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Palaeoecological data indicates land-use changes across Europe linked to spatial heterogeneity in mortality during the Black Death pandemic

The Black Death (1347–1352 ce) is the most renowned pandemic in human history, believed by many to have killed half of Europe’s population. However, despite advances in ancient DNA research that conclusively identified the pandemic’s causative agent (bacterium Yersinia pestis), our knowledge of the Black Death remains limited, based primarily on qualitative remarks in medieval written sources available for some areas of Western Europe. Here, we remedy this situation by applying a pioneering new approach, ‘big data palaeoecology’, which, starting from palynological data, evaluates the scale of the Black Death’s mortality on a regional scale across Europe. We collected pollen data on landscape change from 261 radiocarbon-dated coring sites (lakes and wetlands) located across 19 modern-day European countries. We used two independent methods of analysis to evaluate whether the changes we see in the landscape at the time of the Black Death agree with the hypothesis that a large portion of the population, upwards of half, died within a few years in the 21 historical regions we studied. While we can confirm that the Black Death had a devastating impact in some regions, we found that it had negligible or no impact in others. These inter-regional differences in the Black Death’s mortality across Europe demonstrate the significance of cultural, ecological, economic, societal and climatic factors that mediated the dissemination and impact of the disease. The complex interplay of these factors, along with the historical ecology of plague, should be a focus of future research on historical pandemics.

begun to put sub-Saharan Africa on the Black Death map 9 , to fill in lacunae in our understanding of the pandemic's Mediterranean and European spread 10,11 , and to explore the pandemic's origins in central Asia and dissemination in east Asia, drawing on evolutionary biology and palaeogenetics 12 . But while multiple disciplines have reassessed the pandemic's spatiotemporality, its mortalityestimates of which have drawn attention to the Black Death for centuries-remains underexplored and limited almost entirely to unidisciplinary, written-source-based approaches.
Medieval mortality data are scarce and highly fragmentary. Nineteenth-century historians of the Black Death based their assertion that the pandemic claimed 25% or more of European lives on assessments of qualitative narrative sources 13 . Since the mid-twentieth century, historians have painstakingly built-up multiple instructive case studies of Black Death mortality for regions comparatively well-endowed with administrative sources allowing for statistical analysis (for example, regions of England, France, Italy and the Netherlands) [14][15][16] . Some of these case studies have argued for a death toll in the range of 50% or more. These numbers have been increasingly considered representative of the Black Death's broader mortality [1][2][3][4]9,12,14 . Although it has been suggested that regional variation characterized the demographic crisis the Black Death caused, little evidence-historical, archaeological or environmental-has been employed to substantiate such thinking 8,17,18 . As a result, case studies from better documented regions have been employed as proxies for Black Death mortality in European regions where direct evidence for the pandemic is nonexistent (for example Bohemia and Finland), slight (single sentences or vague passages; for example Moravia, Hungary and Scotland) or available but neither detailed nor quantitative (most regions, including areas of England, France, Italy and the Netherlands) 1 .
By treating case studies of relatively well-documented regions as predictive of the pandemic's death toll in all regions the pandemic touched, histories of the Black Death implicitly rest on the untested assumption that plague mortality was uniform across regions regardless of local cultural, ecological, economic and societal contexts, and therefore that the prevalence of Y. pestis, an ecologically and epidemiologically complex disease 19 , was comparable across Europe. This methodology has lent itself to estimates of an aggregate European death toll upwards of around 50% (approximately 50 million deaths) 1,4 , though studies accounting for regional source scarcities have estimated mortality to have fallen below that mark 8,17,20 . Problematically, many of the quantitative sources drawn upon to build cases studies of Black Death mortality relate to urban contexts, which owing to their crowding, generally poor sanitation and quite possibly heavier disease burdens, may have suffered higher plague mortality than rural areas 21,22 , and in the mid-fourteenth century upwards of 75-95% of the population of every European region was rural 23 . Although every pandemic, plague or not, is distinct, a Black Death mortality across Europe of approximately 50% vastly exceeds the demographic losses sustained during the third plague pandemic in the late nineteenth century-the plague pandemic for which the most mortality data exists-including in China and India, which were then severely affected 24,25 .

