My research is broadly aimed at understanding the interactions that structure plant communities in space and time. These range from competition, facilitation, interactions with soil microbes, and climatic and environmental pressures. My research also focuses on understanding the mechanisms through which global change factors, including climate change, nitrogen deposition, and invasive species, influence native plant communities and the consequences of these global changes for ecosystems in the future. I study ecology in a framework that is founded in ecological theory and utilizes a broad range of experimental, survey, and modeling techniques.

1. Invasion biology

-Dispersal vectors and risk assessment of noxious weed spread

medusaheadandcattlePlant invasions are occurring on a global scale at unprecedented rates, and often their spread is not tracked until they become widely distributed and problematic. Being able to reconstruct the spatial history of invasions and understanding dispersal patterns of invaders is essential in identifying important corridors of invasive spread. I am currently on a USDA postdoc fellowship studying how cattle may act as dispersal vectors of the noxious weed medusahead (Elymus caput-medusae) in California grasslands. I am using population genetics to understand how medusahead spread across the state and will eventually use this information for invasion risk assessment.

-Litter feedbacks in invasive cattail success

Invaded systems are commonly associated with a change in ecosystem processes and a decline in native species diversity; however, many different causal pathways linking invasion, ecosystem change, and native species decline could produce this pattern.  For my Ph.D. I studied these interactions in the aggressive wetland invader hybrid cattail, Typha x glauca, in Great Lakes coastal marshes.  I found that cattails affect ecosystem processes by elevating nutrients and reduce native plant diversity through litter production, which produces positive feedbacks that benefit cattail growth.  This implies that management of cattails must include not only reducing live cattail abundance, but also eliminating cattail litter and addressing high soil nutrient levels.

2. Climate change effects on plants and microbes in the alpine

-Interactive effects of warming, snowpack, and N deposition on tundra communities

Global change is multifaceted, comprising simultaneous changes in many environmental factors, and these factors may have non-additive effects on plant communities.  Moreover, these global change factors can influence species both by directly affecting performance and by modifying the competitive environment. I am collaborating with Katharine Suding, Isabel Ashton (National Parks Service), Jonas Knape (UC Berkeley) to study these non-additive effects in the alpine tundra.   In this study, we incorporate components of global change into population dynamic models and fit them to experimental data to determine how global change factors affect population dynamics and how species interactions are altered by global change.

-Nitrogen deposition effects on plant-soil interactions

I am collaborating with Katharine Suding (UC Berkeley), Robert Sinsabaugh (University of New Mexico), and Andrea Porras-Alfaro (Western Illinois University) to investigate how increased N deposition affects plant-fungal interactions. Human activities have played a major role in increasing nitrogen (N) availability throughout the world.  In many plant communities, N enrichment facilitates plant productivity but negatively affects species diversity.  The mechanism behind this diversity decline may be increased competition among plants; however recent studies suggest that the soil microbial community also plays a role.  Most plants form symbiotic associations with mycorrhizal and endophytic fungi, and these soil microbes and their interactions with plant hosts can change with increased N availability.  We are finding that plants that do well in N enriched conditions may allocate more carbon to their fungi, and plants that do poorly may be colonized by parasitic or pathogenic microbes.

-Woody encroachment in the alpine tundra

Global change is causing a shift in tundra vegetation in that woody plants are encroaching into herbaceous-dominated communities.  This drastic change in functional composition of the tundra has consequences for ecosystem function, notably carbon storage.  Quantifying the rates of shrub encroachment and determining which tundra community types are being invaded is essential for predicting future trajectories in tundra vegetation as well as assessing the implications this will have for the tundra as a carbon sink.  I mentored Adam Formica, an undergrad at Columbia University, as an REU student at the Mountain Research Station, Niwot Ridge, CO in 2011.  We are using aerial photographs from the 1946-2008 and shrub biomass and carbon data to answer these questions.

3. Feedbacks and the balance between competition and facilitation

-Competitive hierarchies and negative feedbacks

Models of local stable coexistence require negative feedbacks, i.e. intraspecific interactions must be more negative than interspecific interactions. However, most competition experiments have found evidence for competitive hierarchies.  For my Ph.D. I studied competitive interactions among four common species in the dry sand prairie in Michigan, at two life history stages, germination and adult growth.  I found that patterns in the relative strength of conspecific and heterospecific competition depend on life-history stage, with hierarchies in effects on germination but negative feedbacks among adults.

-Time-lagged plant interactions

Both facilitative and competitive interactions occur simultaneously among plants, and the net balance between them can vary over time. Despite this, recent model-fitting studies have found that negative interactions predominate. This suggests that more complex models may be necessary to uncover facilitation. For my Ph.D. I fitted models including seasonality, interannual variation, and time lags to survey data to test for patterns in positive and negative interactions among plants in a Michigan dry sand prairie.  I found that immediate (direct) interactions were primarily competitive, however, they were more facilitative in the drier, harsher summer compared to the wetter fall/spring.  Interestingly, all species had positive conspecific lagged interactions for both seasons.  I think these lagged interactions might indicate effects from litter or past storage in rhizomes.