Authors

Anastasia Nikulina¹, Fulco Scherjon^1,2

¹Faculty of Archaeology, Department of World Archaeology, Human Origins Group, Leiden University, Einsteinweg 2, Leiden, 2333CC, the Netherlands

²MONREPOS Archaeological Research Centre and Museum for Human Behavioural Evolution, Römisch-Germanisches Zentralmuseum, Schloss Monrepos, Neuwied, 56567, Germany

Abstract

In the complex history of human-environment interactions, a fingerprint of hunter-gatherer impact on landscapes precedes agriculture and industrialization. Ethnographic observations show that foragers can substantially modify their surroundings via vegetation burning for diverse objectives, such as driving game, stimulating the growth of edible plants, and clearing pathways (Scherjon et al., 2015). Based on archaeological evidence, researchers have identified traces suggestive of human-induced vegetation burning by hunter-gatherers during the Early–Middle Holocene (∼11,700–6000 BP) (e.g., Bos and Urz, 2003; Innes et al., 2013). Furthermore, the earliest evidence thus far of a local-scale impact of anthropogenic fire use was discovered at the Neumark-Nord site in Germany, dated to the Last Interglacial (Eemian, ∼130,000–116,000 BP) (Roebroeks et al., 2021). The data quality is not necessarily that different between the Early–Middle Holocene and the Eemian evidence (Nikulina et al., 2022), though the quantity of possible Holocene cases is substantially larger. In addition, landscape dynamics are influenced by various natural and cultural processes at different spatio-temporal scales. Therefore, it is often challenging to identify whether foragers’ activities were limited to local-scale changes only or entailed regional or even (sub-)continental changes. Modelling approaches such as spatially explicit agent-based modelling (ABM) provide opportunities to explore the interactions of complex system components and propose scenarios of system functioning, especially in cases where real-time experiments are not possible. In this vein, our study focuses on multiple drivers of landscape change within a system-based approach, including natural and human-induced fires, herbivory and climatic impacts during the Early Holocene and the Last Interglacial in Europe. In this research we developed a spatially explicit HUMLAND (HUMan impact on LANDcapes) ABM for the following overarching research question: “How might fires set by hunter-gatherers have influenced vegetation in Europe during the Last Interglacial and the Early Holocene periods?”

The starting point of our study is the comparison of CARAIB (CARbon Assimilation In the Biosphere) model output with pollen-based reconstructions obtained via REVEALS (Regional Estimates of Vegetation Abundance from Large Sites). CARAIB represents theoretical potential natural vegetation distribution, driven by climatic conditions only. REVEALS shows the observed past vegetation cover shaped by different drivers including climate, humans, megafauna and natural fires. 

CARAIB and REVEALS are different modelling approaches, with dissimilar outputs, and there is no accepted protocol to compare and to integrate them into a single ABM. Hence, we developed our own approach whose details can be found in Nikulina et al., 2024. Comparison of CARAIB and REVEALS datasets indicated a substantial and interesting difference between the two modelled vegetation reconstructions. This significant difference between climate-based and pollen-based reconstructions shows that the observed vegetation cover is not solely a product of climatic impact; other factors may have also played an important role in shaping vegetation in the study area. 

To better understand the impact of various factors, including human-induced vegetation burning, we integrated outputs from CARAIB and REVEALS, as well as other spatial datasets (digital elevation model, distribution of large rivers and lakes, and estimates of megafauna plant consumption) into HUMLAND, conducting simulations with this combined dataset to explore these differences. The developed continental ABM was implemented in NetLogo 6.2.2. The temporal resolution of the model is one year. Simulations stop after 1000 steps. The model includes four most influential and widespread types of impact on vegetation: climate, anthropogenic fires, thunderstorms, and megafauna plant consumption (Whelan, 1995; Bond and Wilgen, 1996; Pringle et al., 2023; Nikulina et al., 2024). 

Each simulation step starts with climatic impact, which defines vegetation regeneration after fire events and megafauna vegetation consumption. Following this, anthropogenic impact is assessed via the five associated parameters: number of hunter-gatherer groups (the number of groups during one simulation run), accessible area (area within which people move and burn), openness criteria to burn, and the percentage of hominin groups that relocate their campsites and frequency of this action. The sensitivity analysis indicated that the two latter parameters do not significantly impact the intensity of human-induced vegetation changes and overall HUMLAND output (Nikulina et al., 2024). After humans, natural fires impact vegetation, and the intensity of their impact is determined via the number of thunderstorms per simulation step. The actual natural burning due to thunderstorms and fire spread after that or after human-induced fires depend on the probability of ignition, which is calculated by the time passed since the last disturbance event, using natural fire return intervals (FRI). These were obtained via 2002–2020 MODIS (MODerate resolution Imaging Spectroradiometer) burned area data from the MCD64A1 C6 product. Finally, megafauna increase vegetation openness and modify PFT distribution via plant consumption. 

The first application of HUMLAND used to produce a preliminary scenario for one Early Holocene time window (9200–8700 BP) suggested that hunter-gatherers and climate were the most influential factors in driving continental-level vegetation changes, while natural fires and megafauna activities had less impact (Nikulina et al., 2024). Our modelled results represent general patterns at the continental level, and it is possible that humans may have had a smaller impact compared to climate, megafauna, and natural fires in some regions. Despite that, the observed general pattern is that early anthropogenic impact on the environment was the principal non-climate factor affecting landscapes during the Early Holocene, in line with evidence obtained in other parts of the world (Ellis et al., 2021). 

References

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