Ramon Vallejo

Are wildfires a disaster in the Mediterranean basin? - A review

Evolutionary and paleoecological studies suggest that fires are natural in the Mediterranean basin. However, the important increase in the number of fires and area burned during the 20th century has created the perception that fires are disasters. In the present paper, we review to what extent fires are generating ecological disasters in the Mediterranean basin, in view of current fire regimes and the long-term human pressure on the landscapes. Specifically, we review studies on post-fire plant regeneration and soil losses. The review suggests that although many Mediterranean ecosystems are highly resilient to fire (shrublands and oak forest), some are fire-sensitive (e.g. pine woodlands). Observed erosion rates are, in some cases, relatively high, especially in high fire severity conditions. The sensitive ecosystems (in the sense of showing strong post-fire vegetation changes and soil losses) are mostly of human origin (e.g. extensive pine plantations in old fields). Thus, although many Mediterranean basin plants have traits to cope with fire, a large number of the ecosystems currently found in this region are strongly altered, and may suffer disasters. Post-fire disasters are not the rule, but they may be important under conditions of previous human disturbances.

Pines and oaks in the restoration of Mediterranean landscapes of Spain: New perspectives for an old practice — a review

Pines have been extensively used for land restoration in the Mediterranean basin and in other parts of the world, since the late 19th century. The theoretical basis supporting pine utilisation was its stress-tolerant and pioneer features, and their attributed role of facilitating the development of late-successional hardwoods in the long-term. In the present work, the use of pines and hardwoods in forest restoration is discussed in the frame of the current disturbance regime and social demands for Mediterranean forests. Large pine plantations have recently disappeared because of their sensitivity to fire (e.g., Pinus nigra) or because of the short fire-intervals (e.g., Pinus halepensis). Combined pine and oak plantations are proposed for degraded land restoration on the basis of the complementary features of both groups of species. Seeding and containerised seedling plantation, soil amendments and plantation techniques to reduce transplant shock are evaluated for reforestation under water-stressing conditions, on the basis of several experiments performed in eastern Spain. Both P. halepensis and Quercus ilex are tested.

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.

Analysis of the mycorrhizal potential in the rhizosphere of representative plant species from desertification-threatened Mediterranean shrublands

An evaluation of the mycorrhizal status of desertification-threatened ecosystems has been recommended as a first step in rehabilitation/restoration approaches based on revegetation strategies using arbuscular mycorrhizal (AM) technology. Representative desertified semiarid areas were selected from southeast Spain where the vegetation is dominated by grasses, with Stipa tenacissima usually present, and with some patches of the shrubs Pistacia lentiscus, Rhamnus lycioides, Olea europaea subsp. sylvestris and Retama sphaerocarpa. The objective of this study was to evaluate the mycorrhizal potential in these soils, the contribution of the different species established to the mycorrhizal potential of the soils and to assess the main mycorrhizal propagules involved. There were more AM fungal propagules in the rhizospheres of all the shrub species studied compared with adjacent fallow soils, suggesting that AM propagules can be considered as a functional component of the resource islands developing around plant roots. R. sphaerocarpa and O. europaea had a higher capacity to enhance the development of mycorrhizal propagules in their rhizospheres than R. lycioides and P. lentiscus. Correlation analyses showed that the number of spores of the most representative AM fungal species, i.e. Glomus constrictum, and the total length of extraradical AM mycelium are the propagule sources which were best correlated with the mycorrhizal potential in terms of the number of “infective” AM propagules in the rhizosphere of the target plant species. The contribution of AM symbiosis to the potentiality of S. tenacissima as nurse plant was site dependent. Diversity of AM fungi present in the test area is rather low, indicating the high degree of degradation of the ecosystem. At most, only four AM fungal spore morphoecotypes were consistently detected in the rhizosphere of the target plant species.

Fire severity and seedling establishment in Pinus halepensis woodlands, eastern Iberian Peninsula

Given the observed heterogeneity in fire severity produced within wildfires, we asked to what extent this heterogeneity might affect post-fire regeneration. For this purpose, we studied the post-fire dynamics of Pinus halepensis (Aleppo pine) in the eastern Iberian Peninsula. Sampling was stratified on the basis of fire severity. We defined three fire severity classes based on the degree of consumption of the pine canopy. The results suggested that there is no clear relationship between seedling density and fire severity; however, mortality was lower and growth (height, shoot biomass and root biomass) was higher in the high severity class. These results can be explained by soil processes: Sites in the high fire severity class may have sustained higher fire intensities, resulting in higher soil organic matter mineralisation and higher ash deposition, and thus in higher post-fire soil fertility. This higher fertility would produce faster growth in pine seedlings. Independent of the severity class, seedling mortality was higher in quadrats (50 × 50 cm) with higher cover of the perennial grass Brachypodium retusum (Poaceae), suggesting a possible competitive effect. For all plots in all 3 severity classes, spatial analysis suggests an aggregate seedlings pattern, but with independence from the position of the adult (source) trees.

