Burning season and vegetation coverage influenced the community-level physiological profile of Mediterranean mixed-mesogean pine forest soils.

Knowledge of forest soil ecology is necessary to assess vulnerability to disturbances, such as wildfires, and improve its microbial diversity and functional value. Soil microbiota play an important role in forest soil processes and are a key driver of postfire recovery, but they are very vulnerable to heat. According to future scenarios for climate and land-use change, fire regimes will undergo transformations in semiarid terrestrial ecosystems, mainly in the Mediterranean Basin. To develop tools for forest management in fire-prone areas, i.e., fire prevention, we assessed the impact of prescribed burnings on soil microorganisms in Mediterranean mixed pine forests. We hypothesised that low severity fire burns would not influence the functional diversity of soil microorganisms, although the burning season could influence that response due to seasonal variations in its vulnerability. We used the Biolog EcoPlate System to record soil biological indicators and assess the effect of the prescribed burning season (early or late season) on bacterial communities, including the soil-plant interphase. The soil microbiome response differed significantly according to vegetation coverage but prescribed burning season was not directly related. Burning increased the proportions of soil organic matter and soil organic carbon, and also promoted cation-exchange capacity and total phosphorus, which were higher following spring burns. Microbial richness and the Shannon-Weaver diversity index both showed a positive correlation with vegetation cover. However, microbial richness was triggered after burning uncovered patches of vegetation. We also noted differences in the usage pattern for the six substrate groups defined in our study: the use of carboxylic acids, amino acids and carbohydrates was higher in unburned plots and those subject to late burns, whereas amino acids did not predominate in early burn plots.

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.

Biochemical responses of Pinus pinaster trees to fire-induced trunk girdling and crown scorch: secondary metabolites and pigments as needle chemical indicators.

The biochemical responses to varying levels of localized heat damage to stems and crowns were studied in Pinus pinaster trees in Galicia, Spain. The objective was to ascertain the utility of secondary metabolites (total polyphenols. condensed tannins, astringency, free and esterified phenolic acids) and pigments (chlorophylls and carotenoids) as chemical indicators of localized fire damage. The study involved three levels of girdling by trunk heating (0%, 60%, and 75%), three levels of crown scorching (0%, 50%, and 75%), and all combinations of those trunk and crown treatment levels. Secondary metabolites and pigments were analyzed in needles before, during, and up to 8 months after fire treatments. High levels of polyphenols, condensed tannin, and pigments occurred in situations where the sole treatment was 75% crown scorching. Low levels of hydroxycinnamic acids were the result when the treatment was 75% trunk girdling. These responses occurred in the first two months after fire. Thereafter, normal values were observed. Low values of chlorophyll a/b ratio and high levels of free protocatechuic and esterified syringic acids were found to be long-term indicators of trunk girdling damage combined with crown scorching.