Plant communities on harsh soils such as serpentine have been observed to be less sensitive to climate change. However, although disturbance from fire is an important part of grassland, chaparral, and forest communities, the resistance of these communities to climate change has primarily been studied in an undisturbed context. We compare two fires that burned fifteen years apart in a Mediterranean chaparral system to test whether shifting climate patterns—including a historically severe drought—had the same negative impacts on diversity of post-fire communities as it has on unburned communities, and whether those impacts varied by soil type. We sampled species richness in 250 m2 plots (n = 70), and within those plots measured shrub cover on 50 m line intersect transects and herbaceous cover in five 1m2 quadrats.
Results/Conclusions
We found lower shrub regeneration post-fire, including higher mortality and lower shrub seedling density on both soils types (mortality z = -2.1, p = 0.04; seedling t = -2.1, p = 0.04), and lower post-fire growth of coppicing stems on sandstone soils but not serpentine soils (t = 2.6, p = 0.01). In contrast to the shrub response, herbaceous community richness and cover after the 2015 fire was comparable to that after the 1999 fire, with an increase in species richness on burned serpentine soils following a particularly wet year (pseudo-F = 13.7, p < 0.001). This increase in herbaceous richness contrasts with nearby undisturbed chaparral and grassland communities, which did not recover diversity after the same wet winter. The decreases in shrub regeneration are consistent with increasing heat and drought stress during this fifteen year period. Disturbance-following communities on harsh soils may be more resilient to climate change than those on more benign soils, but slower recruitment from woody species may lead to eventual type conversion even on harsher soils.