The editor’s choice for our May issue is ‘Simulating past and future fire impacts on Mediterranean ecosystems‘, by Christoph Schwörer et al. Here, Associate Editor Anping Chen discusses the importance of this research:

Characterized by a distinct climate with hot, dry summers and mild, wet winters, the Mediterranean region is also often associated with frequent and intensive wildfires. The fire regime, together with the unique climate, shapes plant community dynamics and ecosystem functions. Vegetation in this region is well-adapted to co-exist with prolonged droughts and frequent wildfires, with many species possessing thick, waxy leaves and deep root systems to conserve water. Yet, the climate-wildfire-vegetation nexus is not always unidirectional. In areas where vegetation forms a closed canopy, the microclimate tends to be wetter, reducing the frequency of wildfire ignition. This results in the presence of alternative stable states in Mediterranean climates. Open habitats promote wildfires, which suppress forest trees like oaks, maintaining an open landscape. Conversely, closed oak forests experience fewer fires due to their damp microclimate, allowing these forests to persist. This dynamic interplay between vegetation structure and fire frequency underscores the complexity of ecosystem responses to climate and fire regimes in Mediterranean environments.

This complexity also poses significant challenges in predicting the future of Mediterranean ecosystems. The Mediterranean landscape is a mosaic of diverse ecosystems, ranging from open shrublands to closed forests, each with varying degrees of resilience to fire and climate change. The current balance between these ecosystems could be disrupted at different thresholds of warming and fire intensity, leading to abrupt and possibly irreversible shifts in vegetation composition. Importantly, while it is generally anticipated that the region will experience a warmer climate with more frequent and intense fire regimes, there is little information on when such tipping points might be reached. Understanding these critical thresholds is essential, as crossing them could result in biodiversity loss, altered ecosystem services, and significant impacts on local communities. Predictive models and long-term ecological data are essential for better understanding these dynamics and developing strategies to mitigate the potential adverse effects on Mediterranean ecosystems.

However, long-term observation data is scarce, especially considering that vegetation shifts often require millennia to fully manifest. This scarcity poses a challenge for understanding and predicting the long-term impacts of climate change and fire regimes on Mediterranean ecosystems. In this context, paleoecological data provides a unique opportunity for validating model predictions of long-term vegetation changes. By examining pollens, plant macrofossils and microscopic charcoals, and other proxies, scientists can reconstruct past climate conditions, fire events, and vegetation changes over hundreds and thousands of years. These historical insights allow researchers to compare past ecosystem responses to current and future climate scenarios, enhancing the accuracy of predictive models. Incorporating paleoecological data into contemporary studies helps bridge the gap left by the lack of long-term observational data, offering a more comprehensive understanding of the dynamics governing Mediterranean ecosystems and informing more effective conservation and management strategies.

Published in Journal of Ecology, Schwörer and colleagues offered a nice example comparing palaeoecological data over the past 8000 years from Sardinia with output from a process-based dynamic vegetation model. Their results indicate that historical vegetation shifts from heath (Erica) shrublands to mixed evergreen-broadleaved forests dominated by holly oaks (Quercus ilex) driven by a climate-induced fire regime shift. Through simulations of vegetation dynamics under varying fire regimes, they successfully reproduced Holocene vegetation trajectories and identified mechanistic tipping points. Their future projections reveal that, without an immediate reduction in greenhouse gas emissions, there will be an expansion of fire-prone Mediterranean maquis and an increase in fire occurrences. Additionally, high anthropogenic ignition frequencies and the planting of non-native, highly flammable trees could lead to a shift back to fire-adapted heath shrublands. However, their simulations also suggest that if global warming can be kept below 2°C, holly oak forests will be able to persist, effectively reducing fire occurrences and impacts, making them a valuable target for restoration in Mediterranean ecosystems. Schwörer et al. demonstrated that past climate-driven fire regime shifts were primary drivers of vegetation changes, resulting in alternative stable states persisting over centuries. Projected future climate change, which exceeds Holocene variability, could lead to significant vegetation changes and increased fire risks, necessitating new fire management strategies to preserve current ecosystem services.

This study is a significant contribution to fire ecology in Mediterranean environments by combining long-term palaeoecological data with a process-based mechanistic model to produce fire risk simulations for future climate change scenarios. With the LandClim vegetation model simulations, the study effectively captures fire as an emergent property of the landscape under varying climate conditions and simulated vegetation dynamics. The integration of these methods allows for a comprehensive description of how climate-induced fire regime shifts influence vegetation changes over time. The study’s innovative approach offers valuable insights into predicting and managing future fire risks in Mediterranean ecosystems, providing a robust framework for understanding the complex interactions between climate, fire, and vegetation.

The analysis has important implications for the Paris Accord goal of limiting warming to 1.5 or 2 degrees Celsius. By focusing on the Holocene Epoch, whose Climatic optimum occurred between 5000–3000 BC when average global temperatures were 1–2 °C warmer than today, this research provides a valuable historical analogue for how ecosystems may respond if the Paris Accord targets are breached. Importantly, the research demonstrates that even under the warmer climate of the Holocene, holly oak forests would not be driven out by heath shrublands in the absence of frequent fire disturbances. It was the intensified fire regimes, rather than the elevated temperatures alone, that primarily drove the dominance of heath shrubs. Moreover, the study predicts that under both high fire disturbances and warming beyond Holocene variations, even heath shrublands will be replaced by more xeric (drought-tolerant) shrublands or open woodlands. This suggests that future climate scenarios characterized by extreme warming and increased fire activity could lead to significant shifts in Mediterranean vegetation, resulting in landscapes dominated by species adapted to even harsher conditions. This research thus underscores the urgency of adhering to the Paris Accord targets to prevent disruptive ecological transformations, loss of biodiversity, and degradation of ecosystem services.

As one of the first studies integrating paleoecological data and process-based model simulations, this research inevitably has several limitations. For instance, fire is prescribed in the model rather than being an output derived from modeled processes of fire weather and fuel accumulation, which can affect the accuracy of fire dynamics representation. Additionally, there is an inherent challenge in directly validating model simulations with paleoecological data due to the temporal and spatial scales involved, as well as the limited data size. Furthermore, the model does not account for land use changes and human disturbances. Compared to the Holocene period, these factors are critical in contemporary landscapes, where human activities greatly influence fire regimes, vegetation patterns, and overall ecosystem health. Despite these limitations, the study provides valuable insights and a foundation for future research to improve the integration of paleoecological data and dynamic vegetation models, ultimately enhancing our understanding of ecosystem responses to climate change and fire regimes.

Read the full article online: Simulating past and future fire impacts on Mediterranean ecosystems