Soil-vegetation relationships in Mediterranean forests after fire

P.1-13Background Wildfires are one of the major environmental concerns in Mediterranean ecosystems. Thus, many studies have addressed wildfire impacts on soil and vegetation in Mediterranean forests, but the linkages between these ecosystem compartments after fire are not well understood. The aim of this work is to analyze soil-vegetation relationships in Mediterranean burned forests as well as the consistency of these relationships among forests with different environmental conditions, at different times after fire, and among vegetation with different functional traits. Results Our results indicate that study site conditions play an important role in mediating soil-vegetation relationships. Likewise, we found that the nature of soil-vegetation relationships may vary over time as fire effects are less dominant in both ecosystem compartments. Despite this, we detected several common soil-vegetation relationships among study sites and times after fire. For instance, our results revealed that available P content and stoichiometry (C:P and N:P) were closely linked to vegetation growth, and particularly to the growth of trees. We found that enzymatic activities and microbial biomass were inversely related to vegetation growth rates, whereas the specific activities of soil enzymes were higher in the areas with more vegetation height and cover. Likewise, our results suggest that resprouters may influence soil properties more than seeders, the growth of seeders being more dependent on soil status. Conclusions We provide pioneer insights into how vegetation is influenced by soil, and vice-versa, in Mediterranean burned areas. Our results reflect variability in soil-vegetation relationships among study sites and time after fire, but consistent patterns between soil properties and vegetation were also detected. Our research is highly relevant to advance in forest science and could be useful to achieve efficient post-fire management.S

Do Fire Regime Attributes Affect Soil Biochemical Properties in the Same Way under Different Environmental Conditions?

This article belongs to the Special Issue Forest Post-Fire Regeneration[EN] Global change is altering fire frequency and severity in many regions across the world. In this work, we studied the impact of different frequency and severity regimes on the soil biochemical properties in burned areas with different environmental conditions. We selected three sites dominated by pine ecosystems along a Mediterranean-Transition-Oceanic climatic gradient, where we determined the fire frequency, and severity of the last wildfire. Four years after the last wildfire, we established 184 4 m2 plots. In each plot, we collected a composed soil sample from a 3 cm depth, and measured several ecological variables potentially affected by the fire frequency and severity (cover of bare soil, cover of fine and coarse plant debris, cover of vegetation, and vegetation height). From each soil sample, we analyzed the enzymatic activities corresponding to the biogeochemical cycles of carbon, nitrogen, and phosphorus (β-glucosidase, urease, and acid-phosphatase, respectively), and the microbial biomass carbon. The results indicated that fire frequency only played a significant role in soil biochemical properties at the Mediterranean and Transition sites. Specifically, we found that increases in frequency contributed to increased urease and phosphatase activities (at the Transition site), as well as microbial biomass carbon (at the Mediterranean and Transition sites). In relation to burn severity, we found opposite patterns when comparing the Mediterranean and Oceanic sites. Specifically, increased severity significantly decreased β-glucosidase, urease, and microbial biomass carbon at the Mediterranean site, whereas at the Oceanic one, severity significantly increased them. Burn severity also decreased microbial biomass carbon at the Transition site. Our results also indicated that, overall, fire frequency determined the studied ecological variables at the Mediterranean and Transition sites, but clear indirect effects on biochemical properties due to changes in ecological variables were not found. This study adds to the knowledge on the impact of shifts in fire regimes on soils in the current context of changeSIThis study was financially supported by the Spanish Ministry of Economy and Competitiveness; the Spanish Ministry of Science, Innovation and Universities; and the European Regional Development Fund (ERDF), in the framework of the GESFIRE (AGL2013-48189-C2-1-R) and FIRESEVES (AGL2017-86075-C2-1-R) projects, and by the Regional Government of Castile and León in the framework of the FIRECYL (LE033U14) and SEFIRECYL (LE001P17) project

Interactions between large high-severity fires and salvage logging on a short return interval reduce the regrowth of fire-prone serotinous forests

