Post-fire resilience of restored coastal sage scrub and grassland communities

California coastal sage scrub (CSS) and native grassland systems are
experiencing increased fire frequencies and extreme droughts due to
climate change. These stressors, in addition to historical and ongoing
disturbances, make native-dominated communities vulnerable to
vegetation-type conversion to communities of non-native grassland. The
challenge is to restore resilient native plant communities under these
shifting conditions. Our study site, Bee Flat Canyon in the Santa Ana
mountains, contains CSS, grasslands, and other plant communities degraded
by historic agricultural and rangeland use. The area was restored by the
Irvine Ranch Conservancy (IRC) over 10 years and burned shortly after
project completion in the 2020 Silverado fire, presenting a unique
opportunity to assess restoration resilience to fire. We studied how
restored CSS communities recovered compared to unrestored degraded and
intact areas post-fire, whether physiological traits influence post-fire
cover of CSS species, and how restored grassland changed over time in
relation to temperature, precipitation, and fire. We found no significant
difference between restored and intact CSS sites for abundance and
survivorship of crown sprouting shrubs and cover of native/non-native forb
seedlings, indicating that restored sites were as resilient as intact
sites. Degraded sites remained in a degraded state post-fire and contained
almost zero native shrubs. There were differences in CSS plant community
composition amongst degraded, intact, and restored CSS sites such that
restored sites contained some native species that were seeded but absent
from intact sites and vice versa. Plant physiological traits did not
correlate with cover of CSS species, indicating that traits were not
predictors of fire response. In restored grassland, cover did not
significantly correlate with temperature and precipitation. Forbs and
non-native grasses recovered post-fire, but the native
grass Stipa pulchra continued to decline, possibly due
to competition. Grassland plant community composition shifted
significantly toward a higher dominance of non-native grasses in the
second year post-fire compared to previous years, indicating a need for
non-native control measures after fire. From these assessments, we
recommend the IRC to (1) continue monitoring and managing weeds in CSS and
grasslands, (2) enhance the diversity of their restoration seed mixes with
additional native species, and (3) develop a post-fire recovery plan to
limit the reestablishment of non-native species and support resilience of
native species.