Post-fire recovery and microsite differentiation among ten fern species in the Santa Monica Mountains of southern California

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Sarah A. Holmes, Stephen D. Davis and Helen Holmlund, Natural Science Division, Pepperdine University, Malibu, CA

Background/Question/Methods
The Santa Monica Mountains (SMM) in southern California are a mediterranean-type ecosystem, exposed to extended summer drought and frequent wildfires. Although most fern species worldwide require abundant water, previous studies have shown that fern species in the SMM use niche segregation to reduce competition for limited water resources, with considerable variation in minimum seasonal leaf water potential, root depth, gas exchange, and frond phenology (evergreen, deciduous, and desiccation-tolerant “resurrection” ferns). Although prior work has characterized fern adaptation to drought in the SMM, the impacts of recent wildfire on fern populations remain unknown. In November 2018, the Woolsey fire burned nearly one-half of the SMM, providing a natural experiment to characterize fern microsites and post-fire recovery. We hypothesized that the distribution and abundance of fern species would be differentially impacted by the Woolsey fire, with the resurrection ferns most at risk for localized extinction, as they have the shallowest root system. Furthermore, we hypothesized that fern microclimate (aspect, slope, and sunlight) would reflect niche segregation and partitioning of limited resources. To test these hypotheses, we surveyed ten fern species in six inland sites of the SMM (three burned and three unburned sites).

Results/Conclusions
Most ferns grew along north-facing slopes, avoiding direct sunlight. However, there were few significant differences in sun exposure (percent of maximum PPFD) with respect to species and no significant differences in sun exposure with respect to frond phenology (p > 0.05, n = 225 ferns). This lack of differences in sun exposure among species and frond phenology type may indicate a high degree of plasticity with respect to microsite, as PPFD ranged from 0% to 80% of maximum sunlight (open-canopy control site). The ten species varied in the breadth of their distribution; evergreen dehydration-tolerant Dryopteris arguta was abundant at all six sites, while three species (Adiantum capillus-veneris, Pteridium aquilinum, and Woodwardia fimbriata) were restricted to sites with perennial streams. The Woolsey fire did not affect the distribution of fern species or frond phenology types in recently burned vs unburned sites, as most ferns resprouted from an underground rhizome post-fire. We conclude that fern species in the SMM may be well adapted to rapid recovery post-fire, perhaps aided by patchy burn intensity and adaptation to a broader range of microsites than previously realized.