P. 54-63New fire disturbance regimes under accelerating global environmental change can have unprecedented consequences for ecosystem resilience, lessening ecosystem natural regeneration. In the Mediterranean Basin, fire-dependent obligate seeder forests that are prone to increasingly frequent stand-replacing fires and then salvaged logged repeatedly can be vulnerable to additional disturbances for decades. In this study, we investigated, for the first time, the cumulative and interactive effects of two large high-severity fires at a short (<15-year) return interval and the subsequent burned timber harvesting with biomass removal on the post-disturbance recovery of such forests. We further assessed the type and amount of the material legacies (deadwood) that persisted through the different post-disturbance successional trajectories, as well as the influence of these legacies on forest regeneration. The early recovery of the studied forests after two consecutive large fires and post-fire logging was, in the first place, driven by fire repetition, which led to reduced seedling recruitment and enhanced regrowth of resprouter shrubs. Despite no interactive effects between fire and logging were detected after a single large fire event, two repeated fires followed by salvage harvesting had a greater negative impact than two fires alone (synergistic effects) on seedling establishment; while a lower positive impact (subadditive effects) on the recovery of resprouter shrubs. There was also an interaction modification effect in which fire repetition worsened the per-unit impact of salvage logging on forest regeneration. Nonetheless, the residual legacies, i.e., fine and coarse woody debris (unburned needles, downed branches, pieces of deadwood, and burned pine cones) that remained after the manual harvesting of the burned trees, aided seedling re-establishment and hindered the regrowth of the shrubby understorey. These findings indicate that high-intensity salvage logging after two large high-severity fires at a short return interval is inadvisable in fire-prone serotinous pine forests, unless it explicitly retains the key material legacies that help tree natural regeneration and enhance ecosystem resilience to the next disturbance.S

The role of fire frequency and severity on the regeneration of Mediterranean serotinous pines under different environmental conditions

P. 59-68Fire frequency and burn severity may increase in pine forests in the Mediterranean Basin under the warmer and drier climate projected for this region. Our study aims to evaluate the role of fire frequency and burn severity in the post-fire recruitment and development of Mediterranean serotinous pines under different environmental conditions. Two pine forests representing contrasting climatic conditions and soil types that support serotinous pines in the Iberian Peninsula and affected by large wildfires in summer 2012, were selected. In these two study sites, we determined the number of wildfires between 1978 and 2012 and the burn severity of the last fire (2012 at both sites) through the dNBR spectral index. Three and four years after the wildfires, we sampled the density, cover and height of pine seedlings and the cover of woody understory species in 1296 1 m2 plots. The results indicated that the density and cover of pine seedlings was low after two fires combined with high severities, as well as after three fires, regardless of burn severity. Seedling recruitment after three fires was particularly threatened in the most arid study site (0.01 seedlings m−2), resulting in low seedling cover (0.01%). Seedling height decreased with fire frequency in both study sites, and with burn severity owing to fire-induced shifts in soil fertility and microclimatic conditions. There was a significant negative effect of the cover of woody understory species on the recruitment and cover of pine seedlings. Our results suggest that the effects of increasing fire frequency and severity on pine regeneration may be aggravated under arid conditions. Additionally, this study encourages forest managers to avoid the occurrence of frequent crown fires in order to prevent the loss of serotinous pine forest, and provides useful information to predict the scenarios in which post-fire restoration actions would be helpful.S

Soil Resistance to Burn Severity in Different Forest Ecosystems in the Framework of a Wildfire

[EN] Recent changes in fire regimes, with more frequent, extensive, and severe fires, are modifying soil characteristics. The aim of this study was to evaluate the effect of burn severity on the resistance of some physical, chemical, and biochemical soil properties in three different forest ecosystems affected by a wildfire in the northwest of the Iberian Peninsula. We evaluated burn severity immediately after fire using the Composite Burn Index (CBI) in three different ecosystems: shrublands, heathlands, and oak forests. In the same field plots used to quantify CBI, we took a composite soil sample to analyse physical (mean weight diameter (MWD)), chemical (pH; total C; total organic C (TOC); total inorganic C (TIC); total N; available P; exchangeable cations Na+, K+, Mg2+, and Ca2+; and cation exchange capacity (CEC)), and biochemical (β-glucosidase, urease, and acid phosphatase enzyme activities) properties. The resistance index of each property was then calculated. Based on our results, the values of the soil chemical properties tended to increase immediately after fire. Among them, total C, TOC, and exchangeable Na+ showed higher resistance to change, with less variation concerning pre-fire status. The resistance of chemical properties was higher in the oak forest ecosystem. MWD decreased at high severity in all ecosystems, but soils in shrublands were more resistant. We found a high decrease in soil enzymatic activity with burn severity, with biochemical properties being the least resistant to change. Therefore, the enzymatic activity of soil could be a potential indicator of severity in forest ecosystems recently affected by wildfiresSIThis study was financially supported by the Spanish Ministry of Economy and Competitiveness in the framework of the FIRESEVES (AGL2017-86075-C2-1-R) project; and by the Regional Government of Castilla and León in the framework of the SEFIRECYL (LE001P17) project. The European Regional Development Fund also provided funding for the present study. S.H. was supported by a predoctoral fellowship from the Regional Government of Castilla and León and the European Social Fund (EDU/574/2018

