Stephanie Yelenik: Current Research
Plant-soil Feedbacks and Invasive Plants
Plant-soil feedbacks have received much recent interest in the literature, especially in relation to invasive plants. It has been postulated by researchers that the ability to create a positive soil feedback (or, to “culture” a soil environment that is has a net benefit over the soils “cultured” by other species) may lead to the aggressive invasiveness of some non-native species. Research in this area has largely been carried out in greenhouse pot studies, in the absence of natural field conditions or competition. My dissertation research focuses on the role of plant-soil feedbacks in the dynamics between native California coastal sage scrub and invasive annual grasses on Santa Cruz Island. I am interested in 1. how native and invasive species differentially alter soil nutrient cycling and microbial communities, 2. how these changes in soils feedback to alter plant performance, 3. how large the effects of plant-soil feedbacks are relative to the effects of competition in the field. I am using a combination of field experiments, greenhouse experiments, and field soil sampling to answer these questions. In addition, I am also collaborating with Jonathan Levine, Elizaveta Pachepski, Bruce Kendall, and Janneke Hille Ris Lambers to formulate a model that looks at the effects of plant-soil feedbacks on invasive spread.
The Role of Microbes in Interspecific Plant Competition for Nitrogen
The domination of California grasslands by Mediterranean annual grasses is one of the most spectacular examples of the displacement of a native flora by nonnative plant species. Janneke Hille Ris Lambers, in collaboration with Jonathan Levine, established a manipulative experiment in the Santa Ynez Valley to ask why exotic annuals are so common when native annuals are rare. Part of this experiment used an R* framework to investigate the role of resource competition in the extirpation of native by exotic annuals. In reference to soil nitrogen, Tilman’s R* theory suggests that if exotic annuals are better competitors for nitrogen than native annuals, then exotic species should have lower levels of N in monoculture plots at the end of the growing season. Although unimportant for the overall R* theory, the assumed mechanism behind this pattern is plant draw down. In other words, the species that draws N down to the lowest level should be the best competitor, and gain dominance in mixture plots. This is a contentious statement, however, because N dynamics in soils are often mediated through microbial communities. Indeed, the dominant paradigm in California grasslands states that microbes are better competitors for N than plants. Thus, it should follow that the level of N at the end of the growing season is due to plant species effects on microbial communities and N dynamics, and not simply plant uptake of nitrogen. I have been working in collaboration with Ben Colman to investigate C and N dynamics in these monoculture and mixture plot soils, as well as C and N in plant and microbial biomass to tease apart the roles of plants and microbes within resource competition in California grasslands.
Ecosystem Impacts of Invasive Plants
My Masters research with William Stock and Dave Richardson at the University of Cape Town, South Africa, focused on the impacts of N-fixing invasive species on ecosystem function in the fynbos. Fynbos is an extremely diverse vegetation type that makes up a large portion the Cape Floristic Region and is found in very low N soils. These communities have been invaded by a suite of N-fixing plants from around the world, including Acacia saligna (a perennial woody species) and Lupinus luteus (an herbaceous annual). Therefore, the biodiversity found in fynbos vegetation is under threat of the legacy these aggressive invasive plants leave in the soil as increased levels of nitrogen. I was interested in 1. comparing how an annual and perennial species of N-fixer can differentially affect nutrient cycling, 2. how the active clearing of invasive plants can further impact changes in N cycling by increasing soil light and temperature, 3. whether increases in N cycling rates could lead to secondary invasions of weedy species. This research was especially timely as South Africa had taken large measures to clear its invasive plants through the job creation initiative “Working for Water”.