimentary charcoal data from 49 sites across the Peninsula covering part or all of the past 12,000 years to construct the fire history for the region. We compare these records to the summed probabilities of radiocarbon-dated archaeological sites, which provides an index for changes in human impact on land use and land cover due to the growth or decrease in human population through time. This reconstruction is based on 8200 radiocarbon dates covering the timespan between12000 and 3500 uncal BP. Our analyses confirm that the broad trends in fire history are well aligned with the likely impact of climate changes during the Holocene. The charcoal records indicate a rapid increase in fire at the end of the Younger Dryas, a reduction in fire during the middle Holocene as a result of wetter conditions across the Peninsula, and an increase in fire concordant with the increased aridity registered during the interval after 3000 yr BP. However, finer-scale temporal variations are superimposed upon these broadscale changes. Similarly, although the most pronounced change in population reflects population growth associated with the onset of agriculture in the mid-Holocene, the summed probability record of population shows considerable finer-scale temporal variation. In addition to analyses of the temporal correlations between the two data sets, we consider whether there are distinct geographic patterns that could provide additional insights into the relationship between human activities and fire across Iberia.

Fire is an important disturbance process, having significant socio-economic consequences on the one hand, while fulfilling a vital ecological role on the other. Across fire-prone ecosystems, different fire regimes can be found, reflecting a combination of climatic factors and of different plant species characteristics. Ecosystem flammability and fuel load are the most evident and well-studied aspects of fire regime, with only recently attention being devoted to plant traits associated with fire adaptation and post-fire response. The aim of this research is to understand the role that plant traits have in driving fire regimes in different fire-prone ecosystems across the world. A mathematical, mechanistic model was developed representing vegetation dynamics, including stochastic fires and different plant fire-responses. We observe that differences in combinations of plant traits are an important factor in determining alternative ecological states. This is driven by differences in how plants determine fire occurrence and in relation to competition between plant species. Differing plant communities under the same climatic conditions can occur when the most competitive plant types do not have a strong resistance to fires, leading to different ecological and fire regime states for example in some tropical savannas and forests, or in Boreal forests. Conversely, when the dominant plant type has a very strong, post-fire response (at individual level), as e.g. in Mediterranean forests, only one ecological state is possible. This research can help improving understanding of changes in fire regime in the future to assist in fire management efforts, and underlines the importance of including plant fire-responses when modelling fire ecosystems under climate-change scenarios.

Although it is well known that mean annual rainfall (MAR) and rainfall seasonality have a key role in influencing the distribution of tree and grass cover in African tropical grassy biomes (TGBs), the impact of intra-seasonal rainfall variability on these distributions is less agreed upon. Since the prevalent mechanisms determining biome occurrence and distribution change with MAR, this research investigates the role of intra-seasonal rainfall variability for three different MAR ranges, assessing satellite data on grass and tree cover, rainfall and fire intervals at a sub-continental scale in sub-Saharan Africa. For MAR below 630 mm y −1 , rainfall frequency had a positive relationship with grass cover; this relationship however became mostly negative at intermediate MAR (630–1200 mm y −1 ), where tree cover correspondingly mostly increased with rainfall frequency. In humid TGBs, tree cover decreased with rainfall intensity. Overall, intra-seasonal rainfall variability plays a role in determining vegetation cover, especially in mesic TGBs, where the relative dominance of trees and grasses has previously been largely unexplained. Importantly, the direction of the effect of intra-seasonal variability changes with MAR. Given the predicted increases in rainfall intensity in Africa as a consequence of climate change, the effects on TGBs are thus likely to vary depending on the MAR levels.