Persistent hot and dry conditions play an important role in vegetation dynamics, being generally associated with reduced activity. In the Mediterranean region, ecosystems are adapted to such conditions. However, prolonged and intense heat and drought or the occurrence of compound hot and dry events may still have a negative impact on vegetation activity. This work aims to study how the productivity of Mediterranean vegetation is affected by hot and dry events, examining a set of severe episodes that occurred in three different regions (Iberian Peninsula, Eastern Mediterranean and Western Europe) between 2001 and 2019. The analysis relies on remote sensing products, namely Gross Primary Production from MODIS to detect and monitor vegetative stress and LST from MODIS and SM from ESA CCI to evaluate the influence of temperature and soil water availability on stressed vegetation. Of all events, the 2005 episode in the Iberian Peninsula was the most significant, affecting large sectors of low tree cover areas and crops and leading to reductions of annual plant productivity in affected vegetation of ~47 TgC/year. The obtained results highlight the influence of land-atmosphere coupling on vegetation productivity and clarified the role of warm springs on vegetation activity and soil moisture that may amplify summer temperatures. The functional recovery of affected vegetation productivity after these episodes varied across events, ranging from months to years. This work highlights the influence of hot and dry events on vegetation productivity in the Mediterranean basin and the usefulness of remote-sensing products to assess the response of different land covers to such episodes.

Wildfires are a serious threat to ecosystems and human. In Portugal, during 2017, a catastrophic fire season burned more than 500 000 hectares and caused the death of more than 100 people. Previous studies have shown that hot and dry fuel conditions promoted widespread propagation of wildfires. However, burned area (BA) and mega-fires, such as the 2017 ones, depend not just on favourable meteorological conditions, but also on fuel accumulation and dryness. In this study, we will assess the influence of spring meteorological conditions on fire season BA, through their effects on fuel accumulation and dryness. Using satellite-based data, we show that the association of higher temperatures and water availability in spring can increase the risk of summer wildfires propagation, flammability and intensity through their influence on vegetation gross productivity. This study highlights the important role of fuel accumulation during the growing season in fire-prone regions like Portugal. Our results imply that fuel management may be an effective way to mitigate extreme fire seasons associated with warmer and drier conditions in the future.

Mediterranean European countries, including Portugal, are considered fire-prone regions, being affected by fire events every summer. Nonetheless, Portugal has been recording large burned areas over the last 20 years, which are not only strongly associated with hot and dry conditions, but also with high fuel availability in the ecosystems. Due to recent catastrophic fire seasons, Portugal has been implementing preventive policies during the pre-fire season, which, in turn, can optimize combat strategies during the fire season. In this context, our study contributes to fire prevention by identifying the regions with the highest potential to burn. The application of a Principal Component Analysis (PCA) to a range of climatological, ecological, and biophysical variables, either provided by remote sensing or reanalysis products, and known to be linked with diverse fire-vulnerability factors, allows the objective identification of the regions with the highest susceptibility to burn. The central and southernmost areas of Portugal present a stronger signal in the PCA, suggesting a likely high exposure to future fire events. The fuel accumulation over several months, in conjunction with elevation and fire weather conditions, are the terms out of the retained PCs that can explain most of the variability. The quality assessment performed for the burned areas in 2022 showed that they occurred in highly susceptible areas, highlighting the usefulness of the proposed methodology.

<p>Fires are a natural part of many ecosystems, constituting nonetheless a serious threat to ecosystems and humans. Portugal is recurrently affected by wildfires, being considered a fire-prone region within the Mediterranean basin. However, during the last twenty years, several fire seasons recorded a large extension of burned areas, and the catastrophic fire season of 2017 stands out by recording more than 450,000 hectares of burned area and causing the death of more than 100 people.</p> <p>Hot and dry fuel conditions were pinpointed as the main drivers of the widespread propagation of wildfires. Therefore, this work aims to assess the impact of the compound or cascading extreme events on vegetation, and also if there was a selectivity of fires for more stressed vegetation. We analyse the climatological and ecological conditions during the pre-fire season, in order to understand its effects on fire season burned areas through fuel accumulation and dryness.</p> <p>The study highlights the importance of fuel accumulation during the growing season in fire-prone regions like Portugal. Moreover, anomalous hot and dry conditions during summer, in conjunction with strong fuel accumulation during the months preceding fire season, enabled to clarify why the 2017 fire season were so outstanding. Additionally, under the context of climate change, fire seasons as the one occurred in 2017 in Portugal can be more frequent. Therefore, our results highlight that fuel management can be an effective way to mitigate extreme fire seasons and point to the need of implementation of fire prevention policies, especially regarding biomass accumulation control during pre-fire season.</p> <p>This study was supported by national funds through FCT (Fundação para a Ciência e Tecnologia, Portugal) under the project FIRECAST (PCIF/GRF/0204/2017) and by the 2021 FirEUrisk project funded by European Union’s Horizon 2020 research and innovation programme under the Grant Agreement no. 101003890.</p>