Resilience of Mediterranean communities to fire depends on burn severity and type of ecosystem

[EN] Background: Burn severity plays an important role in shaping vegetation recovery in Mediterranean ecosystems. In the present study, we aimed to evaluate the influence of burn severity on short-term vegetation resilience in different ecosystems. We selected the Cabrera wildfire (northwest Iberian Peninsula), which affected shrubland, heathland,broomland, and oak woodland ecosystems in 2017. Immediately after the fire, we established 249 field plots within the burned area, in which burn severity was quantified by the application of the Composite Burn Index, differentiating three burn severity categories: low, moderate, and high. Moreover, we fixed 136 plots in unburned areas at two different maturity stages: young (unburned for the last 10 years) and old (unburned for the last 20 years) vegetation. Two years after the wildfire, we evaluated the total percentage cover of vegetation in four vertical strata ranging from 0 to > 4 m, as well as the cover of each woody species and total herbaceous vegetation in the lowest stratum (0–0.5 m). Resilience of the 2017 burned areas was interpreted in terms of the difference in vegetation cover and species composition in relation to the two different maturity stages. Results: The results showed that the lowest stratum was the most resilient in the short term. In fact, all ecosystems presented high resilience of this stratum in low-severity areas. In shrublands and heathlands, this was mainly the consequence of the regeneration of herbaceous vegetation, as the dominant woody species did not fully recover in any of the burned situations (at least 21% and 11% less cover in shrubland and heathland burned plots compared to areas of young vegetation). Specifically, the resilience of this stratum was higher in broomlands and oak woodlands, mainly under moderate and high severities. In these ecosystems, woody dominant species recovered with respect to young vegetation at the 0–0.5-m level. Despite this, burn severity had a negative impact on the short-term resilience of the uppermost strata in broomlands and oak woodlands (cover values close to 0%). Conclusions: The effects of burn severity on short-term vegetation resilience differed among type of ecosystems and vertical strata, so these results may constitute a starting point for the evaluation of the influence of burn severity and vegetation composition and structure on ecosystem resilience[ES] Antecedentes: La severidad de los incendios juega un papel importante en la forma en que se recupera la vegetación de los ecosistemas mediterráneos. En el presente estudio, nos enfocamos en evaluar la influencia de la severidad de los incendios en la resiliencia de la vegetación a corto plazo. Para ello seleccionamos el incendio de Cabrera (noroeste de la Península Ibérica), que afectó ecosistemas de aulagar, brezal, escobar y robledal en 2017. Inmediatamente después del fuego, establecimos 249 parcelas dentro del área quemada, donde la severidad del fuego se cuantificó mediante la aplicación del índice de quema compuesto (Composite Burn Index), diferenciando tres categorías: baja, moderada y alta. Además, fijamos 136 parcelas en áreas no quemadas diferenciando dos estadios de madurez: vegetación joven (sin quemar en los últimos 10 años), y vegetación vieja (sin quemar en los últimos 20 años). Dos años después del fuego, evaluamos el porcentaje total de cobertura de la vegetación en cuatro estratos verticales en un rango desde 0 a > 4 m, así como la cobertura de cada especie leñosa y del total de la vegetación herbácea en el estrato más bajo (0–0,5 m). La resiliencia de las áreas quemadas se interpretó en términos de diferencia en la cobertura de la vegetación y en la composición de especies con respecto a los dos estadios de madurez analizados. Resultados: Los resultados demostraron que el estrato más bajo fue el más resiliente en el corto plazo. En efecto, todos los ecosistemas presentaron una alta resiliencia en este estrato en las áreas quemadas a baja severidad. En los matorrales y brezales esto fue principalmente consecuencia de la regeneración de la vegetación herbácea, ya que las especies leñosas dominantes no se recuperaron en ninguna de las situaciones quemadas (los aulagares y brezales quemados presentaron al menos una cobertura un 21% y 11% menor que las áreas de vegetación joven). Específicamente, la resiliencia de este estrato fue más alta en escobares y robledales, principalmente bajo severidades moderadas y altas. En estos ecosistemas, las especies leñosas dominantes se recuperaron con respecto a la vegetación joven en el estrato de 0 a 0,5 m. A pesar de ello, la severidad de la quema tuvo un impacto negativo en la resiliencia de corto plazo en el estrato más alto de escobares y robledales (con valores de cobertura cercanos a 0%). Conclusiones: Los efectos de la severidad de la quema en la resiliencia de la vegetación a corto plazo difirió entre los tipos de ecosistemas y los estratos verticales; por lo tanto estos resultados pueden constituir un comienzo para evaluar la influencia de la severidad de la quema, y de la composición y estructura de la vegetación en la resiliencia de los ecosistemasSIThis research was funded by the Spanish Ministry of Economy and Competitiveness in the framework of the FIRESEVES (AGL2017-86075-C2-1-R) project and by the Regional Government of Castilla and León in the framework of the WUIFIRECYL (LE005P20) project. The European Regional Development Fund also provided funding for the present study. S.H. was supported by a predoctoral fellowship from the Regional Government of Castilla and León and the European Social Fund (EDU/574/2018)

