Many native Californian plant species associate with mutualistic arbuscular mycorrhizal fungi, pictured here through the microscope.

How Will Above- and Below-ground Linkages Influence Community Response to Environmental Change?

Plants can have a profound influence on the physical, chemical, and biological properties of soils. Soil properties, including soil microbial communities, also exert a strong influence on plant performance and growth. These plant-soil feedbacks can play an important role in structuring plant communities, and may also mediate ecosystem response to environmental change. In southern California, many native plant species form relationships with symbiotic mycorrhizal fungi, which supply plants with nutrients and water in exchange for carbon from photosynthesis. This key symbiosis may also protect plants from drought stress. Previous research has shown that this relationship can be impacted by elevated nitrogen deposition, and that the resulting changes to soil fungal communities may benefit nonnative species over natives. I explore if alterations to plant-soil feedbacks can help explain nonnative plant invasion under drought and nitrogen deposition in California'a coastal sage scrub.

Interactive effects of nitrogen deposition and drought-stress on plant-soil feedbacks of Artemisia californica seedlings

Nitrogen (N) deposition and drought are major drivers of global change that will influence plant-soil feedbacks. In a study previously published in Plant and Soil, I investigated how N availability, N- impacted soil communities and drought affect feedback in seedlings of a drought-deciduous mycorrhizal shrub, Artemisia californica. Seedlings were inoculated with soil from either a high or low deposition site or sterilized inoculum and grown with or without supplemental N and under well-watered or drought-stressed conditions. I found that inoculum, N and water had interactive effects on feedbacks. Seedlings grown in low deposition inoculum exhibited a neutral to positive feedback under drought and had the highest root to shoot ratios and mycorrhizal colonization. Seedlings inoculated with high N-deposition soil experienced a positive feedback when N fertilized and well-watered, but plants allocated large amounts of biomass to shoots and had a negative response to drought. I found N-impacted communities may reduce mycorrhizal colonization and allocation to roots and provide less protection against drought. These results highlight the context dependency of plant-soil feed- backs and the potential for climate change and N deposition to have interactive effects on these relationships.

Seedlings of California Sagebrush, or Artemisia californica. This species is a dominant of the costal sage community.

Nitrogen enrichment contributes to positive responses to soil microbial communities in invasive plant species

Increased resource availability and feed- backs with soil biota have both been invoked as potential mechanisms of plant invasion. Nitrogen (N) deposition can enhance invasion in some ecosystems, and this could be the result of increased soil N availability as well as shifts in soil biota. I recently published a study exploring these dynamics in Biological Invasions. In a two- phase, full-factorial greenhouse experiment, I tested effects of N availability and N-impacted soil communities on growth responses of three Mediterranean plant species invasive in California: Bromus diandrus, Centaurea melitensis, and Hirschfeldia incana.  I found that while these species may differ in responses to soil biota and N, growth responses to soils conditioned by conspecifics appear to be most positive in all species under high N availability and/or in soil communities previously impacted by simulated N deposition. My results suggest N deposition could facilitate invasion due to direct impacts of soil N enrichment on plant growth, as well as through feedbacks with the soil microbial community.

I used controlled greenhouse experiments to further explore mechanisms that operate in the field.