Toward an Improved Understanding of the Role of Transpiration in Critical Zone DynamicsBhaskar Mitra1, Shirley A. Papuga1,2, Michelle L. Cavanaugh3, Joseph R. McConnell4, Nate Abramson2, Greg A. Barron-Gafford5, Erik P. Hamerlynck3, Peter Troch2, Paul Brooks2
1. School of Natural Resources and Environment, University of Arizona, Tucson, AZ 857212. Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721
3. Southwest Watershed Research Center, USDA-ARS, Tucson, AZ 857194. 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