BDP and Black Death mortality
Here, we pioneer an alternative approach (discussed in detail in Methods). BDP allows us to evaluate the Black Death's mortality across Europe using quantitative palaeoenvironmental datasets, which can be employed for spatial statistical comparisons (Fig. 1). Our dataset consists of fossil pollen counts from 261 radiocarbon-dated sediment cores from 19 present-day European countries ( Fig. 2; Supplementary Data 1). Pollen data can be used to assess past demographic dynamics as human pressure on the landscape in the pre-industrial period was directly dependent on the availability of rural labour. We focus on the period between 1250 and 1450 ce (comparing 100 years before and after the Black Death; 100 years representing roughly four generations, a time period during which pre-industrial populations could not recover from potentially high plague mortality 23 ) for which 1,634 pollen-analysed sediment samples are available (Supplementary Data 2).
In order to compare trends between different European bioclimatic zones, we assembled pollen types (taxa) representing different plants from species to family level into four standardized summary indicators, based on their subsistence value for human economy or their ecological needs as reflected in two major ecological indices, the Ellenberg light indicator and Niinemets and Valladares shade tolerance scale: (1) cereals; (2) herding (pasturelands); (3) fast forest secondary succession (pioneer shrubs and trees growing on former fields/pastures within 5-10 years of abandonment); (4) slow forest

Results
Validity test using Sweden and Poland. We validated our BDP approach by examining two well-studied, but contrasting, regional case studies of the Black Death's mortality, in Sweden and Poland. An earlier multidisciplinary analysis discovered significant contraction in cereal cultivation, as well as a more general economic and demographic decline, in the uplands of southern Sweden following the Black Death 26 . By contrast, historians have long demonstrated that central Europe, particularly Poland, experienced economic growth over the fourteenth century, related to the centralization of royal power following a period of partition, few wars in the central provinces of the country, large-scale colonization of uncultivated lands and the development of cities 27 . Our BDP approach independently corroborates these trajectories. We validate the ability of our pollen datasets to reflect the extent of the Black Death's mortality by comparing them to historical data of national tax payments made to the pope (Peter's pence) [28][29][30] (Fig. 3). This validity test lends further support to our primary focus in this study on cereal cultivation, as argued in the Methods.

Four major scenarios of landscape and demographic change.
Having validated our approach, we analysed the pollen dataset for all of Europe. We focused on contrasting the four summary pollen indicator values on a regional scale for subperiods of 100 years before and after the Black Death. While we discuss the results of this analysis here ( BDP employs bootstrapping to evaluate statistical significance of the differences between the pre-and post-Black Death subperiods on a regional level. In this way, we identified four scenarios of post-Black Death agricultural change based on the cereal pollen indicator (Fig. 4): (1) substantial and statistically significant increase, reflecting arable expansion and limited Black Death mortality (top panel); (2) modest and statistically insignificant increase, suggesting stability or slow-paced agrarian growth (upper middle panel); (3) modest and statistically insignificant decrease, suggesting stagnation or some contraction of agrarian activities, possibly stemming from more limited demographic losses or economic disruption caused by the plague (lower middle panel); (4) substantial and statistically significant decrease, reflecting arable contraction and pronounced Black Death mortality (bottom panel). Scenarios 1 and 2 falsify the theory that Black Death mortality was significant everywhere. Scenario 3 and 4 demonstrate Black Death mortality was devastating in some regions.
Changes in the herding indicator reveal trajectories similar to those of cereals in most cases or no change. We discovered only one region (southwest Germany) where there occurred a statistically significant decline in cultivation in parallel with a statistically significant increase in herding (some indication of this is also apparent in Greece), suggesting a shift to livestock production related to agrarian labour shortages and less demand for grain 31 . The fast forest succession indicator increases in a statistically significant way in central Italy and central France, confirming field abandonment. In central Italy, this is accompanied by discernible reforestation (statistically significant increase in the slow forest succession indicator), attesting to significant regional Black Death mortality and slow demographic and economic recovery.
To confirm the robustness of our results, BDP combines statistical with independent spatial approaches (

