Fire is an important element in many ecosystems and in the Earth system (Bowman et al., 2009; Resco de Dios, 2020). It impacts vegetation dynamics, ecosystem functioning and biodiversity (Harrison et al., 2010; Ward et al., 2012; Keywood et al., 2013). It also affects climate through vegetation changes and the release of trace gases and aerosols. Fire directly impacts socio-economic assets (e.g. Stephenson et al., 2013; Thomas et al., 2017) and has deleterious effects on human health though the release of smoke and particulates into the atmosphere (e.g. Johnston et al., 2012; Yu et al., 2020). These impacts make it important to understand what controls the incidence and severity of fires.

Analyses of fire regimes during the satellite era have shown that multiple factors play a role in determining the occurrence of fire, including climate and fire weather, vegetation properties and human activities (e.g. Harrison et al., 2010; Brotons et al., 2013; Bistinas et al., 2014; Knorr et al., 2014; Andela et al., 2017; Forkel et al., 2019a, b; Kuhn-Régnier et al., 2020). However, the satellite record only covers a short time period (ca. 20 years), and the impact of anthropogenic changes to land cover on suppressing fire during this interval is strong (Andela et al., 2017). Reconstructing changing fire regimes during the pre-industrial Holocene (12 000 BP to ca. CE 1850) provides an opportunity to investigate the controls on fire over timescales when human influences on the landscape, including fire regimes, were more localised and less profound than they have become during the industrial era.

Sedimentary charcoal, preserved in lakes, peatbogs and other anoxic environments, has been widely used as an indicator of past changes in fire regimes (Marlon et al., 2008, 2016; Power et al., 2008; Daniau et al., 2012; Vannière et al., 2016; Connor et al., 2019). Evaluations that combine charcoal-inferred palaeofire reconstructions with past hydrological, vegetation and archaeological data support the idea that there are strong relationships among climate, fire, vegetation and human activities (Carrión et al., 2007; Marlon et al., 2008; Gil-Romera et al., 2010; Turner et al., 2010; Vannière et al., 2011; López-Sáez et al., 2018; Morales-Molino et al., 2018). However, charcoal records only provide a semi-quantitative index of fire activity rather than quantitative estimates of burnt area or biomass loss. Attempts to calibrate the charcoal record to provide quantitative estimates of proximity or area burnt are either site-specific (Duffin et al., 2008; Hennebelle et al., 2020) or rely on modelling (Higuera et al., 2007). Furthermore, although the number of charcoal records is increasing, there are still comparatively few sites compared to other types of palaeoenvironmental data, and this can make it difficult to make regional reconstructions of changing fire regimes.

Although the occurrence of fire is influenced by multiple factors, analyses of present-day fire relationships globally using satellite-derived data have shown that vegetation properties determining fuel availability are the strongest determinants of fire occurrence (Bistinas et al., 2014; Forkel et al., 2019a, b; Kuhn-Régnier et al., 2020). This suggests that palaeovegetation data could provide a way of reconstructing burnt area in the past, particularly at times when human influences on land cover were less important. This would also allow us to capitalise on the more extensive site networks for palaeovegetation.

In this study, we present a new method to reconstruct quantitative changes in fire regimes over the Holocene. We relate the relative scale of modern charcoal abundance to absolute burnt area using a conversion factor derived from a generalised linear model (GLM) of fire probability based on burnt area data. We then derive quantitative relationships between pollen assemblages and inferred burnt area using tolerance-weighted weighted averaging partial least-squares regression with a sampling frequency correction (fxTWA-PLS; Liu et al., 2020). The vegetation–burnt area relationship is then used to reconstruct changes in burnt area through time from pollen assemblages, including at sites with no charcoal record. We use the Iberian Peninsula as a test case. The Iberian Peninsula is the most fire-affected region in southern Europe (Jesus et al., 2019; Molina-Terrén et al., 2019). Although the modern fire regime is partly driven by human activities, the patterns also reflect the strong climate gradients across the region. Although much of the Iberian Peninsula has a typical Mediterranean climate, parts of the region are influenced by proximity to the Atlantic Ocean or the Mediterranean Sea and by the mountainous topography, giving rise to complex weather and climate patterns and large gradients in vegetation diversity (Loidi, 2017). We reconstruct fire regimes across the Iberian Peninsula through the Holocene and discuss the implications of the reconstructed changes.

