• Energy Research

Climate has a strong influence on fire activity, varying across time and space. We analyzed the relationships between fire?weather conditions during the main fire season and antecedent water-balance conditions and fires in two Mediterranean-type regions with contrasted management histories: five southern countries of the European Union (EUMED)(all fires); the Pacific western coast of the USA (California and Oregon, PWUSA)(national forest fires). Total number of fires (#8805;1 ha), number of large fires (#8805;100 ha) and area burned were related to mean seasonal fire weather index (FWI), number of days over the 90th percentile of the FWI, and to the standardized precipitation-evapotranspiration index (SPEI) from the preceding 3 (spring) or 8 (autumn through spring) months. Calculations were made at three spatial aggregations in each area, and models related first-difference (year-to-year change) of fires and FWI/climate variables to minimize autocorrelation. An increase in mean seasonal FWI resulted in increases in the three fire variables across spatial scales in both regions. SPEI contributed little to explain fires, with few exceptions. Negative water-balance (dry) conditions from autumn through spring (SPEI8) were generally more important than positive conditions (moist) in spring (SPEI3), both of which contributed positively to fires. The R2 of the models generally improved with increasing area of aggregation. For total number of fires and area burned, the R2 of the models tended to decrease with increasing mean seasonal FWI. Thus, fires were more susceptible to change with climate variability in areas with less amenable conditions for fires (lower FWI) than in areas with higher mean FWI values. The relationships were similar in both regions, albeit weaker in PWUSA, probably due to the wider latitudinal gradient covered in PWUSA than in EUMED. The large variance explained by some of the models indicates that large-scale seasonal forecast could help anticipating fire activity in the investigated areas.

Interspecific differences in tree growth patterns with respect to biotic and abiotic factors are key for understanding forest structure and dynamics, and predicting potential changes under climate change. • Repeated observations from the Spanish Forest Inventory (SFI) were used to parameterize maximum likelihood estimators of tree growth as a function of tree size, competition indices and climate for Pinus pinaster, P. sylvestris, Quercus ilex and Q. pyrenaica. • Significant responses to both biotic and abiotic factors were found, with interspecific differences in species performance along competition, temperature and precipitation gradients. Q. ilex was the species most tolerant to competition while P. pinaster was the species most sensitive to climatic variation. Species relative positions shifted along gradients of these factors with rank reversals in species performance along size, competition and climatic gradients. • The results based on average growth matched previous forestry classifications and experimental studies on relative growth rate (RGR). • When examining growth along studied abiotic and biotic gradients, a mismatch was found between species performance ranks as predicted by our models and information derived from previous knowledge. Those discrepancies highlight the relevance of ontogeny and environmental heterogeneity in defining species performance along competition gradients. © 2008 INRA EDP Sciences.

- Key messageFire activity has decreased in the last decades in Spain as a whole and in most provinces. However, fire riskfactors have increased. Wildfires are burning preferentially treeless areas. Flammable pine areas burn less, while the lessflammable oak forests burn more. This decreasing fire activity coincides with increasing fire suppression resources. - ContextClimate and other fire risk factors are changing across the world, including the Mediterranean region. It is important todetermine how fire activity is responding to such changes. - AimsTo identify spatio-temporal changes in fire frequency and size, main tree species burned, and their association withchanges in climate, land-use and land-cover, and fire suppression. - MethodsAnalysis of trends, breakpoints, and association among the time series of fire variables and fire risk factors during1980–2013 in Spain at country and provincial levels. - ResultsWildfires in Spain have been decreasing in number, burned area, mean, and largest fire sizes. Fires are more frequentlyburning treeless areas than forests. Pine forests are burning less, while oak forests are burning more. Fire suppression resources,particularly aerial ones, increased significantly. Risk factors like the mean summer fire weather index, and the forested and theartificial areas all increased. - ConclusionDespite increasing fire risk factors, wildfires have decreased at the country as a whole and in most provinces inSpain during the last 34 years