Shrub fuel characteristics estimated from overstory variables in NW Spain pine stands

Abstract

Understanding the relationships between forest overstory and the understory shrub layer is important for predicting biomass changes in response to forest management. The shrub fuel stratum is a critical fire hazard in the Mediterranean region, hence managing the canopy cover to control the understory could be an option for managing fire hazard. In this study we determined the relationship between overstory and understory shrub variables for Pinus pinaster Ait., Pinus radiata D. Don and Pinus sylvestris L. in a large geographical area (NW Spain) using data from the Spanish National Forest Inventory. CHAID and quantile regression procedures were used for obtaining mean and maximum response models, respectively, of shrub characteristics (cover, height and fine fuel load). Overstory variables, especially stand basal area, explained the variation in the mean response of shrub variables. According to maximum response models, maximum shrub development was also limited by overstory variables in P. pinaster and P. radiata stands. This does not apply to P. sylvestris, suggesting that altitude or other non-measured environmental factors are more active limiting constraints for shrub development in stands of this species. Simulation of surface fire behaviour and crown fire initiation was exemplified from P. pinaster data, using quantile regression models as inputs. Fire behaviour outputs revealed that the control of understory shrubs through high overstory basal area can decrease surface fire intensity. Nonetheless, the effect of overstory basal area on crown fire initiation is negligible, and is unlikely to increase fire suppression effectiveness, at least in young stands. Moreover, the potential for crown fire spread and intensity is expected to increase in high-density stands because of the higher canopy bulk density. Forest management can benefit from fire behaviour models, namely in the design of silvicultural treatments targeting crown fire hazard mitigation.