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

Diane Thomson, W.M. Keck Science Department, Claremont, CA, Wallace M. Meyer III, Biology, Pomona College, Claremont, CA and Isobel F. Whitcomb, W. M. Keck Science Department, Claremont, CA

Background/Question/Methods
Non-native plant invasions, changes in fire regime, and increasing drought stress all pose major threats to biodiverse Mediterranean-climate shrublands. These factors also interact in important ways. Fire and drought can create opportunities for non-native species to establish dominance before native shrubs recover, while non-native plant invasion in turn may promote higher fire frequencies. Yet post-fire monitoring and experimental tests of non-native plant effects are rarely integrated into the same studies. We carried out demographic monitoring of the common native shrub Artemisia californica in a southern California sage scrub fragment for 7 years after fire, 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 plant removal, and their interactions on seedling and adult survival. We then developed an individual-based model (IBM) of post-fire recovery from these demographic data.
Results/Conclusions
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. In the IBM simulations, non-native removal focused in the second and third year after fire yielded almost as much benefit as removal for all of the first five years. Our findings reinforce the high vulnerability of sage scrub 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.