Fire Severity in a Large Fire in a Pinus pinaster Forest is Highly Predictable from Burning Conditions, Stand Structure, and Topography

Identifying what factors control fire severity in large fires is critical for understanding fire impacts and planning pre- and post-fire management. Here, we determined the role of pre-fire stand structure, directional topography, and burning conditions on fire severity in a large fire (12,697 ha) in Central Spain that burned a Pinus pinaster forest on July 2005. Fire severity was estimated using RdNBR based on Landsat 5 TM images. Forest stand structure was reconstructed by systematically sampling the burned area (n = 236). Burning conditions were established using weather information and a map of fire progression, based on which fire rate of spread and propagation direction were calculated. Topographic features in the direction of the fire-front were derived from a digital elevation model. Boosted regression tree (BRT) analysis was employed to relate each group of variables or the entire set to RdNBR. Fire severity was best explained by burning conditions (cross-validation correlation [CVC] 0.56), followed by pre-fire stand structure (CVC 0.34), and directional topography (CVC 0.17). Combining the three sets of variables, CVC increased to 0.71. Higher fire severity occurred in areas burning upslope, with high fire rate of spread, with heterogeneous and dense stands of P. pinaster and Quercus pyrenaica in the understory, receiving high solar radiation, among other characteristics. Fire severity was the result of interactive relationships between burning conditions, pre-fire stand structure, and directional topography. Thus, determining factors controlling fire severity from static stand structure or topography, as is often done, may not be appropriate.