As rising temperatures exacerbate abiotic stressors and disrupt pollinator and herbivore phenologies, plant-plant interactions, such as nurse plant facilitation, will likely have increased effects on their biological communities. Nurse plants mitigate stress gradients for other plant species, especially in xeric climates, through shading, mobilization of nutrients, and other means. Nurse plants often receive benefits from facilitated species, but because they lack means of punishing cheaters, they may also harbor invasives and native competitors. Unlike previous studies, we hypothesize that California buckwheat (Eriogonum fasciculatum) is a nurse plant that achieves indirect means of punishing cheaters through a combination of 1) facilitation of California sagebrush (Artemisia californica) via acidic root exudates that mobilize nutrients otherwise bound to calcareous soils and 2) tolerance of Artemisia’s herbicidal leaf leachates. Such an interaction helps explain the shrubs’ nearly exclusive coexistence in each other’s immediate proximity in their nutrient- and water-stressed coastal sage scrub (CSS) habitat.
A facilitated defense experiment was conducted in the greenhouse to minimize the confounding effects of shading, water competition, and herbivores. Since non-native forbs are displacing CSS grasses, we investigated the shrubs’ interactions by placing them in direct competition with the invasive, fungicidal forb black mustard (Brassica nigra).
Results/Conclusions
E. fasciculatum facilitated A. californica while tolerating Artemisia’s negative effects observed in B. nigra. E. fasciculatum lowered soil pH 6.3%, corresponding with a 96.3% increase in A. californica’s shoot DW when no B. nigra was present. Interestingly, the presence of B. nigra weakened facilitation to a statistically intermediate 27.3%. In the absence of E. fasciculatum, A. californica reduced B. nigra growth rate 64.6%, final shoot DW by 78.6%, and inflorescence 595.8%. While E. fasciculatum reduced most of A. californica’s negative effects on B. nigra, E. fasciculatum and A. californica together reduced survival of B. nigra by 133% compared to pots with only E. fasciculatum and B. nigra. A. californica did not reduce the biomass of E. fasciculatum whether B. nigra was present or not. However, B. nigra increased E. fasciculatum mortality 100% in the absence of A. californica and 500% when A. californica was present. Taken together, B. nigra weakened E. fasciculatum’s tolerance and facilitation of A. californica, which suggests that the shrubs’ positive interactions are mediated by mycorrhizae, and threatened by the fungicidal forb.