Wildfire effects on soil properties in fire-prone pine ecosystems: Indicators of burn severity legacy over the medium term after fire

P. 147-156The aim of this study was to determine the effects of burn severity on soil properties (chemical, biochemical and microbiological) in fire-prone pine ecosystems three years after fire. To achieve these goals, we selected two large wildfires that occurred in summer 2012 within the Iberian Peninsula: the Sierra del Teleno wildfire, which burned 119 km2 dominated by Pinus pinaster forests developed over acidic soils, and the Cortes de Pallás wildfire, which burned 297 km2, part of them dominated by Pinus halepensis ecosystems with calcareous soils. We classified the burned areas into low or high burn severity categories using spectral indices. Three years after the wildfires, we distributed 56 field plots proportionally to the extent of each severity category. In each field plot, we collected samples of mineral soil from a depth of 0–3 cm. We analysed soil chemical (pH, electrical conductivity, organic carbon, total nitrogen, available phosphorus) biochemical (β-glucosidase, urease and acid phosphatase enzymatic activities) and microbiological (microbial biomass carbon) properties in each soil sample. The relationship between burn severity and soil properties was analysed by a Permutational Multivariate Analysis of Variance and Generalized Linear Models. The results showed a significant influence of the original ecosystem and of burn severity on the overall soil status over the medium term after fire. Available P content increased with burn severity in the acidic soils of the P. pinaster ecosystem. However, the three enzymatic activities and microbial biomass carbon decreased with burn severity in both types of pine ecosystems. β-glucosidase, urease and microbial biomass carbon showed common patterns in relation to burn severity in the two different Pinus ecosystems (acidic and calcareous soils), and therefore we suggest that they could be potential indicators of the burn severity legacy on soils over the medium term after fire in fire-prone pine Mediterranean forests. Available P and acid phosphatase could be potential indicators in the P. pinaster ecosystem. This study provides useful knowledge for developing hazard reduction and restoration strategies after large wildfiresS

Changes in a Humic Cambisol heated (100–500 °C) under laboratory conditions: The significance of heating time

