Day Five: More Research

I'll cut right to the research that I did today.  I will attach the summaries.  I read a lot of articles today, but only deemed a few important enough to my project to necessitate a summary. Article reading is all I did today, other than monitoring the dogwood.  The phenology camera still has not come in.

A review of whole-plant water use studies in trees: Wullschleger Stan et al. 1998

The article described reviewed fifty two studies previously conducted on whole-plant water use, detailing the different techniques used in doing so.  The techniques vary from modern energy-balance and heat dissipation methods to older techniques such as weighing lysimeters, large-tree photometers, ventilated chambers, and radioisotopes.  The accuracy of water-use measurements between these techniques varies a great deal, and as such, the variation in trees studied ranged from 10 kg day^-1 to 1,180 kg day^-1, with the majority of measurements falling between 10-200 kg day^-1.  The merits and drawbacks of each category of technique was evaluated.  Weighing lysimeters are sensitive to small changes, but they are very expensive to maintain.  Large-tree photometers can often change the amount of water that the tree originally would have used.  Radioactive tracers like tritium are being phased out due to environmental and regulatory concerns.  Heat-pulse and heat-dissipation techniques are gaining widespread acceptance because of their relative inexpensiveness.  Drawbacks of the heat-pulse and heat-dissipation techniques include empirical calibration inaccuracies, and variation in flux through the depth of the sapwood, but many of these concerns are being addressed through new methods of applying the said techniques.  The article recommends the continued use of heat-pulse and heat-dissipation methods in the future to estimate whole-tree water use.

Physiological impacts: There are five main places the article elaborates on in terms of physiological impacts of measuring whole-tree water use.

Stomatal and boundary layer control of whole-tree water use:

Measuring whole-tree water use gives several benefits in evaluating the role of the stomata in controlling transpiration.  Having quantitative values allows scientists to make meaningful hypotheses about several factors.  In fact, Jarvis et al. (1986) elaborates by introducing a dimensionless decoupling coefficient (Ω) which measures the relative amount that the stomata affects the whole-tree transpiration rate.  The closer Ω is to zero, the more control the stomata has on the rate of transpiration of the whole tree.  Conversely, the closer Ω is to one, the less control the stomata has on the rate of transpiration.  For a tree with large leaves (Tectona grandis has a leaf size of 700 cm²), Ω approaches one and the stomata conductance is so high that it cannot influence the rate of transpiration in the tree.

Whole-tree estimates of hydraulic conductance:

Hydraulic conductance is the measure of water flow from the soil to the trunk via the roots of the tree.  With the advent of easily measurable whole-tree water use, hydraulic conductance is measurable simply by a conversion described by , where  is hydraulic conductance,  is an estimate of current transpiration, and  is the difference between maximum and current leaf water potential.  Estimating hydraulic conductance is important physiologically because it allows researchers to evaluate the benefits of certain designs of tree growth.  For example, scientists can assess the trade-offs between  and vulnerability to xylem cavitation – something that would be controlled through evolution.

Coordinated control of stomatal and hydraulic conductance:

Quantitative measurements of whole-tree water use can help scientists assess the correlation between stomatal and hydraulic conductance in trees.  There is substancial evidence that stomatal and hydralulic conductance are positively correlated – when  increases, stomatal conductance and leaf transpiration also sharply increase.  This shows that stomata respond quickly and sharply to changes in water transport efficiency.

Sapwood water storage:

In large trees such as oak, there is a considerable lag between changes in transpiration and changes in water flux at the base of the tree.  This lag shows that there is a significant storage of water inside the tree.  By measuring at both ends of the tree, it is possible to quantitatively determine the quantity of stored water inside the tree.  This measurement can help scientists measure the drought-resistance of certain trees. 

Granier’s Thermal Dissipation Probe (TDP) Method for Measuring Sap Flow in Trees: Theory and Practice: Ping Lu, Laurent Urban, Ping Zhao, et al.

This article thoroughly reviews the thermal dissipation method first described in Granier 1987.  This method is one of the most popular ones to determine whole-tree water use in modern day.  This is because of the Granier method’s simplicity, high degree of accuracy, and relatively low cost.  A comprehensive review of the technique will help refine future methods of measurement using the TDP method.  Several practical issues such as the determination of non-flow values, natural gradient, wounding effect, reversed sap flow, and flux scaling are discussed.

The original Granier method was published by Andre Granier in French.  Therefore, there have been many misunderstandings on the technique and misuse of the methods.  The purpose of the article is to clarify many of these misunderstandings and give a clear overview of the technique, including how to scale measurements from a point to cross section, and then to the tree level.

The theory behind the technique is as follows: the equilibrium of the system will be changed as the sap flux rate changes.  This changes the convection between the two sensors and changes the difference in voltage between the two.  Practically, the system consists of a pair of sensors embedded in the trunk of a tree.  One probe is heated while one is not.  The heated probe is embedded 10-15cm above the reference probe.  The reference (unheated) probe has two wires inside creating a thermocouple.  The two wires are made out of copper and constantan.  The heater probe has four wires consisting of a thermocouple and a heater.  The thermocouple is a copper-constantan pair, while the heater is a constantan-constantan pair.  The two thermocouples are joined together through the constantan wire, and linked to the data logger through the copper wires.  The two heater wires are routed to a 12v power supply where they will receive a constant 0.2 watts of power.  Now the circuit is complete, and by measuring the difference in voltage between the two probes, one can see the sap flux rate.  The conversion formula is .  The length and diameter of the probe is important in this empirical formula – any changes to the shape of the probe will most likely change the formula.  However, Granier calculated that a change in the tree should not change the formula that is applied.  Also, unlike the heat-pulse method, there is no real harm in varying the distance between the heater and reference probe, as long as the reference probe is not affected by the heat propagated by the heater probe.  An optimal distance, as shown in literature, is between 10-15 cm.

Determination of :

The parameter  is defined as the maximum difference in temperature between the two probes when the rate of sap flux is zero.  In the past, scientists assumed that the value of sap flux at night was zero, and simply calibrated the value of  each and every night.  We now know through different studies that the sap flux at night time is not zero.  This poses a problem – what is the true value of  every night?  The answer to this problem is not incredibly simple, but it is possible to estimate the true value of  at any given time.  Determining nights when the sap flux rates will be closest to zero can be done by looking at different variable such as Vapor Pressure Deficit (VPD) and Photosynthetically Active Radiation (PAR).  When a baseline is determined, scientists can then estimate the value of  through a linear regression.  Granier proposed in his 1987 paper to calculate the local maxima of  over a 10 day period, and then average the values to calculate a value – this has been used with some limited success.  Linear regression is the preferred method to calculate this.

Calculation of whole-tree sap flow from a point-value:

Scaling is a known issue with the Granier Thermal Dissipation Probe method.  Measurements by the probes can only determine the exact rate of flux at a specific location.  Scaling to the cross sectional level, the tree level, and the stand level relies on several calculations.  The current method to determine this is to find the centroid of the sap flux curve from the outer edge of the xylem to the sapwood, and then integrate this over the entire area of the cross section of the tree.  This can be further integrated to obtain the complete water use of the tree.  Additionally, one can take the curve of the sap flow for different depths all the way to the heartwood, and use calculus methods to rotate this curve around the center axis of the tree to get a rotated solid that will have the total calculated volume of water flow.