Self-organized criticality of wildfires ecologically revisited

Abstract Wildfire cumulative frequency–area distributions of Mediterranean landscapes are examined for agreement with self-similar (fractal) behavior. Our results support landscape-specific restricted scaling regions of 1.5–3.5 orders of magnitude in size, which are delimited by breakpoints or ‘cut-offs’. By identifying the extent of such regions in the fractal frequency–area distribution of wildfires, fractal statistics may give a deeper insight into the scale-invariant dynamics of fire spread, whereas the observed cut-offs may be related to changes in the process–pattern interactions that control wildfire propagation at the landscape scale.

Modelling the effects of landscape fuel treatments on fire growth and behaviour in a Mediterranean landscape (eastern Spain)

The number of large fires increased in the 1970s in the Valencia region (eastern Spain), as in most northern Mediterranean countries, owing to the fuel accumulation that affected large areas as a consequence of an intensive land abandonment. The Ayora site (Valencia province) was affected by a large fire in July 1979. We parameterised the fire growth model FARSITE for the 1979 fire conditions using remote sensing-derived fuel cartography. We simulated different fuel scenarios to study the interactions between fuel spatial distribution and fire characteristics (area burned, rate of spread and fireline intensity). We then tested the effectiveness of several firebreak networks on fire spread control. Simulations showed that fire propagation and behaviour were greatly influenced by fuel spatial distribution. The fragmentation of large dense shrubland areas through the introduction of wooded patches strongly reduced fire size, generally slowing fire and limiting fireline intensity. Both the introduction of forest corridors connecting woodlands and the promotion of complex shapes for wooded patches decreased the area burned. Firebreak networks were always very effective in reducing fire size and their effect was enhanced in appropriate fuel-altered scenarios. Most firebreak alternatives, however, did not reduce either rate of fire spread or fireline intensity.

Effects of land use and eventual fire on soil erodibility in dry Mediterranean conditions

The Mediterranean countries of Europe have suffered deep land-use transformations during this century and the expansion of wildfires after generalized land abandonment in the marginal uplands. Differences in soil properties due to land use may lead to different responses of the ecosystem in case of fire. We analyze how soil erodibility in a cropland and oldfields of different age after abandonment (bushland, shrubland and pine forest) would be affected by wildfire. Soil samples taken in each land use were analyzed to characterize soil erodibility. From fuel load measures, the range of temperatures to be attained in case of a fire were estimated for the topsoil. From these temperatures and from the information available in the literature, we predicted the changes in soil properties produced by the fire and estimated soil erodibility for the burned lands. Changes in soil erodibility were slight in the field crop with low fuel load, and increased according to biomass and forest floor accumulation. As the initial soil erodibility was inversely related to fuel load, the fire would increase the values of soil erodibility of the different land uses up to close to those of the field crop. In addition, the fire would produce very similar soil erodibility values for the different land uses, and therefore a homogenizing post-fire soil response in the landscape.

Microsite and mycorrhizal inoculum effects on the establishment of Quercus coccifera in a semi-arid degraded steppe

Abstract Alpha grass (Stipa tenacissima) steppes are widely distributed within the semi-arid areas of the Mediterranean Basin, and represent a degraded stage of vegetation. We evaluated the effect of S. tenacissima tussocks and nursery inoculation with mycorrhizae on the survival of the native shrub Quercus coccifera. Experiments were carried out in three steppes located in semi-arid southeastern Spain. Survival during the first months after plantation was significantly higher in the surroundings of S. tenacissima tussocks than in open areas between the tussocks. We did not find any effect of nursery inoculation on seedling survival. This effect may be related to the fact that the used fungal strain did not survive the drought summer conditions in the study area. Our results support the idea of a facilitative effect of S. tenacissima on introduced shrubs, which may be related to improved environmental conditions in tussock microsites. However, this effect may not be enough for the establishment of seedlings in years with below-average rainfall and a strong summer drought.

Modelling the Ecological Vulnerability to Forest Fires in Mediterranean Ecosystems Using Geographic Information Technologies

Forest fires represent a major driver of change at the ecosystem and landscape levels in the Mediterranean region. Environmental features and vegetation are key factors to estimate the ecological vulnerability to fire; defined as the degree to which an ecosystem is susceptible to, and unable to cope with, adverse effects of fire (provided a fire occurs). Given the predicted climatic changes for the region, it is urgent to validate spatially explicit tools for assessing this vulnerability in order to support the design of new fire prevention and restoration strategies. This work presents an innovative GIS-based modelling approach to evaluate the ecological vulnerability to fire of an ecosystem, considering its main components (soil and vegetation) and different time scales. The evaluation was structured in three stages: short-term (focussed on soil degradation risk), medium-term (focussed on changes in vegetation), and coupling of the short- and medium-term vulnerabilities. The model was implemented in two regions: Aragón (inland North-eastern Spain) and Valencia (eastern Spain). Maps of the ecological vulnerability to fire were produced at a regional scale. We partially validated the model in a study site combining two complementary approaches that focused on testing the adequacy of model’s predictions in three ecosystems, all very common in fire-prone landscapes of eastern Spain: two shrublands and a pine forest. Both approaches were based on the comparison of model’s predictions with values of NDVI (Normalized Difference Vegetation Index), which is considered a good proxy for green biomass. Both methods showed that the model’s performance is satisfactory when applied to the three selected vegetation types.