Woody plant encroachment into karst terrain: implications for regional cycles of carbon, water and energy

James L. Heilman, Texas A&M University

Co-investigators: Marcy E. Litvak, University of New Mexico
Kevin J. McInnes, Texas A&M University
M. Keith Owens, Texas Agricultural Experiment Station

Abstract

Broad Project Objectives: The overall goal is to develop an understanding of how carbon, water and energy exchange in semi-arid savannas on karst terrain respond to rainfall and vegetation change. Specifically, we wish to determine if woody encroachment into rain-limited savannas on the Edwards Plateau, TX, increases carbon sequestration and water use, and reduces sensitivity of carbon, water and energy exchange to rainfall and drought, as is widely presumed. An underlying assumption for increased sequestration and reduced sensitivity to rainfall and drought is that deep root systems provide a more stable supply of water to woody plants than what is available to grasses. Data from our three AmeriFlux sites on the karst Plateau challenge that assumption.

Hypotheses to be Tested: 1) Net ecosystem exchange of carbon, and productivity are highest in Plateau savannas having intermediate densities of woody cover; 2) Species specific carbon uptake and transpiration responses to rainfall and water deficits on the karst Plateau are independent of the physical characteristics of the root zone profile; and 3) Encroachment by Ashe juniper into Plateau savannas and grasslands reduces sensitivity of carbon sequestration to rainfall and drought over the long term.

Location of Research: Research will be conducted on the Freeman Ranch near San Marcos, Texas.

Methods: Ecosystem-scale fluxes of carbon, water, and energy in response to rainfall and water deficits will be measured by eddy covariance at seven sites (three AmeriFlux sites, four satellite sites) having distinct differences in woody cover and physical characteristics of the root zone. A portable eddy covariance system will be used at the satellite sites, and these measurements will show how carbon, water and energy exchange deviate from those at the AmeriFlux sites as a result of differences in ecosystem structure. Sap flux and gas exchange (leaf-level and chamber) measurements will be used to evaluate responses of woody plants and grassland components of the ecosystems to rainfall and water deficits. Measurements of water content profiles will help answer the question of whether deep roots provide sufficient water to maintain photosynthesis and transpiration during drought, thereby reducing sensitivity to rainfall and water deficits.

Accomplishments/Deliverables: The research will provide quantitative information on carbon, water and energy exchange in response to rainfall and water deficits at seven sites typifying the diversity of ecosystems on the Edwards Plateau, and on how ecosystem structure and rainfall contribute to spatial and temporal variability of carbon, water and energy cycles.