Discussion
The Black Death was a diverse and entangled phenomenon. Figure 6 visualizes the spatial distribution of the four trajectories of post-Black Death landscape change from Fig. 4, demonstrating that the Black Death's mortality varied significantly between European regions. The pandemic was immensely destructive in some areas, but in others it had a far lighter touch. Strikingly, BDP identifies a sharp agricultural decline in several regions of Europe, independently corroborating analyses of historical sources that suggest high mortality in regions of Scandinavia, France, western Germany, Greece and central Italy 1 , and lending further validation to our approach. At the same time, there is much evidence for continuity and uninterrupted agricultural growth in central and eastern Europe, and several regions of western Europe, particularly in Ireland and Iberia. In this way, BDP invalidates histories of the Black Death that assume Y. pestis was uniformly prevalent, or nearly so, across Europe and that the pandemic had a devastating demographic impact everywhere.
While we have centred our study on estimating the impact of the Black Death on the landscape, plague recurred in several regions within the 100-, 50-and 25-year periods of our analyses. It is not implausible that some recurrences, notably the so-called pestis secunda of the late 1350s and early 1360s 32 , could have been more devastating in a few regions we studied than the Black Death and that some of the arable contraction we detect may be attributable to both the Black Death and early second-pandemic recurrences. That the results of our 100-, 50-and 25-year period analyses were   largely uniform, strongly suggests that if early plague recurrences contributed considerably to the landscape change, the earliest of those recurrences, occurring within the first 25 years, were the most significant. Of course, if the pestis secunda partially accounts for the arable contraction and forest succession shown here for parts of Europe, our results would further call into question the demographic toll of the Black Death, even where we have discerned significant arable contraction, as in parts of France, Italy and Scandinavia. At the same time, if we are to include the earliest recurrences, where we have discerned little change or arable expansion, the significance of both the Black Death and those earliest recurrences is limited. These remarks aside, our approach has shown conclusively that the Black Death did not significantly alter land use everywhere or affect all regions equally. The significant variability in mortality that our BDP approach identifies remains to be explained, but local cultural, demographic, economic, environmental and societal contexts would have influenced Y. pestis prevalence, morbidity and mortality. Ongoing transformations of rural economy in many European regions would have also modified the plague mortality, or-to an extent-the amplified impact it had on the landscape. Importantly, regional population densities cannot explain the complex landscape dynamics we discern (as visualized in Supplementary Fig. 8), nor the geographical location of the 261 coring sites (see Supplementary  Fig. 9, showing the averaged elevation of our sites). The spread of the pandemic depended on numerous factors, which would have generated compound effects, feedback loops and regionally unique outcomes. Plague is an ecologically and epidemiologically complex zoonotic disease, maintained by sylvatic rodents and their fleas, and transmittable to and between people via multiple pathways, including commensal and sylvatic rodent flea bites, respiratory secretions, direct contact with infected animals, human ectoparasites (fleas and lice) and fomites 19 . The behaviour of Y. pestis hosts and vectors, and their capacity to efficiently transmit the pathogen, is partly constrained by complex interactions with seasonal climate variability and local ecological conditions, both in anthropogenic and rural environments (cities, villages and fields versus mountains, forests or wetlands 32 ). Regional variation in population density and distribution, ectoparasite burdens, living conditions, and commensal rodent populations and their fleas, undoubtedly mattered. Local climatic contexts were also shown to have strongly determined third plague pandemic dynamics in Asia 33,34 . In Europe, where the Black Death spread over several years (1347-1352 for the regions considered here), different seasons and annually variable climatic conditions may have influenced Y. pestis prevalence and the pandemic's mortality. Furthermore, while hypothesized links between early-fourteenth-century famines and the Black Death remain to be substantiated, and while Y. pestis is often lethal in lieu of antibiotics 19 , it has been shown that Black Death mortality was selective and the immunological and nutritional heterogeneity of the populations the pathogen interacted with would have ensured uneven toll 35,36 .
That climatic, cultural, ecological and economic factors shaped regional Black Death outcomes is well illustrated by the initial phases of the pandemic in Europe. Cereal trade is thought to have been instrumental for the introduction of the pandemic to Mediterranean Europe, and along established conduits of commerce and communication, ecological factors, associated contingency effects and historical path dependency mattered from the outset. Before the pandemic arrived in the Crimea, the volume of cereal trade between Italy and the Black Sea was sizable, yet blocked by embargo 11,37,38 . High demand in southern Europe for Black Sea cereals from 1345 onwards was associated with a period of excessive precipitation and cooling 39    coasts resumed once the situation improved in 1347, and by early 1348 Venetian merchants had filled many Italian granaries with Black Sea produce 11,37,39,43 and introduced plague to Europe. Plague outbreaks disseminated from major cereal ports in southern and north-western Italy from January 1348 1,44 . To the contrary, plague hardly spread in north-western Italy, which was independent from overseas cereal imports 37,43,45 . Local circumstances shaped the outcome of the pandemic on regional scales from the outset. In summary, our BDP approach shows Black Death mortality was far more spatially heterogeneous than previously thought. This significant variation in Black Death mortality may be explained by the pathogen's entanglement with a dynamic nexus of cultural, ecological, economic, societal and climatic factors that determined its prevalence and the pandemic's mortality in any given region. That the pandemic was immensely destructive in some regions, but not all, falsifies the practice, not uncommon in Black Death studies, of predicting one region's experience on the basis of another's. Regional mortality outcomes must be reconstructed using local sources, including BDP as proxies of changes in cultural landscapes. As a few well-documented case studies of the Black Death's destructive mortality in Europe have informed estimates of the pandemic's mortality not only in other European regions, but also in several regions of Africa and Asia, our findings have significant implications both for the wider history of the Black Death and for how historical disease outbreaks are reconstructed.

