Effects of invasive species on post-fire seedling establishment in Ceanothus spinosus

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Nina R. Duchild, Michael T. Nicolas, Addison Deisher and Helen I. Holmlund, Natural Science, Pepperdine University, Malibu, CA, Mari R. Irving, University of California Santa Barbara, Stephen D. Davis, Natural Science Division, Pepperdine University, Malibu, CA, Helen I. Holmlund, Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, CA, Frank W. Ewers, Biological Sciences, California State Polytechnic University, Pomona, Pomona, CA

Background/Question/Methods
The chaparral ecosystem of the Santa Monica Mountains of Southern California is adapted to natural wildfire regimes. In November 2018, the Woolsey Fire burned 39,234 ha of land, approaching one-half the area of the Santa Monica Mountains. Following the Woolsey Fire, we compared recovery of the chaparral shrub Ceanothus spinosus at two distinct burn sites on Pepperdine University’s campus in Malibu, CA. One site had undergone fuel reduction treatment by the thinning of shrubs and experienced a “cool burn,” which was not hot enough to kill seeds of exotic grasses and forbs. The other site was a high density stand of chaparral shrubs, relatively pristine, bearing a much higher fuel load which generated a high intensity “hot burn” that killed seeds of exotics. We hypothesized that native shrub seedling success in the cool burn site would be hindered by competition from prevalent exotic species, resulting in lower seedling physiological performance and availability of water and light compared to seedlings at the hot burn site. Beginning in February 2019, we compared post-fire seedling height, seedling mortality, shading, leaf area index, shoot water potential, photosynthetic rates, rooting depth, root nodulation, as well as soil water potential, soil respiration, and soil nutrient levels.
Results/Conclusions
Early in the first dry season post-fire, indicators of plant growth such as photosynthetic rates, root length, shoot length, and predawn water potential showed no significant differences between the hot and cool burn sites at α = 0.05. However, in October 2020, seedlings in the cool burn were significantly taller than those in the hot burn (n=14-16 quadrats per group, p < 0.05). The hot burn seedlings also had significantly lower water potentials (-5.8 ± 0.4 MPa) compared to those in the cool burn (-2.0 ± 0.1 MPa; mean ± SE, n=9, p<0.05). This difference in seedling shoot water potential may be attributed to the significant difference in soil water potential between sites in October 2020 (n=12 per group, p<0.05). We concluded that the C. spinosus seedlings in the cool burn site did not face significant competition from exotics during the first summer post-fire due to high soil moisture associated with unseasonably late rainfall. Contrary to our hypothesis, the seedlings in the hot burn ultimately experienced a greater water deficit than those in the cool burn, possibly due to the summer dormancy of the exotic grasses. Our results suggest that exotic grasses may not reduce water availability to shrub seedlings.