The central premise of our approach is that fire frequency is one of the factors that influences vegetation assemblages (see the Supplement) and therefore that specific aspects of differences in vegetation assemblages – identified by a numerical technique that can isolate the effects of any one controlling factor on taxon composition – can be used to reconstruct fire. The vegetation–fire relationship can be derived by comparing changes in pollen assemblages and charcoal records through time. However, since the charcoal records from different sites consist of different size fractions, and the records must be normalised to facilitate comparisons, it is necessary to derive site-specific conversion factors between modern charcoal abundance and present-day burnt area fraction. This calibration is then applied to the charcoal record in order to derive an estimate of the palaeoburnt area for each pollen sample.

We have shown that it is possible to derive trends in burnt area through time by applying a quantitative relationship between pollen assemblages and charcoal-derived burnt area to palaeovegetation records from the Iberian Peninsula. Our analyses exploit the multivariate nature of vegetation and hence pollen assemblages. Vegetation patterns, and the distribution of individual species, are controlled by many factors including seasonal temperature and precipitation regimes, disturbance (including wildfires) and human activities. Pollen-based palaeoclimate methods have long exploited the multivariate nature of pollen assemblages to reconstruct different aspects of climate (see, for example, the discussion in Bartlein et al., 2011). The canonical correspondence analysis (CCA) shows that there is sufficient information in the pollen assemblages to assess the independent contribution of fire to vegetation assemblages. The overall relationship between pollen and charcoal-derived burnt area is reasonably strong (R2 = 0.44), reflecting the importance of vegetation properties (gross primary production and non-tree cover) in driving the occurrence of fire – as seen in the GLM analysis of satellite-derived modern burnt area patterns. The overwhelming importance of vegetation properties in influencing modern fire occurrence is consistent with results from global analyses (e.g. Moritz et al., 2012; Pausas and Ribeiro, 2013; Bistinas et al., 2014; Forkel et al., 2019b). Nevertheless, the GLM analysis shows that climate factors, in particular the occurrence of dry intervals, are important controls on modern fire patterns in Iberia. Again, this is consistent with global analyses of the modern drivers of fire occurrence.

We have used fxTWA-PLS (Liu et al., 2020) to make the burnt area reconstructions because this technique reduces the compression bias characteristic of many other reconstruction techniques by accounting for differences in the tolerance of individual taxa in an assemblage and for the frequency of the reconstructed variable in the training data set. However, although the bias is apparently reduced, there is still an overestimation at the low end and an underestimation at the high end of the burnt area range. This is reflected by the extremely high burnt area values reconstructed for some sites in northern Iberia in the recent millennium which exceed the upper limit of the low-compression zone. This remaining bias may also be explained by the comparatively small sample size (1106 binned samples) compared with the much larger data set of 6458 samples used by Liu et al. (2020) to carry out climate reconstructions for Eurasia. It would be useful to test whether the problem of compression bias in the reconstruction of burnt area could be overcome by expanding the training data set to cover a wider range of vegetation types and fire regimes.

Although palaeoburnt area reconstructions have only been obtained from a limited number of records, they nevertheless show interesting patterns over the past ca. 15 kyr. The high fire intervals at the beginning of the record, between 14–13 ka and between 12–11 ka, correspond to the Bølling–Allerød (14.6–12.9 ka) warming interval and to rapid warming at the end of the subsequent Younger Dryas (12.9–11.7 ka) cold phase (Alley et al., 1993). A similar response to these climate events has been seen in charcoal records from eastern North America (Marlon et al., 2009). Burnt area is less than today through the Early and Middle Holocene (10–5 ka), an interval when pollen, speleothem and lake records suggest the Mediterranean region was wetter than today (Prentice et al., 1996; Magny et al., 2002; Bartlein et al., 2011; Roberts et al., 2011). Reconstructions of fire activity anomalies (FAAs) for the south-eastern part of the Iberia Peninsula also indicate low-level fire activity in the Mid-Holocene between 7.5 and 6 ka (Gil-Romera et al., 2010). Burnt area continuously increases during the Medieval Warm Period (MWP; 1–0.7 ka) and peaks at 0.6 ka, consistent with the warm and dry conditions recorded during this period in the Iberian Peninsula (Moreno et al., 2012). During the Little Ice Age (LIA; 0.6–0.1 ka), the reconstructed fire indicates a sharp downturn, which may be associated with subsequent cold and wet climate (Ramos-Román et al., 2016; Abrantes et al., 2017). Thus, the broadscale patterns of trends in reconstructed burnt area are consistent with known Holocene climate changes in this region.