Methods
Pollen-inferred landscape change and pre-industrial demography. Recently, data derived from tree rings or ice cores have been employed to approximate changes in human economic activity related to past epidemics, as well as to warfare and climatic variability 46,47 . However, none of these proxies is directly related to human demography or provides a basis to estimate variation in the Black Death's mortality on a regional scale across Europe (to date only a single archaeological study using pottery as a proxy for demographic change on the national level, focusing on just a single country-England-has appeared 48 ).
In recent years, pollen data have been proven to be closely related to demographic variability. Most importantly, detailed comparisons of historical documentary data on population trends and landscape changes as revealed by pollen data have been carried out on a local scale and a close link between changes in European pollen data and changes in European local demography over the past millennium has been demonstrated on multiple occasions, that is, during the period and region of our concern here 49,50 . A strong link between long-term demographic trends as visible in regional settlement numbers and macro-changes in land cover (deforestation/afforestation) have also been confirmed for ancient Greece 51 . Additionally, a recent publication successfully employed pollen data to test the extent of the mortality associated with the sixteenth-century Spanish and Portuguese empires' colonization of tropical regions in the Americas and Asia 52 . However, as of now there is no method to quantify past demographic trends in absolute numbers based on palaeoecological data. Consequently, we also focus in this paper on relative changes in historical societies' populations and test the now common idea that the Black Death caused enormous mortality across Europe (with many scholars now arguing for a mortality exceeding 30% and upwards of 50% of the population within a few years) (see also Fig. 1). Using our BDP approach, we conclude this hypothesis is not maintainable. Our evidence for demography-related landscape changes (or lack thereof) negates it. Our main indicator is cereal pollen. In pre-industrial economies, rural labour availability (hence rural population levels) and the spatial scale of cereal cultivation were directly related. An increase in the extent and intensity of cereal cultivation-as reflected in pollen data-would have required not only a predilection and demand for cereals, but also greater availability of labour and thus population growth or significant immigration. The maintenance of existing agricultural activity, in turn, would have required relatively stable population levels [53][54][55] . The uniform ~50% mortality postulated for the Black Death across Europe should have resulted in a large and significant decline of cereal cultivation and parallel forest regrowth across Europe, as previously demonstrated for mid-fourteenth-century Sweden 26 and singular sites in some regions of western Europe 56 . This result agrees with the fact that Black Death mortality could be high among people at productive age, as illustrated for England 57,58 . Moreover, even in the case of England, a comparatively commercialized and adaptive rural economy in mid-fourteenth-century Europe, the loss of 50% of the population led to a significant decline in the total area under cultivation (as documented by heterogeneous written sources) 59 . In Italy, another well-developed economy at that time, the expansion of large estates following the Black Death also did not compensate for the general loss of cereal productivity 60 . This effect, high mortality driving arable contraction, must have been yet more pronounced in more subsistence-oriented and less adaptive economies, with limited surplus production, such as in regions of the Iberian Peninsula, Germany, Sweden and particularly east-central Europe. Importantly, palaeoecological evidence for arable contraction may be indicative, to some extent, of not only rural population decline but also urban population decline in the region, as there is evidence in some areas, following the pandemic, of rural-to-urban migration, of country-dwellers repopulating urban centres 10 . Possibly less common was intraregional rural migration, as marginal lands were abandoned for better quality soils, which were more likely to remain under cultivation 26,61 .
Therefore, cereal pollen remains our most potent pollen indicator related to demographic changes in pre-industrial European societies. Other pollen indicators, reflecting rewilding and reforestation (secondary ecological succession) of cereal fields abandoned as a result of significant mortality, or the transformation of cereal fields into pastures, which required less rural labour and thus also could have been a response to high plague mortality, play a secondary role in our analysis and provide further support for our conclusions.
BDP data collection. Existing online palynological databases (the European Pollen Database (EPD) 62 www.europeanpollendatabase.net, and the Czech Quaternary Palynological Database (PALYCZ) 63 , https://botany.natur.cuni.cz/palycz/), as well as personal contacts of the study authors and a systematic publication search were employed to identify palynological sites in Europe reaching the required chronological and resolution quality for the study of the last millennium. In order to enable statistical analysis, we included only sites clustered in well-defined historical-geographical regions, excluding isolated sites even if the quality of a site's data was very good. Data of sufficient quality and amount from regions for which the Black Death is well-studied, notably central and northern England and the Low Countries, is not presently available; to the best of our knowledge, for each of these regions there currently is not more than a single isolated site 56 , which does not allow for the application of statistical approaches.
In total, 261 pollen records with the average temporal resolution of 58 years and 14 C-age control (or varve chronology), have been collected. The age-depth models of the sequences have been provided by authors in original publications, by the EPD or developed through the Clam package (version 2.3.4) of R software for the purpose of this study. The analytical protocol for pollen extraction and identification is reported in the original publications. The Pollen Sum includes all the terrestrial taxa with some exceptions based on the selection done in the original publications. The full list of sequences, exclusions from the Pollen Sum, age-depth models and full references are reported in Supplementary Data 1.
The taxa list has been normalized by applying the EPD nomenclature. In this respect, the general name Cichorioideae includes Asteraceae subf. Cichorioideae of the EPD and PALYCZ nomenclatures, which primarily refers to the fenestrate pollen of the Cichorieae tribe 64 . Ericaceae groups Arbutus unedo, Calluna vulgaris, Vaccinium and different Erica pollen types, whereas deciduous Quercus comprehends both Q. robur and Q. cerris pollen types 65 . Rosaceae refers to both tree and herb species of the family. Finally, Rumex includes R. acetosa type, R. acetosella, R. crispus type, Rumex/Oxyria and Urtica groups U. dioica type and U. pilulifera.

