Measurements and Instruments

TDR (Time-Domain Reflectometry) probes

Checking TDR readings from deep in the soil shaft.

Our measurements of soil moisture using Time-Domain Reflectometry are a cornerstone of our rainfall exclusion experiment. We manufacture and callibrate our own sensors, making it possible for us to install hundreds of sensors to monitor soil moisture changes both over the suface of each parcel at the sample gridpoints, and beneath the ground down to 11 meters depth in each of our ten soil shafts. The principle of operation is relatively simple. Sensors consist of three stainless-steel rods fixed in a resin disk and attached to a length of coaxial cable. An electronic device called a 'cable tester' is connected to each sensor, and emits an electronic pulse which goes down the central rod , travels through the soil and is received by the other rods; the delay in signal propagation through the soil is correlated to soil moisture content.

Soil Resistivity Profiles

Programming the Sting soil resistivity instrument

To complement our point measurements of soil moisture using TDR, we also collected semi-annual transects of deep soil moisture in both parcels using the Advanced Geosciences Inc. Sting soil resistivity meter. Steel rods are inserted into the soil and attached to electrodes at 5-meter intervals along a 255-meter cable laid to bisect each parcel; a radio signal is emitted by each electrode in turn, and received by all the other electrodes. The two-dimensional soil resistivity profile generated by this technique can be used as a surrogate for soil moisture at much deeper depths than our soil shafts can reach.

Soil resistivity profiles for the two parcels were already dramatically different in July 2002, after only two years of rainfall exclusion.

Granier sapflow monitoring system

Granier sapflow probes in place

Granier sapflow probes allow us to monitor the flow of moisture from the soil up into tree trunks in the form of sap. A Granier probe consists of two large hypodermic needles inserted into the tree 10cm apart along a vertical axis; both probes have thermocouples sealed inside, and the top probe also has a small electrical heating element, which warms the surrounding cambium layer; sapflow is measured as a function of the difference in temperature between the two probes. Higher sapflow volume dissipates the heat more quickly so the temperature differential is smaller between probes; low sapflow results in increased temperature around the heated probe. We monitor sapflow in this way in 25 trees in each parcel. The trees were carefully selected to represent important species in the forest, and to include both sub-canopy and taller emergent tree species. Where possible we have pairs of trees of the same species and similar size in each parcel, to allow us to monitor the differences in sapflow as the experimental parcel soils become drier.

Measurements of leaf respiration

We use three different methods to measure leaf respiration - a LiCor LI-6400 photosynthesis monitoring machine, a Delta-T AP-4 Porometer, and a Scholander pressure bomb. All three methods allow us to measure leaf physiological processes that relate to water use and photosynthesis. When trees undergo moisture stress, their first line of defense is to close the stomata in their leaves to prevent moisture loss. This restricts their photosynthesis rates, but it is an essential survival mechanism. At regular intervals, one or more of these techniques was used to compare leaf physiology between the experimental and control parcel.

Both the LiCor and Delta-T are automated instruments: a central processor is attached to a sampling wand with a cable and tubing; a cuvet (small chamber) is enclosed around a portion of a leaf, and the device then samples the rate of exchange of CO2, Oxygen, and water vapor under given light conditions, and provides a measurement of rates of conductance or photosynthesis. The measurement does not harm the leaf, allowing us to revisit the same leaf multiple times.

Preparing to test a leaf using the Scholander pressure bomb.

The Scholander pressure bomb is a much simpler but more destructive and labor-intensive means of detecting the status of a leaf. Leaves are collected and prepared by slicing the tip of the petiole (leaf stem) with a razor, then inserting it through a gasket in the cap of the 'bomb', which is simply a heavy metal chamber designed to withstand great pressure. When the cap is placed over the chamber the leaf is fully enclosed with only the tip of its petiole exposed, and then high pressure is applied to the chamber. When moisture begins to escape out of the tip of the petiole, the pressure is recorded. The pressure reading gives an indication of how closed the leaf stomata are in response to moisture stress.

Measurements of Microclimate

Above the tree canopy on the main tower in our experimental parcel, we monitor rain falling on our parcel, solar radiation, temperature, and humidity. At numerous other points scattered through both parcels we have small Hobo dataloggers which record temperature and relative humidity. These measurements allow us to get a sense of how different the microclimate is between the two parcels as the soil dries out.

Solar panels, Temperature/Relative Humidity, rain gauge, and solar radiation instruments above the canopy.

Monitoring the forest's response

We continuously check key indicators of the health of our forest, to determine the magnitude and nature of the impacts of our artificial drought.

Canopy closure is estimated to be one of the most important factors in maintaining forests resistant to fire, and one of our hypotheses was that canopy would thin markedly as a result of our experiment. We measure this monthly using both a Spherical densiometer and a LiCor LAI-2000 Leaf Area Index instrument, over the 144 sample grid points in each parcel.

Measuring Leaf Area Index

Hanging around, counting leaves.

We monitor how well trees are growing using 'dendrometers,' spring-loaded aluminum belts that are permanently strapped around over 1,100 trees in both parcels. These bands are made to fit each tree and have a pair of notches cut close together at the start; as the tree trunk grows in diameter, the gap between the notches grows, and is recorded.

We conduct seasonal evaluations of reproductive function and other phenology events - flowering, fruiting, generation of new leaves or new branches, as well as death. With our database of thousands of plants, we have a very large sample to compare between treatment and control parcels.

Soil trace gas emissions

Lastly, we measure changes in the release of important trace gases due to alterations in soil microbial processes provoked by the drought. Around each of the 10 soil shaft is an array of 8 plastic rings 25cm in diamter, inserted into the soil surface. Samples of the gases emitted from the soil are collected by placing collection chambers over these rings and either passing the air through instruments in-situ for analysis of NOx and CO2, or carrying syringe samples back to the laboratory for to monitor the flux of Methane from the soils. These data allow us to estimate how severe, basin-wide drought might affect global balances of these important greenhouse gases.

Setting up the gas analysis 'backpack'.

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©Woods Hole Research Center, 2007