Mexican mixed-species forest shows resilience to high-intensity fire

In the aftermath of a high-intensity wildfire in La Michilía Biosphere Reserve, Mexico, an initial study suggested a shift from an oak-pine forest to a grass-savanna ecosystem. We conducted repeated measurements on sixty permanent plots 1, 5, 10, and 20 years after the original wildfire at paired burned and unburned study sites to capture spatial and temporal dynamics in forest composition and structure. We found that the burned site regained most pre-wildfire characteristics two decades after the wildfire. The ongoing regeneration in the burned site suggests that despite the remaining differences, the site is approaching a complete recovery, with forest characteristics analogous to the unburned site. Our findings indicate that the combination of seeders' wildfire resistance and resprouters’ post-wildfire sprouting strategies in mixed-species forests provide high resilience to high-intensity wildfire. Moreover, protecting La Michilía as a biosphere reserve and heightened public awareness of the natural environment likely played an indispensable role in facilitating the recovery of the post-wildfire ecosystem. 

Methods
We re-sampled sixty permanent plots in the buffer zone of the MBR at approximately 2,245 m elevation in the burned and unburned study sites five (2001), ten (2006), and twenty years (2017) after the wildfire (Fig. 2). The 2017 measurement was conducted in early March, ahead of the 2017 growing season, so we considered it to represent the 2016 growth. Center plots were established in 1996 and remeasured in 1997 on a 100 x 100 m grid spacing (Fig. 3). For each 200 m2 circular fixed-area plot, we re-measured live tree overstory characteristics (species, condition class, diameter at breast height, and total tree height) and cut trees by species, and their diameter at stump height. We also measured dead woody biomass, litter (distinguishable plant parts like bark and twigs scattered on the forest floor), and duff (partially decomposed organic matter found between the litter layer and the A1 soil horizon, with minimal similarities to the original plant structure), along a planar transect from the center plot in the direction established in 1996. Additionally, we tallied tree species regeneration across height classes (less than 40 cm, between 40.1 and 80 cm, and between 80.1 and 130 cm) over a nested 40 m² circular plot. For the complete procedures of field sampling methods, please refer to the Field Sampling section in Fulé et al. (2000).