Understory fuel load and structure eight to nine years after prescribed burning in Mediterranean pine forests

• Understory volume and cover were lower in burned than in unburned plots, 8–9 y. postfire.
• Paired differences in understory volume subject to fire were higher in sites with more canopy cover.
• Crown fire under moderate weather conditions was predicted to be lower in burned plots due to less woody phytovolume.
• Both burned and unburned plots were dominated by woody obligate resprouters.

Prescribed burning is being widely used in fire-prone forests to modify stand structure, reduce risks of severe wildfire, and increase ecosystem resilience to natural disturbances. This study focuses on changes to understory cover, phytovolume, and species richness eight to nine years after underburning, by comparing the understory structure of burned and paired unburned (100 m2) plots in eight Mediterranean pine stand locations (Catalonia, Spain). Taking into account the assumptions of the space-for-time substitution in our study design, phytovolume was significantly lower in burned plots compared to the unburned plots. These differences varied, however, across localities, ranging from negligible changes to a reduction of more than 90%. Differences attributable to management were greater in forest plots with higher overstory cover, which was assessed by hemispherical photograph analysis. This result suggests that the lack of light availability may limit the reestablishment of understory in managed dense stands. Crown Fire Initiation and Spread (CFIS) simulations indicated that these changes in understory structure would only decrease the behavior of a potential wildfire when occurring in low or moderate weather conditions. In both burned and unburned plots, woody obligate resprouters represented more than 60% of the total cover, while facultative resprouters or obligate seeders accounted for less than 20%. No differences were detected in the richness of resprouter, seeder, graminoid or legume functional groups between burned and unburned plots. Our results support the application of prescribed fires to reduce surface fuels in the studied forest types, and the hypothesis that fuel load reduction is most effective in a forest with a closed overstory.