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