BDP summary pollen indicators.
In order to connect changes visible in the pollen data to human demographic trajectories, we assembled four summary pollen indicators that describe specific landscapes related to human activity. They reflect different degrees of demographic pressure on the landscape (cereal cultivation, pastoral activities, which are less-labour intensive than cereal cultivation, abandonment and rewilding) as well as different durations of land abandonment that might have occurred post-Black Death. Our indicators account for the fact that Europe is a continent rich in natural heritage, with a wide range of landscapes and habitats and a remarkable wealth of flora and fauna, shaped by climate, geomorphology and human activity. In order to ensure uniform interpretation of the indicators, we relied on criteria that can be applied to all European landscapes regardless of their local specificity. Cereals and herding are directly related to human activities and are barely influenced by spatial differences. More complex is the succession of natural plants with their ecological behaviour and inter-species competition. For this reason, we relied on existing quantitative indicators of plant ecology.
The Ellenberg L -light availability indicator 66 provides a measure of sunlight availability in woodlands and consequently of tree-canopy thickness, reflecting the scale of the natural regeneration of woodland vegetation after cultivation or pasture activities 61 . Nonetheless, ecological studies have suggested that geographic and climatic variability between different European regions can influence the Ellenberg indicator system [67][68][69][70][71] . The original indicators were primarily designed for Central Europe 58  Please note our summary indicators are not designed to reflect the entirety of the landscape and reconstruct all of its different components. Rather, they are a means of approximating changes in the landscape related to the types of human activities, and their intensity, as much as they relate to demographic changes in human populations using and inhabiting these landscapes.

