Wildland-urban interface typologies prone to high severity fires in Spain

Due to complex interactions between climate and land use changes, large forest fires have increased in frequency and severity over the last decades, impacting dramatically on biodiversity and society. In southern European countries affected by demographic challenges, fire risk and danger play special relevance at the wildland-urban interfaces (WUIs), where decision-making and land management have strong socio-ecological implications. WUIs have been historically typified according to both fire occurrence probability and settlement vulnerability, but those classifications lack generality regarding fire regime components. We aim to develop an integrated and comprehensive scheme for identifying the WUI typologies most at risk to fire severity across large territories. We selected fourteen large wildfires (over than 500 ha) occurred in Spain (2016-2021) containing different WUI scenarios. First, based on a building cartography and a multi-temporal series of Sentinel-2 imagery, each WUI was delimited and spatially characterized according to building density and pre-fire fuel characteristics (type, amount, and structure). Afterwards, a decision tree regression model was applied to identify the most relevant pre-fire vegetation parameters driving burn severity. The combined effect of the selected pre-fire vegetation drivers and the building density patterns on fire severity was evaluated using linear mixed models. Finally, the WUI typologies most prone to high burn severity were recognized using Tukey post-hoc tests. Results indicated that building density, land cover class and vegetation cover fraction determined fire severity in areas close to human settlements. Specifically, isolated, scattered and sparsely clustered buildings enclosed in a high-cover shrub matrix were the WUI typologies most susceptible to high-severity fires. These findings contribute to the development of appropriate strategies to minimize the risk of severe fires in WUIs and avoid potential losses of multiple ecosystem services valuable for society.

Key factors controlling microbial community responses after a fire: Importance of severity and recurrence

Wildfires are a major problem in Mediterranean forest ecosystems, affecting the same area year after year. Their severity is increasing, partly due to climate change and hence, every now and then, virulent fires of high severity spread ravage this region. The aim of this study was to evaluate the influence of fire regime (recurrence, severity) in soil microbial community structure analyzing the phospholipid fatty acid (PLFA) and the microbial functional diversity assessing the level physiological profiling technique (CLPP). Samples of a soil affected by a high severities wildfire and a soil affected by a low severity experimental fire were heated under laboratory conditions at different temperatures to simulate different fire severity. To simulate fire recurrence, the heating treatment was repeated after one month of incubation. The fire severity was estimated as the amount of heat supplied to samples by degree-hour methodology. A marked impact of fire regime on soil microorganisms was detected; the microbial community response varied depending on previous history of fire and the magnitude of changes in PLFA pattern and CLPP, was related to the amount of heat supplied to the samples. Wildfires had a greater impact on microbial community structure than subsequent soil heating in the laboratory. The total biomass and the biomass of specific groups of microorganisms decreased notably as a consequence of wildfire and minor changes were detected due to the experimental fire and soil heating under laboratory conditions. The results clearly showed the usefulness of PLFA pattern to study the effect of fire regimes and associated direct and indirect changes in soil microorganisms and in soil quality. The data also indicated that the degree-hour methodology rather than maximum temperature is adequate to simulate fire severity and evaluate the impact of thermal shock on soil ecosystems.