Plants Microclimate.pdf

CHAPTER 3 PLANT MICROCLIMATE M.B. JONES 3.1 General introduction effect of one cannot be known without specifying...

CHAPTER 3 PLANT MICROCLIMATE M.B. JONES 3.1 General introduction effect of one cannot be known without specifying the state of the others. The productivity of plants is ultimately Microclimate is the complex of environmental dependent upon the influence of the microclimate variables, including temperature, radiation, on plant processes such as photosynthesis, humidity and wind, to which the plant is exposed. respiration, transpiration and translocation. In It is the climate near the surface of the earth and it order to understand how plant processes respond is different from the weather forecasters' to the microclimate we need to be able to measure macroclimate or local climate because of the the various components of the microclimate in the influence of the earth's surface and, most natural environment. In recent years a whole importantly, the presence of vegetation. Plants are range of micrometeorological instruments has "coupled" to their microclimate because a change been designed for this purpose2,3'4, and in the in one brings about a change in the other, and this following sections some of these will be described results from an exchange of force, momentum, along with the principles upon which the energy or mass1. Two important types of coupling measurements are based. The use of these are (i) radiative coupling, where energy is instruments, and the analysis and interpretation of transferred through electromagnetic vibration; data collected with them, requires some and (ii) diffusive coupling, where heat, water understanding of environmental physics. vapour and C 0 2 are exchanged across the Recommended text books on this subject include boundary layer of the plant. The radiant flux those by Monteith5, Campbell6, Woodward and incident on a plant is coupled to the temperature Sheeny7 and Jones 8. Details of the characteristics of the plant by its absorptivity. If leaf absorptivity of these instruments are listed in an Appendix to is high then the leaf temperature is tightly coupled this book. to incident radiation, and vice versa. Diffusive coupling across the boundary layer can be viewed as an analogue of an electrical circuit where energy 3.2 Radiation - solar and long wave in the form of a charge moves from a high to a low "potential" (measured as a voltage) at a rate (the 3.2.1 Introduction current) which is inversely proportional to the resistance (Figure 3.1). Radiative and diffusive The ultimate source of energy for components of the microclimate are themselves photosynthesis and bioproductivity is solar coupled so that, for instance, energy from energy. Plants intercept solar energy for electromagnetic radiation can be consumed in photosynthesis but normally less than 5% is used evaporating water in transpiration. Consequently, in this process; the rest of this energy heats the radiation, air temperature, wind and humidity all plant and surrounding organisms, so that solar interact simultaneously with the plant, so the energy also determines the temperature at which 26  PLANT MICROCLIMATE 27 H20 Boundary lay« Epidermis Mesophyll Fig.3.1. Diffusive coupling at the leaf surface, showing resistances to gas and heat exchange at the surface of a single leaf. ra is the boundary layer, rs the stomatal, rc the cuticular and rm the mesophyll or residual resistance. physiological processes are functioning. Apart radiant fluxes are the units of power (W m - 2 ) from photosynthesis, solar radiation also where the term irradiance (I) refers to the energy influences the plant's growth and development in flux incident on unit surface area. Irradiance is the what are referred to as photomorphogenic and correct radiometrie term for what is commonly phototropic responses. These normally require called "light intensity". Strictly speaking, "light" only very small amounts of energy to bring about is that part of radiation which is visible to a response, and different discrete parts of the humans, so it is not a very appropriate term to use radiation spectrum are involved. in plant research. Radiant energy can be described About 98% of the radiation emitted by the sun either as waves or as discrete packets of energy is in the waveband from 0.3 to 3.0 μΐτι. The energy called photons. When dealing with photochemical spectrum of this radiation before it reaches the processes such as photosynthesis, the number of earth's atmosphere peaks at 0.48 μιτι, which is photons incident in unit time is more relevant than consistent with a radiator or emitter with a the energy content of the radiation. This is known temperature of 6000°K (Fig. 3.2). The flux of as the quantum (or photon) flux density (Q) and is radiation (φ) follows the Stefan-Boltzmann Law, measured in units of mol m - 2 s - 1 where a mole is being proportional to the fourth power of the Avogadro's number (6.022 x 1023) of quanta or absolute temperature of the object: photons. When measuring rates of photosynthesis, it is most appropriate to express φ = σΤ4 the radiation incident on the plane of the leaf in terms of photon flux density. (See Appendix D). where o is the Stefan-Boltzmann constant (5.6 x At the earth's surface solar radiation can be 10"8 W m~2 K"4) and T is in Kelvin. The units for divided into two components based on whether 28 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS the radiation comes directly from the sun (direct) photosynthetically active radiation (PAR). Light or whether it is scattered or reflected by the quanta (photons) within this waveband are almost atmosphere and clouds (diffuse). Diffuse equally effective in driving the light reactions of radiation has a different spectral composition photosynthesis9. The proportion of PAR in total from direct radiation because shorter wavelengths (direct + diffuse) radiation is about 50%; this are scattered by air molecules more than long varies little diurnally or seasonally. ones, giving the blue colour to clear skies. Plants and any other surface on the earth also However, larger particles such as dust and water emit radiation due to the heating of the sun. droplets scatter all wavelengths equally, so the sky