<p>Fire is an integral component of ecological dynamics, playing an important role in biome distribution and biomass variability. Nonetheless, fires can also pose a  threat to both ecosystems and humans, by imposing severe economic and social consequences, and potentially contributing to biodiversity loss, carbon loss and soil erosion, whose effects can last from months to years.</p> <p>The Mediterranean basin is a fire-prone region where vegetation is in general well adapted to fire, with several species showing resistance to fire itself or being able to recover quickly following fire events. However, as a consequence of climate change, more intense and frequent summer hot and dry conditions are expected to occur, which can promote more frequent and severe wildfires, with return periods potentially outpacing recovery times. Understanding recovery dynamics is therefore crucial to assess the impact of changing fire regimes in ecological dynamics and stability of ecosystems. </p> <p>In our study, we use the “Enhanced Vegetation Index” (EVI), remotely-sensed by MODIS sensor with a temporal span of 22 years, to evaluate vegetation dynamics before, during and following large fire seasons. We use a mono-parametric recovery model to assess recovery times in different burn scars across the Mediterranean basin, covering different fire regimes and land cover types. We find a tendency for slower recovery in areas that burned more often, which may indicate a decrease in ecosystems’ resilience in the past 22 years.</p> <p>This study was performed under the frameworks of the 2021 FirEUrisk project (funded by European Union’s Horizon 2020 research and innovation programme under the Grant Agreement no. 101003890) and of the PhD MIT Portugal MPP2030-FCT programme (Grant no.22405886350).</p>

<p>Southern Europe is considered a fire-prone region, and fire events occur here every summer. In this context, large fires have hit Portugal over the last 20 years, due to frequent hot and dry summer conditions, and also to high fuel availability in ecosystems. Moreover, climate change in the Mediterranean basin is expected to increase the severity of fire weather conditions and therefore to increase the occurrence of extreme fire seasons.</p> <p>Recent catastrophic fire seasons have led to the implementation of a set of policies during the months before the fire-season, aiming at fire prevention and suppression, which can in turn increase the combat efficiency of fires during the fire season. Therefore, this work intends to contribute to fire prevention by identifying regions with a high likelihood to burn.</p> <p>A Principal Components Analysis (PCA) was applied to several climatological, ecological, and biophysical variables, related to fire weather, fuel availability, and elevation covering the period from 2001 to 2021. Results allowed to assess the areas where large fires were more likely to occur in 2022. The central and southernmost regions of Portugal showed a stronger signal in the PCA, indicating a likely high susceptibility to future fire events. The association of fuel accumulation since the last fire event with elevation and favourable fire weather conditions explains most of the variability of the first six PCs. These results were compared with the fires that occurred in 2022, and a match between larger burned areas and high signals in the PCA was found, highlighting the usefulness of this methodology.</p> <p>This study was supported by FCT (Fundação para a Ciência e Tecnologia, Portugal) through national funds (PIDDAC) – UIDB/50019/2020, and under the projects FlorestaLimpa (PCIF/MOG/0161/2019).</p>

<p>Wildfires have become a serious threat to ecosystems and human society over the last years of the 21<sup>st</sup> century, with many hectares being destroyed every year globally. The lengthening of the fire seasons and the increase of wildfires risk, which have been promoted by climate change, input many losses on society, economy and mostly in diverse ecosystems. In Portugal, the 2017 catastrophic fire season burned more than 450,000 hectares and caused the death of more than 100 people. In this context, relying on remotely sense products from MODIS collections, our study proposes an analysis of the effect of summer heat and water availability deficit in vegetation productivity decline that led to large fires propagation, especially in June and October of 2017. With the aim to evaluate the magnitude of the impact that compound or cascading extreme events had on the vegetation productivity decline, considering the 2001-2019 historical values, we defined three different classes of pixels that should reflect the conditions before the fire: affected by hot, by dry or by hot/dry conditions. Moreover, we assess the influence of favourable winter/spring meteorological conditions on enhancing vegetation productivity that promote high fuel accumulations susceptible to burn some months later. Our results reinforce the water and energy dependency of the vegetation of the region during the growing season and highlight that the combination of higher temperatures and water availability in spring can trigger summer wildfires propagation, flammability and intensity due to the accumulation of biomass. Considering that the example of 2017 can be more recurrent under the context of climate change, this study also highlights the need to improve the awareness strategies in fire prone regions like Portugal, especially on biomass accumulation control during growing season.</p><p>This study was supported by national funds through FCT (Fundação para a Ciência e a Tecnologia, Portugal) under project FIRECAST (PCIF/GRF/0204/2017) and IMPECAF (PTDC/CTA-CLI/28902/2017).</p>

<p>Mediterranean European countries are considered fire-prone regions, being affected by fire events every summer, and Portugal is among these countries. Moreover, Portugal has been recording large burned areas over the last 20 years. Catastrophic fire season occurrence, associated with hot and dry conditions and high fuel availability in forests, has been recurrently destroying several ecosystems. Furthermore, the Mediterranean basin has been stated with high potential to be one of the most disturbed areas due to climate change, which strongly promotes the increase of fire weather conditions and fire risk and, thereby, the occurrence of more extreme fire seasons.</p> <p>During the last years, Portugal has been implementing new effective policies regarding the prevention of fires during pre-fire season months, improving the investment in combat strategies. In this context, our study contributes to identify the regions with more potential to burn in a specific fire season. Through satellite-based data and reanalysis products, with large temporal extent and moderate to high spatial resolution, we combine a wide range of variables linked, directly or indirectly, with fire, in order to identify the most exposed regions to burn.</p> <p>The application of Principal Component Analysis (PCA) to our range of climatological, ecological and biophysical parameters allowed to assess six different regions with more susceptibility to fire events. The central and the southernmost regions of the country presented a stronger signal on PCA analysis, indicating a higher exposure to future fire events. Fuel accumulation during several months, in conjunction with topography, land cover and fire weather conditions were the terms that explained the most variability of the first six PCAs. Therefore, with these results, our work addresses the key trigger parameters of fires, and the most susceptible areas to burn in Portugal, contributing to enhancing the effectiveness of fire prevention policies.</p> <p>Acknowledgements: This study was supported by FCT (Fundação para a Ciência e Tecnologia, Portugal) through national funds (PIDDAC) – UIDB/50019/2020, and under the projects FlorestaLimpa (PCIF/MOG/0161/2019) and FIRECAST (PCIF/GRF/0204/2017).      </p>