Juan de la Riva

Sensitivity of X-, C-, and L-Band SAR Backscatter to Burn Severity in Mediterranean Pine Forests

Synthetic aperture radar (SAR) data at X-, C-, and L-bands have been investigated to determine the relationship between backscatter and forest burn severity over three sites in Spain. The dependence of SAR backscatter on local incidence angle and environmental conditions has been analyzed. At HH and VV polarizations, the backscatter increased with burn severity for X- and C-bands, whereas it decreased for L-band. Cross-polarized (HV) backscatter decreased with burn severity for all frequencies. Determination coefficients were used to quantify the relationship between radar backscatter and burn severity for given intervals of local incidence angle. For X- and C-band copolarized data, higher determination coefficients were observed for slopes oriented toward the sensors, whereas for cross-polarized data, the determination coefficients were higher for slopes oriented away from the sensor. At L-band, the association strength of cross-polarized data to burn severity was high for all local incidence angles. C- and L-band cross-polarized backscatter showed better potential for burn severity estimation in the Mediterranean environment when the local incidence angle is accounted for. The small dynamic range observed for X-band data could hinder its use in forests affected by fires.

Forest Fire Severity Assessment Using ALS Data in a Mediterranean Environment

Mediterranean pine forests in Spain experience wildland fire events with different frequencies, intensities, and severities which result in diverse socio-ecological consequences. In order to predict fire severity, spectral indices derived from remotely sensed images have been used extensively. Such spectral indices are usually used in combination with ground sampling to relate detected radiometric changes to actual fire effects. However, the potential of the tridimensional information captured by Airborne Laser Scanners (ALS) to severity mapping has been less explored. With the objective of addressing this question, in this paper, explanatory variables extracted from ALS point clouds are related to field estimations of the Composite Burn Index collected in four fires located in Aragon (Spain). Logistic regression models were developed and statistically tested and validated to map fire severity with up to 85.5% accuracy. The canopy relief ratio and the percentage of all returns above one meter height were the most significant variables and were therefore used to create a continuous map of severity levels.

Effects of Fire on Vegetation, Soil and Hydrogeomorphological Behavior in Mediterranean Ecosystems

In the Mediterranean context, wildfire has ceased to be a natural ecological factor and has become an anthropic factor of frequent and intense occurrence. Fires cause environmental alterations based the disappearance or modification of the vegetation cover, soil changes and soil hydrogeomorphological behavior. These transformations are mainly concentrated in the immediate aftermath of the fire and are closely linked with fire frequency, fire severity and the period of the year in which the fires occur. A systematic outline of the effects of fire and the factors implicated in the post-fire environmental dynamic is offered. The role of remote sensing methods on fire effects is well known; this chapter also refers to experiences and works on monitoring temporal patterns of vegetation recovery and mapping erosion-sensitive areas

Backscatter properties of multitemporal TerraSAR-X data and the effects of influencing factors on burn severity evaluation, in a mediterranean pine forest

TerraSAR-X dual-polarized (HH and HV polarization) backscatter data have been investigated to assess the temporal backscatter stability of a burn scar in Spain. Analysis of the main factors influencing burn severity evaluation has been also carried out. The temporal stability of the backscatter was strong, unburned areas showing differences of less than 0.6 dB. For increasing burn severity the backscatter varied by up to 2 dB in highly burned areas ocated on slopes tilted towards the sensor. Heavy rainfall or moist vegetation increased the backscatter up to 1 dB. Steeper look angles resulted in significantly higher backscatter coefficients for HH polarization, while for HV polarization only marginal increase was observed. Association strength between backscatter and burn severity estimates improved with the size of the multi-look window, at the expenses however of spatial resolution. Even better results could be achieved at higher spatial resolution by applying a multi-temporal speckle filtering algorithm.

Papel de los factores bióticos y las sequías en el decaimiento del bosque: aportaciones desde la dendroecología

Sanguesa-Barreda, G., Camarero, J.J., Linares, J.C., Hernandez, R., Oliva, J., Gazol, A., et al. 2015. Role of biotic factors and droughts in the forest decline: contributions from dendroecology. Ecosistemas 24(2): 15-23. Doi.: 10.7818/ECOS.2015.24-2.03. Increasing mortality and decline of forests are affecting several tree species and biomes. These events of forest decline have been related to the interaction of several global change elements, like the increase in drought stress or a higher incidence of biotic stress factors. However, the roles played by biotic stressors have been scarcely explored as related to drought stress. In this paper we retrospectively characterize the effects of these organisms on radial growth of trees already stressed by drought using dendrochronology. The combined effect of mistletoe and droughts reduce tree growth in Scots pine and increase defoliation, being these effects more noticeable in the apex. The pine processionary moth induces growth losses in the affected stands of black pine, but they usually recover two years after the outbreak. We did not find a determinate role of root pathogens on silver fir dieback. Finally, dead Aleppo pine trees infested by bark beetles showed similar growth patterns than living pine trees. This study allows characterizing some of the complex and multiple effects of different biotic stressors on growth and vulnerability of forests subjected to the negative effects exerted by droughts.