P. 237-246La influencia de la temperatura y su duración, que son algunos de los componentes más importantes para determinar los efectos del fuego en suelo propiedades, fueron estudiados en la capa de 0-5 cm de un Humic Cambisol desarrollado durante los conglomerados silíceos y bajo un arbusto de Erica australis. El suelo era arenoso con 8,5 g 100 g-1 de materia orgánica, pH de 4.7 y P disponible aproximadamente 0,16 mg kg-1. El índice de estabilidad en agua (ISA) fue 64.9, que se consideraba bueno. Análisis térmico diferencial (DTA) mostró la presencia de dos picos, uno endotérmico hasta 177 ° C y otra exotérmica entre 229 ° C y 579 ° C. Las muestras se calentaron a 100 ° C, 200 ° C y 500 ° C durante 5, 15, 30 y 60 min después de alcanzar la temperatura seleccionada. Para evitar un gradiente de temperatura en el perfil se utilizó una capa de muestra de 2 cm de profundidad. Calentamiento del suelo a 100 ° C en el cuatro tiempos de calentamiento y 200 ° C durante 5 min no produjo cambios significativos en el pH, materia orgánica, N total, relación C / N, P disponible, cationes extraíbles y WSI. Los suelos calentados a 200 ° C durante 15, 30 y 60 min y 500 ° C durante 5 min, mostró un aumento significativo en P disponible (de 0,9 a un máximo de 17,6 mg kg-1) y cationes divalentes (Ca2 + 4,4 a 46,3 y Mg2 + 1,5 a 8,7 mg kg-1). Para suelos calentados a 500 ° C durante 15, 30 y 60 min, un claro incremento en soluble contenido de Na + (0,5 a 1,8 mg kg-1) y pH (5,5 a 8,1), además de una disminución significativa de la materia orgánica (de 8,7% a 2,4%) se detectó. Como consecuencia de ello se encontró una disminución importante de WSI (del 64,9% al 54,3%)S

Physiological and Regenerative Plant Traits Explain Vegetation Regeneration under Different Severity Levels in Mediterranean Fire-Prone Ecosystems

15 p.In Mediterranean fire-prone ecosystems, plant functional traits and burn severity have decisive roles in post-fire vegetation recovery. These traits may reflect plant fitness to fire regimes in the Mediterranean Basin. The aim of this study was to evaluate the effect of burn severity on post-fire vegetation regeneration through plant functional (physiological and regenerative) traits in two Mediterranean ecosystems: one more humid and colder (Cabrera in León province, NW Spain), and another characterized by a longer summer drought (Gátova in Valencia province, SE Spain). A total of 384 and 80 field plots (2 m 2 m) were fixed in Cabrera and Gátova, respectively. In each burned plot, we quantified burn severity by means of the composite burn index (CBI), differentiating three severity levels (low, moderate, and high), and evaluated post-fire vegetation regeneration one and two years after wildfires. We measured the percentage cover of each species and classified them according to physiological (specific leaf area and N2-fixing capacity) and regenerative traits (reproductive strategy, bud bank location, and heat-stimulated germination). The main results showed that in Cabrera, burn severity had significant effects on vegetation cover independently of plant functional traits. In Gátova, burn severity effects differed among functional traits. In this site, the cover of plants with low specific leaf area and without heat-stimulation and N2-fixing capacity was negatively related to burn severity. On the contrary, the cover of N2-fixers and species with resprouting ability and heat-stimulated germination rose with increasing burn severity. In general, vegetation cover showed a more pronounced increased over time in the more humid area, mainly under the effect of high severity. The results of this research highlighted the importance of the use of plant functional traits as a driver to understand the response of different ecosystems to current fire regimes, which could be relevant for pre- and post-fire management.S

Interactions between mediterranean shrub species eight years after experimental fire

p. 235-241Este trabajo es parte de un estudio más amplio en el que se analizaron diferentes combinaciones de especies para la revegetación de forma experimental quemado brezales. En este caso particular, se determinó si había alguna interacción entre la especie dominante, Erica australis, que se recuperaron por rebrote vegetativo, y la Cytisus sp., ocho años después del fuego y siete años después de la siembra. El estudio detallado se realizó sobre cada metro cuadrado de la cubierta y la altura de ambas especies en tres parcelas 160 m2. La cobertura total superó el 100 % en todas las subtramas sembradas con Cytisus, mientras que no alcanzó el 65% en los otros, cuando ambas especies conviven juntos. Hubo una correlación inversa entre las tapas de ambas especies y una correlación positiva entre la altura Erica australis y de la cubierta y la altura de Cytisus No hubo diferencia significativa en Erica cubren en parcelas sembradas con Cytisus y parcelas no sembradas. Podría haber alguna competencia por la luz, pero ambas especies son capaces de coexistir con alta superposición dosel, por lo menos durante algún tiempo. Cytisus se está expandiendo desde la cabeza de serie argumentos secundarios, lo que sugiere que la cubierta densa de Erica australis no impide el establecimiento de CytisusS