Fire frequency analysis in Portugal (1975 - 2005), using Landsat-based burnt area maps

Abstract

Abstract. Fire frequency in 21 forest planning regions of Portugal during the period 1975-2005 was estimated from historical burnt area maps generated with semi-automatic classification of Landsat Thematic Mapper (TM) satellite imagery. Fire return interval distributions were modelled with the Weibull function and the estimated parameters were used to calculate regional mean, median and modal fire return intervals, as well as regional hazard functions. Arrangement of the available data into three different time series allowed for assessment of the effects of minimum mapping unit, time series length and use of censored data on the Weibull function parameter estimates. Varying the minimum mapping unit between 5 and 35 ha had a negligible effect on parameter estimates, whereas changing the time series length from 22 to 31 years substantially affected the estimates. However, the strongest effect was caused by censored data. Its exclusion led to substantial overestimation of fire frequency and of burning probability dependence on fuel age. We estimated a countrywide mean fire interval of 36 years and an annual burnt area of 1.2%. Regional variations in fire frequency descriptors were interpreted in terms of land cover and land use practices that affect the contemporary fire regime in Portugal.

Wildfire frequency varies with the size and shape of fuel types in southeastern France: Implications for environmental management

“…Moritz et al (2009) have explored the consequences of using censored or uncensored fire data and concluded that disregarding censored data may cause distortion of the real fire hazard. Recently, Oliveira et al (2011) have also tested this effect for FRI during the 1975-2005 period for the whole Portugal and confirmed that censoring fire data has a predominant effect on FRI. It is likely to be especially important when the database comprises few fire intervals (e.g.…”

“…Such differences with our study can be explained by the fact that Portugal is the most fire-prone country in Europe whereas burned areas tend to decrease in France since the 1990's. Oliveira et al (2011) found lower age dependency in regions of Portugal with high portions of shrublands, and higher age dependency for extensively forested regions. This difference with our results can result from the fact that areas with shrublands in Portugal are regularly burned then grazed and have small fires.…”

“…The Maures massif had a short NFR value similar to that of Sierra de Gredos Author-produced version of the article published in Journal of Environmental Management, 2013, 117, 150-161 Original publication available at http://www.sciencedirect.com/ doi : 10. 1016/j.jenvman.2012.12.006 in Spain (64 years, Diaz-Delgado and Pons, 2001) while the Aix-Marseille area had a longer NFR value than that of the similar region of Catalonia (Spain, 133 years, Diaz-Delgado and Pons, 2001) but much lower than those of some areas or Portugal (maximum 762 years, Oliveira et al, 2011). Indeed, the authors have shown that the fire rotation period can range from 31 to 762 years according to the regions of Portugal.…”

“…Such analysis requires accurate fire data during a time period sufficient to let successive fires to burn some locations and to account for temporal heterogeneity, i.e. a minimum of several decades for many MTEs experiencing frequent fires (Oliveira et al, 2011). Few FRI studies of are available in MTEs due to the lack of such fire databases.…”

“…Diaz-Delgado et al, 2001) or Portugal (e.g. Oliveira et al, 2011). This may explain why controversy still persists among land managers in Provence about the role of fuel age and the typical fire return interval for the most common fuel types.…”