Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub

Non-native plant invasions, changes in fire regime, and increasing drought
stress all pose important threats to biodiverse mediterranean-climate
shrublands. These factors can also interact, with fire and drought
potentially creating opportunities for non-native species to establish
dominance before native shrubs recover. We carried out post-fire
demographic monitoring of the common native shrub Artemisia californica in
a southern California sage scrub fragment for 7 years, including several
with very low rainfall. Experimental removals of non-native plants were
included for the first 4 years. We quantified A. californica post-fire
crown resprouting and seedling emergence, and tested effects of
precipitation, non-native plants, and their interactions on seedling and
adult survival. Only 7 A. californica were confirmed as resprouts; almost
all individuals established after the fire from seedlings, with 90% of
emergence occurring in the second growing year after fire (spring 2015).
Higher spring precipitation increased both adult and seedling survival.
Non-native grasses and forbs rapidly recolonized control plots, but the
removal treatment reduced non-native cover by nearly 60%. For seedlings,
non-native removal reduced the probability of dropping leaves by start of
summer drought and increased survival both directly and through positive
interactions with rainfall. Non-native removal also reduced mortality in
smaller adult plants. By 2020, mean A. californica canopy area was nearly
four times greater in non-native removal plots. These findings reinforce
the high vulnerability of sage scrub habitat to post-fire loss of shrub
cover and potential type conversion, particularly with increasing drought
frequency and in stands and species with limited crown resprouting. Yet
they also illustrate the potential for targeted management of non-natives
immediately after fire to promote recovery of native shrubs in this
increasingly endangered community.