Disturbance can influence aspects of forest microclimate other than light.
Characterization of such changes can be of value in interpreting the potential impact of disturbance not only on the growth and survival of tree species, but on
the availability of habitat for other species. In montane forests, for example, disturbance to the forest canopy may reduce air humidity in the understorey, which can have negative impacts on epiphytic mosses and lichens. The techniques and instrumentation required for measurements of microclimate in the field are described by Coombs et al. (1985), Hall et al. (1992), and Pearcy et al. (1990), on which this brief summary is based.
Temperaturecan be readily measured by using a mercury-in-glass thermometer, as used in meteorological stations (Figure 4.9). If recordings are required, either thermocouplesorthermistorsare generally used. Thermocouples are relatively cheap and easy to construct, typically out of copper–constantan or chromel–alumel, taking care to ensure a good junction between the two metals by soldering with either tin or silver. After soldering, the junctions should be cut with a blade under a binocular microscope to ensure that they are as small as possible (Coombs et al.
1985). All thermocouples should be individually calibrated, for example by immersing them in a water bath whose temperature can be controlled. Thermistors are semiconductors, composed of sintered mixtures of metallic oxides, and as for thermocouples, can either be constructed or purchased from a commercial supplier.
Although more expensive than thermocouples, they are relatively robust. When measuring air temperature, the instrument should be shaded by a suitable screen.
Surface temperatures (of leaves, for example) require contact between the sensor and the surface being measured, often achieved by using clips, springs, or tapes.
Humiditycan be measured with a variety of different instruments, including psychrometers, dewpoint meters and electronic capacitance and resistance sensors.
Although the latter are most convenient, they require frequent calibration.
Psychrometersconsist of a pair of thermometers, one of which is covered by a wet sleeve. Evaporation from the wetted sensor cools the thermometer, enabling vapour pressure to be calculated. Small ventilated psychrometers are available for use within or above plant canopies; for example, Delta-T devices 具www.delta-t.co.uk典.
Available electronic sensors include capacitance hygrometers, which measure the change in electrical capacitance caused by water-absorption into a dielectric.
Infrared gas analysers can also be used to measure water vapour concentration;
although very expensive, these instruments are accurate and respond quickly to environmental changes. As for air temperature, instruments for measuring air humidity should be shaded when used in the field, and whichever instrument is used, it should be carefully calibrated.
Soil moisturecan be described in two ways: in terms of the quantity of water present or in terms of the energy status of the water. The gravimetric water content is the mass of water in unit mass of dry soil (kg water/kg soil). Typically the wet mass of the soil sample is determined, then the sample is dried at 100–110 C to constant mass and reweighed. The volumetric water contentis expressed in terms of the volume of water per volume of soil (litres of water per litre of soil). This can be measured by multiplying the gravimetric water content by the soil bulk density (kg of dry soil/litre of soil). The energy status of water in soil can be expressed as the total soil water potential (MPa), which can be divided into the matric, solute and pressure potentials (Rundel and Jarrell 1989). Instruments used for measurement Other aspects of microclimate | 179
of soil moisture include tensiometers,gypsum resistance blocks,soil psychrometers, and neutron probes. Of these, resistance blocks and tensiometers are the cheapest and easiest to use, the former being more suitable for drier soils and the latter for wet- ter soils (Rundel and Jarrell 1989).
Measurements of microclimate made over time can be recorded by using a data logger. A variety of different models are available from commercial suppliers such as Delta-T Devices, 具www.delta-t.co.uk典; Campbell Scientific,具www.campbellsci.com典;
or LI-COR, 具www.licor.com典. Data loggers can be programmed to collect and store information at a variety of different intervals, and for different lengths of time.
Those designed for field use are available with weatherproof cabinets. Key features that differ between models include the number of inputs to which sensors can be attached, memory storage capacity, and battery life. Such data loggers are also Fig. 4.9 A micrometerological station for measuring light availability (using a PAR sensor) and wind speed (using an anemometer), to which a data-logger has been attached to record the measurements made. The instrument has been positioned in an experimental gap created at Harvard Forest, USA. (Photo by John Healey.)
used as the basis of automatic weather stations, which can be used to provide detailed measurements of meteorological variables such as wind speed, wind direc- tion, and rainfall (Figure 4.9). Other sensors that can be attached to data loggers that are used in plant ecology include those used for measuring surface wetness and soil moisture content, as well as the PAR sensors described above.