Frederick Meinzer, Katherine McCulloh
Duke University, USDA Forest Service, University of Wisconsin
Hydraulic capacitance (C) is the amount of water that is released per volume, of wood in this case, per change in water potential. Other associated, and related, measurements include water storage capacity of a tissue or organ and relative water content (RWC)-based measures of C. Although both RWC and water mass- based expressions of C are equally valid, we most frequently use mass-based expressions (kg H2O m-3 MPa-1) so that is what we will refer to here. Capacitance and related terms can be measured in several different ways (e.g., sap flow, dendrometer bands), but the most common method is the psychrometric method, which measures intrinsic C (specific to the tissue being measured) and is described here.
Hydraulic capacitance buffers fluctuations in stem water potentials with changing transpiration. This buffering effect can reduce the likelihood that the plant will experience hydraulic failure during typical diurnal cycles. Within wood, sources of capacitive water can be intercellular spaces, xylem parenchyma or even embolizing conduits. Values of capacitance range from < 100 to >700 kg m-3 MPa-1 and are positively correlated with tree size, but negatively correlated with embolism resistance and wood density.
Screen-cage psychrometers can be obtained from Merrill (part number: 83-3VC) or through Wescor (part number: C30). The psychrometers will need to be attached to a device capable of providing cooling current and measuring mV across the psychrometer. Wescor has a dedicated psychrometer reader (Psypro), although some researchers use a Campbell datalogger (CR7) with an attached psychrometer module. Additionally, various salt solutions are needed for calibration and a microbalance is necessary for weighing tissue samples.
Units, terms, definitions
1. Small pieces (~1 x 1 cm) of wood are used to construct sapwood moisture release curves for calculating capacitance. For these measurements, the samples are vacuum-infiltrated overnight in deionized water. The saturated samples are then blotted on a paper towel to remove excess water, weighed and placed in screen cage thermocouple psychrometer chambers.
2. The chambers are then double-bagged and submerged in a cooler of water for 2-3 hours to allow the sample to equilibrate with the chamber air. After the equilibration period, the millivolt readings are recorded using a digital psychrometer reader or datalogger. Following the measurement, the samples are removed from the chambers, weighed, and allowed to dry on the laboratory bench for approximately half an hour before repeating the process (except for the saturation step).
3. Cooling current (typically 5 ma for 15 seconds) is passed through the psychrometer thermocouple junction and the mV output is recorded by the psychrometer reader or datalogger. This value is converted to MPa based on calibration curves from salt solutions of known water potentials (see below, under notes). Samples are measured repeatedly until water potential values reach ~ -4 MPa or lower depending on the species and wood properties. Samples are then placed in the drying oven overnight before weighing the dry mass.
4. For each of the repeated measurements, the relative water content (RWC) is calculated as:
where Mf is the sample mass for the measurement, Md is the dry mass and Ms is the saturated mass of the sample. From RWC, relative water deficit (RWD) is calculated as 1 – RWC. The product of RWD and the mass of water per unit tissue volume at saturation (Mw) yields the cumulative mass of the water lost at each measurement. Mw is calculated as
where is wood density.
5. Then by graphing moisture release curves, which compare the cumulative mass of water lost versus the sapwood water potential, the capacitance of the sample can be calculated by fitting an asymptotic curve to the data. The capacitance is the slope of the curve, which is continuously changing. If an estimate of capacitance over the normal physiological operating range of stem water potential in situ is desired, a regression can be fitted to the initial, nearly linear, phase of the plot. The inflection point at which the moisture release curve begins to deviate from this line often corresponds to the daily minimum stem water potential when soil water availability is adequate. The slope of this line is an estimate of the average maximum capacitance of the sample (in kg m-3 MPa-1). It can also be helpful to plot the data as -1/sapwood water potential versus RWD to help identify the inflection point at which capacitance diminishes rapidly. In this case, the linear portion of the curve corresponds to the asymptotic portion of the moisture release curve.
Notes and troubleshooting tips
Each psychrometer must be calibrated against salt solutions of known concentration. A piece of filter paper fitted inside the chamber (around the circumference) is wetted with 40μl of salt solution. Solutions of 0.1, 0.3 0.5 and 0.7 M KCl are useful because they correspond to -0.5, -1.3, -2.2 and -3.1 MPa, respectively (at 25 C)
Some of the psychrometric parameters may need to be optimized: cooling current, cooling time, and any potential delays between cooling and reading time may require adjustment. A reader/logger with an output of mV/time can be useful in determining the best parameters. These parameters should be adjusted so that the mV value is read when it is at its maximum on the mv/time plot.
Psychrometers will have a large degree of variability at water potentials less negative than -0.5 MPa.
Dirty psychrometers will give erroneous readings. It is important to make sure that samples do not touch the actual psychrometer (or they could contaminate it) and that the psychrometer chambers are cleaned with deionized water followed by a residue-free electronics cleaning solvent.
Links to resources and suppliers
Brown RW. 1970. Measurement of water potential with thermocouple psychrometers. USDA Forest Service Research Paper.
Meinzer FC, James SA, Goldstein G, Woodruff DR. 2003. Whole-tree water transport scales with sapwood capacitance in tropical forest canopy trees. Plant, Cell and Environment 26: 1147-1155.
Meinzer FC, Woodruff DR, Domec J-C, Goldstein G, Campanello PI, Gatti MG, Villalobos-Vega R. 2008. Coordination of leaf and stem water transport properties in tropical trees. Oecologia 156: 31-41.
Scholz FG, Phillips NG, Bucci SJ, Meinzer FC, Goldstein G. 2011. Hydraulic capacitance: Biophysics and Functional Significance of Internal Water Sources in Relation to Tree Size. In Size- and Age-Related Changes in Tree Structure and Function. Meinzer FC, Lachenbruch B and Dawson TE eds. Springer.
Richards AE, Wright IJ, Lenz TI, Zanne AE. 2014. Sapwood capacitance is greater in evergreen sclerophyll species growing in high compared to low-rainfall environments. Functional Ecology (in press).