Spring blooms of the invasive, annual black mustard (Brassica nigra) in California can be seen from space. B. nigra facilitates its dispersal with isothiocyanate, a secondary metabolite in leaves and root exudates that deters herbivores, kills soil fungi, and inhibits germination of native seed. Along with other invasives, B. nigra displaces native grasses, thereby altering the vegetative landscape and exerting bottom-up degradation of coastal sage scrub habitat. However, our previous research indicates that a facilitated mutual defense species interaction exists between two native shrubs, the nurse plant California buckwheat (Eriogonum fasciculatum) that tolerates herbicidal California sagebrush (Artemisia californica), and provides enhanced recruitment inhibition of other plants. Within the current ecological context of intensely invaded habitat, this interaction can improve community resistance and resilience. For example, no B. nigra was present in transects where the shrubs occupied a shared, immediate proximity. We are conducting a series of three-way greenhouse and field germination experiments in order to determine if the positive interaction between E. fasciculatum and A. californica begins upon germination and if the interaction is strong enough to defend and retake territory from B. nigra.
Results/Conclusions
A preliminary three-way germination competition experiment in the greenhouse shows that A. californica can significantly inhibit germination of B. nigra by 100%, but not germination of E. fasciculatum. However, germination rates for all three species were lower than expected based on previous germinations or reported in U.S. Forest Service nursery guides. The low germination rates are presumably due to five-plus years of severe drought-stress in Southern California. While the 2016-2017 winter rainy season saw above average precipitation, rainfall during the 2017-2018 is 70% below the average and daytime highs are > 6 above the mean. We are currently exploring how below average rainfall exacerbated by above average temperatures is affecting germination competition between E. fasciculatum and A. californica versus B. nigra with a series of field and greenhouse experiments in UC Irvine’s greenhouse and Coastal Sage Scrub Ecological Preserve. If the facilitated mutual defense between E. fasciculatum and A. californica begins upon germination and can endure climate change-intensified seasonal and interannual drought, the native shrubs may provide a cost effective and less labor intensive means of conservation and restoration for what is left of the extensively degraded and invaded habitat patches of California coastal sage scrub.