Toward an Improved Understanding of the Role of Transpiration in Critical Zone Dynamics Bhaskar Mitra 1 , Shirley A. Papuga 1,2 , Michelle L. Cavanaugh 3 , Joseph R. McConnell 4 , Nate Abramson 2 , Greg A. Barron-Gafford 5 , Erik P. Hamerlynck 3 , Peter Troch 2 , Paul Brooks 2 1. School of Natural Resources and Environment, University of Arizona, Tucson, AZ 85721 2. Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721 3. Southwest Watershed Research Center, USDA-ARS, Tucson, AZ 85719 4. Division of Hydrologic Sciences, Desert Research Institute, Reno NV 89512 5. B2 Earthscience, University of Arizona, Tucson, Arizona, USA Background Trees reflect upon the health of the forest . An improved understanding of how tree transpiration varies across the critical zone (CZ) will help to elucidate how the vertical water flux modulates the soil moisture balance which in turn influences the subsurface biogeochemical and landscape evolution process across the CZ. Within the subalpine mixed conifer ecosystems in the Jemez River Basin – Santa Catalina Mountains Critical Zone Observatory (JRB-SCM CZO) (Figures 1A and 1B), we propose to address the following critical questions: 1. What is the role of slope, altitude, forest stand dynamics and plant-physiology in the magnitude and seasonal dynamics of transpiration at these different sites? 2. Can we identify a primary driver or drivers of transpiration across these sites? 3. Can we determine the primary water source used for plant transpiration, and how this varies within the CZO? Biometrics Proposed to be Measured Leaf Area Index (LAI) Diameter at Breast Height (DBH) Tree Age Sapwood Area Tree Height Crown Diameter Figure 1A: Santa Catalina Mountains Critical Zone Observatory Figure 5: Soil moisture and sap velocity time series In 2005 from the Jemez Mixed Conifer site (Small and McConnell, 2008) Figure 10: Jemez Mixed Conifer site (Photo by X.Zapata-Rios) Reference Small, E.E., McConnell, J.R., 2008. Comparison of soil moisture and meteorological controls on pine and spruce transpiration. Ecohydrology 1:205-214. Acknowledgement This project is funded by the NSF National Critical Zone Observatories Program Figure 2: Marshall Gullch Schist (Photo by R.Ruiz ) Table 2: Biometrics proposed to be measured Question 2 Objective - Identification of the primary environmental driver/(s) will reduce the uncertainty in our estimation of the total water budget across different sites at the CZO. Based on our preliminary understanding, we have generated the following hypothesis: 1.Soil moisture is the primary driver of transpiration across all the five CZO sites Figure 7: Time series of sap velocity from Mount Bigelow Mixed Conifer site Figure 6: Mount Bigelow Mixed Conifer Site Figure 8: Jemez Mixed Order Conifer Zero Basin, (Photo by X. Zapata-Rios) Figure 9: Time series of sap velocity and precipitation event at the Jemez Mixed Conifer Zero Order Basin Question 3 Objective: Drawing a link between position of tree along the slope, soil texture and solar radiation vis-à-vis water uptake by plants as shown in Figure 4, we have generated the following hypotheses: 1.Vegetation on slopes exposed to higher solar radiation will need to access deeper soil moisture compared to vegetation on slopes exposed to less solar radiation. 2. Vegetation at granite site will have access to deeper soil moisture compared to trees at the schist site due to the fact that coarse soil texture at the granite site allows the roots to have greater access to deep soil moisture compared to roots at the schist site. Figure 4: Marshall Gulch Schist site (Photo by – R.Ruiz) Figure 1B: Jemez River Basin Critical Zone Observatory Hypotheses: 1.Trees which are positioned downhill will have a higher transpiration rate compared to trees positioned uphill. 2.Interspecies difference in transpiration rate across the five CZO sites will be linked to sap size and will be independent of species specific attributes. Preliminary Results •Soil moisture accounted for the variation in sap velocity for Spruce at the Jemez Mixed Conifer site (Figure 6) for the time period following snowmelt and during the early part of summer but was unable to fully explain the variation during the latter half of the summer period as shown in Figure 5 (Small and McConnell, 2008) . • Time series of sap flux (Figure 7) shows that species density in Mount Bigelow Mixed Conifer (Figure 8) does appear to influence the vertical water flux. • Timing of spring precipitation (Figure 9) in Jemez Mixed Conifer Zero Order Basin (Figure 10) does appear to influence the vertical water flux. Table 1: A brief overview of the five different sites where sap flux sensors have been installed or proposed to be installed Question 1:Figure 2 highlights the CZ across the Marshall Gulch schist site while the conceptual diagram of the system is shown in Figure 3. The hypotheses generated from the conceptual diagram is shown below and some of the biometrics proposed to be measured is shown in Table 2 . WEST SLOPE EAST SLOPE Figure 3: Conceptual diagram of the CZ shown in figure 2