Thursday, August 9, 2007: 1:50 PM
Almaden Blrm II, San Jose Hilton
Episodes of drought-induced tree mortality are a recurrent phenomenon in many forested landscapes. The mechanistic causes of these events remain unknown but are probably related to the interaction of plant hydraulic regulation and carbon gain to cope with scarce inputs of rain and insect attack. We are conducting a rainfall manipulation experiment in Piñon-Juniper woodlands at the Sevilleta LTER in central New Mexico , USA . The goal of the study is to detail the hydraulic architecture of both species to predict plant allocation patterns and assess tree-level water transport capacity and/or failure during drought and after periods of augmented rain. Measurements of sap flow, soil pyschrometry, plant water potential, hydraulic conductance, microclimate and stable isotopes of carbon and water during the summer-fall season of 2006 (pre-treatment) reveal important structural and functional differences between P. edulis and J. monosperma. Following precipitation after an extremely dry spring, transpiration (E) was higher in J. monosperma than in P. edulis, but showed a consistent decaying trend as the wet season progressed. Transpiration in P. edulis was not responsive to soil moisture recharge early in the season suggesting a lag on xylem embolism repair after pronounced drought. Trends in E were correlated to the progressive variation in soil and plant water potential, the differences in hydraulic conductance between both species and the percentage use of precipitation vs. deep stored water in the transpiration stream. Because transpiration cannot exceed hydraulic limits of water uptake and transport imposed by soil hydrological characteristics, xylem properties of steams and roots, and the ratio of absorbing root area to transpiring leaf area (AR:AL), understanding the interaction among these factors lends support for predictions about the likelihood of plant death or survival during drought.