There is a distinct west–east gradient in burnt area across Iberia today, and this gradient of high fire in the west and less fire in the east is also present during other intervals of the Holocene. This pattern is likely related to the regional gradient in fuel availability and drought (Pausas and Fernández-Muñoz, 2012). However, the west–east gradient in burnt area is less pronounced during the Mid-Holocene, consistent with a less pronounced gradient in precipitation and moisture availability shown by other studies (e.g. González-Sampériz et al., 2017; Liu, 2019). Reconstructed patterns in fire were also more homogenous after 1 ka, and again this is consistent with the fact that temperature and humidity gradients were less pronounced at that time than they are today (Sánchez-López et al., 2016).

Our analyses show that climate, and climate-induced changes in vegetation, have influenced the fire regimes of the Iberian Peninsula during the Holocene. However, many studies have suggested that human activities could also have been important (Blanco-González et al., 2018; Connor et al., 2019; Feurdean et al., 2020). Land clearance during the Neolithic agricultural transition has been associated with increases in fire activity in some sites from the Iberian Peninsula (e.g. García-Ruiz et al., 2016; Carracedo et al., 2018). Although initiation of agriculture was not synchronous everywhere, the regional onset of agriculture is registered around 7.5 ka (Zapata et al., 2004; Fyfe et al., 2019; Harrison et al., 2020) when the burnt area reconstructions do not indicate high fire activity. However, the gradual increase in reconstructed burnt area between 5 and 0.6 ka may be an indication of increasing human activity, since the initial increase is broadly consistent with increased population shown by summed probability distributions (SPDs) of radiocarbon dates (Balsera et al., 2015; Lillios et al., 2016; Harrison et al., 2020). Human activities, such as deforestation and appropriation of land for agriculture, may have been an important driver of fire patterns from the Bronze Age onwards (Morales-Molino et al., 2013; Morales-Molino and García-Antón, 2014; González-Sampériz et al., 2017), while the competing effects of land abandonment and fire suppression may have contributed to the changes in burnt area in recent times (Turco et al., 2016; Silva et al., 2019). Nevertheless, our GLM analysis indicates that the intensity of human influence, as measured by crop or grazing land area or by population density, consistently has a negative effect on burnt area under modern conditions. It seems likely that human influence on Holocene fire regimes may have been complex, with agricultural expansion both promoting and suppressing fire occurrence. More detailed comparisons of the reconstructed burnt area and archaeological data are required to test this.

The limited availability of charcoal records has meant that the analysis of past fire regimes has tended to focus on large-scale zonal or continental-scale patterns (e.g. Marlon et al., 2008; Power et al., 2008; Daniau et al., 2010; Vannière et al., 2011). Our new methodology opens up the possibility of reconstructing changes in fire regimes from pollen data and thus of examining finer-scale patterning that might reflect climate or human influences on fire. Spatially explicit reconstructions of burnt area would also be useful to evaluate the simulated response of fire to changing environmental drivers in the past (Thonicke et al., 2005; Brücher et al., 2014; Martin Calvo et al., 2014; Marlon et al., 2016; Kraaij et al., 2020) since comparisons based on qualitative inferences from charcoal are inconclusive (e.g. Brücher et al., 2014).

We have developed a novel method to reconstruct palaeoburnt area quantitatively from vegetation records, based on fire–vegetation relationships derived using fxTWA-PLS and the calibration of modern charcoal against GLM modelling of modern burnt area. We have applied this approach to reconstruct changes in burnt area through the Holocene for the Iberian Peninsula. The good predictive power of the fxTWA-PLS-derived fire–vegetation relationship and the plausibility of the palaeofire reconstructions with respect to known climate changes in the region suggest that this calibration approach could be applied more generally to provide quantitative reconstructions of past fire regimes in other regions where there are limited charcoal data, and pollen data are more abundant.