Research Interests and Current Projects

1.Plant Community Change: Research in my lab has focused on clarifying the importance of biotic interactions, seed availability and the physical environment in determining the success of plant invasions and on putting invasion biology into mainstream ecology by discussing community resistance and 'invasibility' as part of general ecological theory. I and my students use both experimental field and greenhouse studies and observational approaches to understand mechanisms responsible for resistance to invasion and variability in resistance among communities.

 

a. Processes influencing rates and patterns of biological invasion: A major theme in plant ecology has been to try to elucidate factors influencing plant community structure and the direction and magnitude of community change through time. The successful management of intact native plant assemblages depends on understanding community attributes as well as how native and introduced plants will respond to anthropogenically-driven environmental change. Current research in my lab is evaluating the effects of rainfall manipulations on native versus non-native grassland species, the influence of savanna trees on plant species contributions at local and landscape scales including their impact on non-native species, and effects of nutrients on composition of grassland habitats.

b. Impacts of livestock grazing on species composition of California grasslands: California grasslands are notorious for their persistent domination by European annual grasses and forbs. Not only has the lively debate over the pre-contact composition of these grasslands and influence of livestock grazing on compositional change not been resolved, but there also has been little resolution about the current influence of livestock grazing on grassland composition. In addition, the usefulness of fire as a tool to manipulate current composition is unclear. In collaboration with Claudia Tyler at UCSB, we are examining the importance of soil factors (e.g. texture, depth, infiltration rates, C,N,P) in controlling the vegetation response to release from grazing across 14 paired exclosure/control sites at UCSB's Sedgwick Reserve. Our goal is to evaluate the sources of variability in response to release from grazing using sites that have had the same recent history of grazing/release, and experience similar climate but differ in soil properties. We have observed that on the more N rich soils, excluding grazing leads to rapid dominance of the invasive grass, Bromus diandrus (ripgut). On the more N poor soils, there are no strong consistent effects of excluding grazing.

c. Plant/litter feedbacks: Invasive grasses accumulate large amounts of standing dead litter over time. We are exploring how this litter influences growth and recruitment of both native and non-native species in California grassland and in invaded seasonally dry woodlands in Hawaii.

2. Species effects on ecosystem structure and function: An exciting development in ecosystem ecology over the past decade is the rise in interest in the impacts of individual species or groups of similar species on ecosystem processes such as rates or patterns of nutrient accumulation and flux, energy flow, and soil and hydrological processes. My interest in this area is centered around trying to understand the conditions within which a species or group of similar species will have measurable impacts versus when climate, soils or other factors will be override the effects of individual species. I first became interested in this while a graduate student at UC Santa Barbara when I found that the introduced succulent Carpobrotus edulis, has significantly greater effects on soil chemistry in some sites than in others and that the strength of the effect was correlated with soil texture. Since then, my research in this area has focused on the effects of non-native invasive species on disturbance regime and nutrient cycling.

 

a. Exotic grasses & the grass fire cycle: Since 1990 I have worked on the impact of exotic grass species on fire regimes mainly in Hawaii but also in Nevada . This work has focused largely on alteration of fire frequency as an important ecosystem change occurring in semi-arid habitats as a result of the widespread movement of invasive grasses. By changing the distribution and abundance of fine fuels through space and time, introduced grasses have increased the occurrence of fire in ecosystems with little prior history of fire, causing large scale loss of native species and profound alteration of ecosystem functioning. With resource management specialists at Hawaii Volcanoes National Park , I have investigated variation in the outcome of grass-fueled fires across an elevation gradient and also specifically the effects of grass-fueled fires on ecosystem change within seasonally dry mid elevation woodlands. In recent research we are evaluating the trajectory of burned sites that have not reburned despite thirty years of being dominated by non-native grasses. We are evaluating possible windows of opportunity for restoration of native species. Currently I also participate in a Great Basin wide network of scientists setting a research agenda on annual grass invasions and fire across the western USA (http://greatbasin.wr.usgs.gov/GBRMP/bwg.html).

b. Species effects on nutrient cycling: The prevalence of invasive exotic plant species on the landscape has provided a tool for examining how species effects on nitrogen cycling develop. With former graduate student Michelle Mack and undergraduate students, we produced a major body of work on the effect of introduced grasses on nitrogen cycling in Hawaiian woodlands. We found that contrary to our expectations, introduced grasses had little to no effect on nitrogen cycling in otherwise undisturbed Hawaiian woodlands despite their very high abundance and large amount of standing litter. This was because they did not replace native species but rather interacted with them in a competitive and compensatory manner. By contrast, after fire these woodlands become dominated almost exclusively by exotic grasses, nitrogen cycling is speeded up and overall ecosystem N retention greatly decreases. The increased leakiness of this system after fire is due to the loss of native species caused by fire after grass invasion rather than direct inputs from the grasses. We are now going back to these systems that we studied in the 1990s and with postdoc Stephanie Yelenik evaluating how ecosystem nitrogen cycling and retention has changed in the 15 more years that have passed since the initial fires and dramatic community change. Our initial results suggest that although the exotic grasses are still dominant in these burned ecosystems, exotic woody species are beginning to invade. Also in unburned woodlands, exotic grass composition in the understory is changing dramatically. In NSF sponsored research, we are exploring the consequences of these changes for N cycling.

With collaborator Jeff Corbin ( Union College ) we are investigating the effect of grass origin and life form on ecosystem N retention and cycling in a California coastal prairie. We have found that introduced annual grasses that have displaced native perennial species in California grassland cause increased nitrate leaching and decreased nitrogen retention. In addition to comparing annual exotic to native perennial grasses, we are comparing both of these groups to a new group of invaders in coastal prairie, cold season perennial grasses from Europe . We used a combination of field and lab soil and plant measurements and experimental additions of 15N labeled ammonium to evaluate species effects on N cycling in this system.

3. Collaborative studies of ecosystem change:

 

a. Impacts of wildfire on coastal ecosystems: In collaboration with the Santa Barbara Coastal Longterm Ecological Research (http://www.lternet.edu/sites/sbc/) program we are evaluating plant and soil responses to high intensity wildfire events that have occurred within watersheds that have been monitored for the past decade. As part of this NSF sponsored research, we are evaluating soil and plant N retention and recovery after fire as well as working with UCSB geographers to evaluate recovery using a combination of remote sensing and on the ground measurements. A new wildfire related project in the lab is evaluating the impacts of fog on fire coastal region regimes.

b. Nutrient limitation in grasslands throughout the world: As part of the "Nutnet" global network of grassland sites (http://nutnet.science.oregonstate.edu/), we are maintaining a fertilization experiment in exotic species-dominated old fields at the Sedgwick Reserve one hr from campus. Annually we measure biomass and compositional responses to N, P and micronutrient fertilization. Within these study plots we also maintain a litter manipulation and native plant seeding experiment to evaluate the importance of plant litter and nutrients to vegetation composition.

 

 
 
Ecology, Evolution and Marine Biology Department :: Environmental Studies Department :: University of California Santa Barbara