Patrick C. Murphy
Leaf Wax Development in Native Sonoran Desert Plants
Species native to the Sonoran Desert in northern Mexico and southwestern US have adapted to the bi-modal precipitation regime of the North American Monsoon (NAM), driven by winter storms from the Pacific Ocean and summer storms from Baja California. 50-80% of rainfall occurs during the months of July-September, when temperatures are frequently above 40 °C. As a result, plant growth and development is influenced by these summer storms.
However, it is unclear how this influence is reflected in the molecular structure of leaves. What is the timing and magnitude of plant growth in relation to summer vs. winter precipitation? Additionally, with temperatures and vapor pressure deficits at extremes, is growth affected by seasonal stress?
Isotope ratio mass spectrometry can address these questions. Stable isotopes of hydrogen and carbon found in leaf waxes produced by these plants record the environmental conditions the plant is experiencing during its life. δ2H and δ13C of leaf wax alkanes relfect the relative precipitation source and water stress, respectively. Results of this study show that some species respond quite rapidly to environmental changes, while other species seem to have no strong relationship.
With changes to the climate over the past several thousand years, the NAM has also changed in timing and magnitude. A better understanding of how monsoon precipitation is recorded in leaf waxes will improve their usefulness as a biomarker proxy of past climate.
Water Use Efficiency in a High Elevation Semiarid Forest
Of strong interest in western US forests is the controls on water loss - what happens to the precipitation once it falls? After losses such as canopy interception and runoff, a fraction of precipitation is held in the soil and is available to roots and soil microbes. During periods without precipitation, water loss from these forest ecosystems is dominated by evapotranspiration - primarily evaporation from the soil surface and transpiration from the trees. Evapotranspiration at the landscape scale is often measured as a single flux, despite being comprised of two processes. As a result, the relative ratio of evaporation to transpiration is not always empirically known, but instead calculated based on known relationships.
Here, we present the first results of evapotranspiration partitioning at the Mt. Bigelow site in Southern Arizona. Latent energy flux at the community scale is compared to sap flow from a subset of the three dominant conifer species within the footprint of the eddy covariance tower. Additionally, leaf-level measurements of transpiration shows good agreement with the distribution of sap flow for each of the three species.
Masters Thesis - Topographic Aspect Controls on Carbon Assimilation
Capturing measurements of primary productivity is important to this project, as they provide a quantitative measure of how these species are responding to their environment. The Li-Cor LI-6400 shown here is being used to measure photosynthesis and transpiration rates for a Douglas Fir sample in Spring, 2016.
My primary interest when starting with the Barron-Gafford Lab was to investigate the impact of environmental drivers on ecosystem health and productivity. Semi-arid environments, such as those found in Arizona, are particularly interesting regions to study because vegetation has already adapted to tolerate broad variation in enironmental conditions. Changes to these drivers push these species to the edges of their tolerances. Although changing climatic conditions are important to consider, I am also curious about the influence topography has on productivity. Variation in this state factor is not often included in descriptions and models of ecosystem function, especially not small variations. The focus of my thesis project is to determine to what extent topography controls the productivity of a community.
Based on work done through the Critical Zone Observatories, it is known that the movement of energy and material through a system varies depending on aspect. Northern aspects, with less direct solar radiation and generally more moisture, support greater flux of energy and material. But, what is the relation of this to net primary productivity? To address this, we are investigating catchment-level variability in vegetation response to environmental conditions. The study site is a mixed conifer forest in the Santa Catalina Mountains north of Tucson, AZ. The topography of this site is such that native species (P. menziesii, P. ponderosa, P. strobiformis) are found on both the southern and northern aspects of a 1.5 hectare zero-order basin. This allows us to stratify samples by both aspect and species. We are taking leaf-level measurements of productivity for these individuals on a seasonal basis, as determined by temperature and water availability.
We expect to find differences in how these individuals respond to environmental conditions depending on which aspect they are found. In addition, we anticipate differences in these reactions between species in the same catchment. The results of this project will provide a better understanding of the heterogeneity in ecosystem function and response to climate change.
More background on the study site and work being done there can be found on Greg's website.
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We have been eager to share research done at this site with the public. The Daily Wildcat at the University of Arizona wrote an article about the collaborative research done in the Santa Catalina Mountains and the Critical Zone Observatory. The Arizona Daily Star also wrote an article about research done at our site in collaboration with an exhibit at the Flandrau Center. See a video produced by journalism students at the University of Arizona below:
Methane Flux Monitoring in the Arctic
While working at San Diego State University, I contributed to an NSF-funded project seeking to measure year-round trace gas fluxes from the Arctic permafrost in Northern Alaska. Our challenge was to design an eddy covariance system that would allow continuous measurement of methane release in harsh winter conditions. This poster details the setup of our towers.
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