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Projects

1. NSF Ecosystems, Collaborative research: Sensitivity of ecosystems processes to precipitation across a grassland to shrubland vegetation transition in the southwestern U.S., $400,000, Research Grant, PI (Huxman), Collaborator (Williams University of Wyoming). 6-01-04 to 5-31-07,

Between 50 and 90% of North Americas biomes may see significant vegetation composition shifts (Neilson 1993), which arguably represents the most ubiquitous and significant form of global change. We know very little about how those sifts in vegetation type influence ecosystem processes such as CO2 and H2O exchange. For example, the current increase in the density of woody plants in North Americas grassland and savanna ecosystems may be an important contributor to the North American carbon sink (Pacala et al., 2001). Additionally, this increase in woody plant density has been suggested to strongly influence landscape water budgets (Huxman et al., In Press). However, site level studies suggest that changes in carbon exchange dynamics likely depend strongly on climate / soil conditions (Jackson et al., 2002). Considering the anticipated shifts in the timing and magnitude of precipitation in North America and potential vegetation sifts, building an understanding of the linkages between ecosystem vegetation composition, processes such as net ecosystem exchange of CO2 and H2O and variability in climate is warranted.

This project evaluates the following question: In grassland and savanna ecosystems, how does an increase in woody plant abundance influence (1) ecosystem CO2 exchange and (2) evapotranspiration? This question reflects a major knowledge gap in ecology and is constraining our ability to predict the impacts of contemporary, large-scale changes in vegetation occurring in semi-arid and arid lands throughout the globe (House et al., 2003; Huxman et al., In Press). We will evaluate this question by measuring the responsiveness of ecosystem ecosystem photosynthesis, respiration, and evapotranspiration to climatic drivers (precipitation, temperature and light) across a gradient of woody plant density in two physiographic conditions; uplands and riparian zones. Manipulative experiments will be used to evaluate the responsiveness of woody plant, grass and soil components of these systems to different sized summer rainfall pulses. These data collection efforts will be connected by the use of multivariate statistical models to understand the direct and indirect effects of different environmental drivers on ecosystem structure and function. This project is the foundation of a larger effort to understand the ecological complexities of global change in semi-arid ecosystems.

2. NSF Ecology, A model for species interactions: costs and benefits of linked herbivory / pollination in Datura wrightii and Manduca sexta, $450,000, Research Grant, PI (Bronstein), Co-PIs (Huxman & Davidowitz). 9-01-03 to 08-31-06,

We are studying the relationship between a common hawkmoth (Manduca sexta) and the plant upon which it primarily depends in southeastern Arizona (Datura wrightii). Like certain other generalist Lepidoptera, M. sexta acts as both a primary pollinator and damaging herbivore of the same plant species. This study will: (1) identify and quantify costs and benefits of the interaction to both partners, via observational and experimental studies; and (2) explore some of the linkages between costs and benefits to each partner, and cross-linkages between costs to one partner and benefits to its mutualist. These data will allow exploration of the link between the benefit of pollination and the cost of herbivory experienced by the plant, and the cross-link between the cost of herbivory to the plant and the benefit of consumption to the insect.

3. NSF Ecology, Individual Function and Community Processes in Desert Annuals, $450,000, Research Grant, PI (Venable), Co-PI (Huxman). 2-01-05 to 1-31-08,

This proposal is to investigate the functional biology of Sonoran Desert winter annual plants in order to provide insights into their population and community dynamics. Annual plants are a major component of arid land plant communities. Ecologists are interested in the mechanisms that allow species to coexist and hence maintain species diversity. We have previously documented long-term population and community dynamics of a community of Sonoran Desert annuals. This ecosystem has highly variable precipitation. Plants respond to this variability in species-specific ways that result in the idiosyncratic schedules of reproductive success. This partially uncorrelated reproduction contributes to these species coexistence in ways that are now understood mathematically. The goal for this proposal is to understand how this variation in performance among species, which contributes to their coexistence, is a function of their physiology and morphology. We will collect various types of physiological, morphological and stable isotope data on plants growing in their natural habitats. This will uncover variation in functional biology that determines how different co-occurring species experience environmental variation and hence coexist. The previously collected long-term population dynamic data on this community have given us a unique window on how environmental variation can contribute to biodiversity by decoupling the temporal pattern of reproductive success. This project allows us to explain the underlying biology that creates the differential response to variation that is in turn responsible for two important ecological phenomena: species coexistence and diversity. The results of the work are also germane to management plans for desert systems.

4. NSF Ecology, Collaborative Research: Vulnerability of Semi-arid Grasslands to Encroachment by Woody Plants: the Role of Grass Invsions, Seasonal Precipiation, and Soil Type $142,573, Research Grant, PI (Huxman), Collaborators (Williams - University of Wyoming; Weltzin - University of Tennessee). 9-01-04 to 8-31-07,

Within the last 100 years, many grasslands throughout the southwestern United States have been inadvertently converted to dense thickets of woody plants, including the thorny shrub mesquite. These conversions have had important impacts on human activities, biological diversity, nutrient cycling, and wildfire regimes. However, many mesquite-dominated grasslands have themselves been recently invaded by an African grass, Lehmann lovegrass. Invasions by this grass may change the availability of water and nutrients in the soil, and thus shift mesquite shrublands back towards grassland. However, these grasslands would be dominated by a non-native grass with arguably greater impacts on ecological systems than mesquite has had on former grasslands. When predicting how southwestern rangelands might respond to invasions by grasses and woody plants, one must also consider how soil characteristics (e.g., clay content) and potential changes in climate (e.g., changes in precipitation) affect the relative performance, and the ultimate ratio, of grasses to shrubs. By conducting a field experiment in southern Arizona, the investigators will test the hypothesis that grass invasions will interact with soil type and seasonal precipitation to affect the successful establishment of mesquite in this habitat. The response of mesquite introduced as seeds into experimental plots will be used to gauge the potential response of this species to different environments. Concurrent investigations of the cycling and storage of carbon dioxide and water within these plots will facilitate our understanding of interactions between water, soil type, and plant cover. This research explicitly considers both climate change and biological invasions, each of which will continue to affect natural ecological systems and the goods and services that they provide to human society. Research will foster collaborations between scientists, land managers, and students from several institutions, and will promote interdigitation of subdisciplines including soil science, modeling, ecology and atmospheric sciences. The project includes training opportunities for students, including those in teacher training programs and in underrepresented groups.