BDP analytical statistical and spatial methods.
To control for local specificity, pollen percentages of every taxon from each pollen site were standardized. From the taxa percentage in a given year the arithmetic mean calculated for the observations from the period 1250-1450 was subtracted and the result divided by the standard deviation for the 1250-1450 period. Standardized taxa results were assembled for each site into four BDP summary indicators. Since each indicator has different numbers of taxa, the sum of standardized taxa values calculated for a given year and site was divided by the number of taxa in the indicator. For the purposes of replication, this standardized pollen dataset, comprising the four indicators for each sample from each site, is available as Supplementary Data 2.
This dataset has been analysed in two ways, statistically and spatially. For the statistical approach, standardized regional indices of landscape transformation were created for each region by calculating the average value for all sites within the region, for each of the subperiods analysed in the study (1250-1350 and 1351-1450; 1301-1350 and 1351-1400; 1325-1350 and 1351-1375). Differences between means for each subperiod were measured by the use of the bootstrapping based on 10,000 resamples. The 90% and 95% confidence intervals were estimated with the bias-corrected and accelerated method (BCa) 78  For the spatial approach, we employed the Bayesian model AverageR developed within the Pandora and IsoMemo initiatives (https://pandoraapp.earth/) to map the distribution of pollen indices across Europe. AverageR is a generalized additive model that has been described previously 79 . It relies on a thin-plate regression spline 80 to predict new, unseen data using the following model: Where Y i is the independent variable for site i; g(longitude, latitude) is the spline smoother; and ε i ∼ N(0, σε) is the error term.
The spline smoother can be written as X × β where X is a fixed design matrix and β is the parameter vector. Surface smoothing is controlled by employing a Bayesian smoothing parameter estimated from the data and trades-off bias against variance to make the optimal prediction 75 . This parameter β is assumed to follow a normal distribution: β ~ N(0, 1 /δ × λ × P), where P is a so-called penalty matrix of the thin plate regression spline, which penalizes second derivatives 81 . The δ parameter is by default set to 1 but this can be adjusted to suit smoothing needs for each application. In our study δ was set at 0.9 to match the preferred spatial scale of analysis for our dataset (approx. 250 to 500 km).
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Data availability
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Data analysis
Statistical analysis were performed in R 4.0.3. We have used libraries: sf, ggplot2, wBoot, dplyr.
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Study description The regional variation in the Black Death's impact in Europe has been investigated using a new approach, Big Data Palaeoecology (BDP), based on collecting and analyzing existing pollen sequences from 19 European countries (published as well as unpublished).

Research sample
In order to create our dataset, we used existing online palynological databases (the European Pollen Database, www.europeanpollendatabase.net, and the Czech Quaternary Palynological Database, https://botany.natur.cuni.cz/palycz/), as well as personal contacts of the study authors and a systematic publication search. In this way, we identified palynological sites in Europe that meet the necessary chronological and resolution quality criteria for the investigated period of time (1250-1450 CE).

Sampling strategy
Pollen sequence with sufficient resolution (average temporal resolution of 58 years) and 14C-age control (or varve chronology) have been selected and grouped according to regional clusters. Single isolated sites from outside these clusters have been excluded because they do not allow the application of quantitative analysis.