Net Carbon Dioxide Exchange in Live Oak-Ashe Juniper Savanna and C4 Grassland Ecosystems on the Edwards Plateau, Texas: Effects of Seasonal and Interannual Changes in Climate and Phenology

P.I./Institution: James Heilman, Texas Agricultural Experiment Station

Co-P.I.(s)/Institutions: Kevin J. McInnes, M. Keith Owens, Texas Agricultural Experiment Station

Summarize in two to four sentences what activities will be (are being) carried out in the project: Net CO2 exchange (NEE) and its components will be quantified for an oak-juniper savanna and a grassland on the Edwards Plateau using a combination of tower-based eddy covariance measurements, leaf-level measurements, and soil measurements. Surface energy balances and water vapor fluxes will be measured in both ecosystems, along with microclimate and soil water content. Functional relationships will be developed to determine how environmental factors affect NEE, and to determine how contributions of dominant plant species to NEE change with climate and phenology.

Project Abstract: The Edwards Plateau is a large (93,000 km2) environmentally sensitive biome in Texas that is dominated by live oak-Ashe juniper savannas and C4 grasslands growing on relatively shallow soils. Grasslands on the Plateau are disappearing due to invasion by Ashe juniper. The Plateau is home to a number of endemic and endangered plant and animal species, and is the recharge zone for the Edwards Aquifer that supplies water for a number of cities including San Antonio. In spite of the size and importance of the Plateau, CO2 exchange, carbon storage, structure and functioning of its ecosystems are poorly understood. We intend to conduct long-term comparative studies to determine how CO2 exchange in oak-juniper savanna and grassland ecosystems is affected by changes in climate and phenology, and to determine how encroachment by Ashe juniper alters CO2 flux. Root systems of live oak and juniper penetrate cracks in the limestone bedrock of the Plateau, giving them access to water stored in cracks and caves, whereas roots of grasses are restricted to the shallow soils. Thus, net CO2 exchange in oak-juniper savannas should be higher and less sensitive to rainfall than in grasslands, giving oak-juniper savannas a competitive advantage. Objectives are: 1. Quantify fluxes of carbon dioxide, water vapor and energy fluxes in live oak-juniper and grassland ecosystems on the Edwards Plateau in response to changes in climate and phenology of dominant plant species; 2. Quantify leaf photosynthesis, respiration, and stomatal conductance of dominant plant species in response to changes in climate and phenology; 3. Quantify soil respiration in the two ecosystems; and 4. Determine how biophysical mechanisms controlling CO2 flux vary with climate and phenology of dominant plant species. This study is designed to provide fundamental information on CO2 exchange, carbon storage, and ecosystem response to environmental change.

Summary, in "layman's terms," of what will be (is being) done and why: The Texas “Hill Country” is home to a number of endangered plants and animals, and it is the recharge zone for the Edwards Aquifer that supplies water to many cities, including San Antonio. Agriculture in the Hill Country depends on pastures for grazing, and on hunting leases to provide additional income. Grasslands are disappearing because of invasion by juniper, and this invasion may have both positive and negative impacts. One positive consequence of encroachment by juniper may be increased consumption of atmospheric carbon dioxide, which may help to moderate global warming. The research is designed to measure how much carbon dioxide is being stored by live oak-juniper woodlands and grasslands in the Hill Country, and to determine how carbon dioxide storage in woodlands and grasslands changes with climate and soil moisture.