According to Wien's displacement law the appears white when cloud-covered. The amount wavelength at which the maximum amount of of diffuse radiation varies with sun angle and radiation is emitted (Am) decreases as the cloud cover, but even on clear days it contributes temperature of the body increases: 10 - 30% of total solar irradiance. The component of solar radiation used in photosynthesis falls 2897 Am — between 400 and 700 nm and is referred to as 2000 1000 0.2 0.5 1.0 2.0 5.0 10 20 30 100 Wavelength ( ^ m ) S e n s o r ;.Shortwave 4 solar Solarimeter long-wave. Near infrared terrestial Filtered solarimeter I 1 PAR Silicon celli(quantum sensor) Total radiation1 -· Net radiometer Fig.3.2. The spectral energy distribution of (i) the solar flux outside the Earth's atmosphere, (ii) the solar flux at ground level after attenuation by gases in the atmosphere, and (iii) the flux of radiation emitted by the Earth's surface (terrestrial flux). Depicted at the bottom of the figure are the typical ranges of instruments used to measure components of the solar and terrestrial fluxes (adapted from Grace10).  PLANT MICROCLIMATE 29 /////////7777777777//>/?/^ Fig.3.3. An illustration of the short wave (φ5) and long wave (ψ,) radiant energy fluxes between a leaf and its surroundings. where λ is in micrometres and T is in Kelvin. a series of alternate junctions between two Consequently, bodies on the earth's surface emit dissimilar metals, e.g. copper and constantan (see long wave radiation with a peak at approximately Figure 3.4a). When a temperature difference exists 9.7 μπι (Figure 3.2). There are therefore between two sets of thermocouples a voltage is continuous fluxes of radiation from the sun generated which is proportional to the during the day and between the atmosphere, temperature difference. When measuring solar plants and their surroundings at all times (Figure radiation the temperature difference is created by 3.3). Radiation incident on a leaf or plant canopy embedding one set of junctions in a metal clamp can be absorbed, transmitted or reflected. In the protected from incident radiation and the other in PAR region of the spectrum the leaf absorbs 90% a surface exposed to radiation, or by painting the of the incident radiation, whilst in the short-wave hot and cold junctions black and white infra-red region (0.7 - 3.0 μπι) it transmits most of respectively and subjecting them both to the same the radiation. The effect of this is to reduce the radiant energy flux. The surface of the sensor is heat load from wavelengths which are not used in normally protected from wind and rain by glass photosynthesis. However, in the far infra-red, domes whose transmittance restricts spectral leaves are good absorbers; thus (because good sensitivity to the 0.3 to 3.0 μπι region. An example absorbers are also good emitters of radiation) they is the Kipp solarimeter (or pyranometer) using a are able to dissipate excess heat very efficiently in Moll thermopile, which is the standard instrument the long-wave region of the spectrum. in many countries for measuring total (direct + diffuse) and diffuse (using a shade ring) solar radiation. Details of other solarimeters can be 3.2.2 Radiation measurements found in Monteith 3 , Szeich2 and Fritschen and Gay4. When solarimeters are used with special Most instruments used for measuring solar and filters (e.g. Kodak "Wratten" 88), they exclude long wave radiation consist of different forms of visible wavelengths, so the energy in the region thermopile arrangement. A thermopile consists of 0.3-0.75 μηι can be determined by difference. 30 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS Long wave radiation is usually measured using obtained by moving a small sensor repeatedly net radiometers which measure the difference along a track or by using a long linear sensor (tube between the total incoming and outgoing radiation solarimeters and radiometers). These linear fluxes at all wavelengths (net)· When net is sensors are less accurate than flat plate sensçrs measured above a canopy, its value is the net because of greater cosine errors (see below), and radiation absorbed by the canopy. However, the should be used for relative rather than absolute net radiation absorbed by a layer of leaves in the measurements. Errors can be minimised by taking canopy is the difference between net above and measurements in two directions at right angles. In below this layer. The main component of a net addition to the fluxes of energy through the radiometer is a flat black plate: the temperature atmosphere, there is also a vertical transfer of difference between the top and bottom surfaces, energy through the soil which is known as the soil- measured with a thermopile, is proportional to net heat-flux. During daylight hours, the soil irradiance. The two sensing surfaces are protected normally acts as a heat sink, so the soil-heat-flux is from the wind either by continuous ventilation or positive; at night, it becomes negative and of by inflatable domes of polythene (which is similar absolute magnitude to daylight values. It transparent at all wavelengths). may range from 2% of net for dense canopies to For measurements within plant canopies, where more than 30% of net in open canopies. The radiation distribution is uneven, averages can be vertical transfer of heat by conduction through the Table 3.1. Some Radiation Terms Term Symbol Meaning Units Radiation or Radiant — Energy transferred through space in the form of joule (J) energy electromagnetic waves or quanta Radiant Flux — The amount of radiant energy received, emitted or J s 1 or transmitted per unit time watts (W) Radiant flux density φ The radiant flux through unit area of a plane surface Wm~2 Irradiance I The energy flux incident on unit area of a plane surface Wm~2 Photon — A quantum of light A mole (mol) is 6.022 x IO23 quanta or photons — Quantum flux density Q The number of quanta incident on unit area of a plane mol m 2 s ' surface Photosynthetically PAR Radiation within the band 400-700 nm mol m -2 s"1 Active Radiation or Wm"2 Short wave radiation H rays (a), the slar zenith angle and the solar irradiance on the Earth's surface (I), where where ρ is the density and cp the specific heat of I0is the irradiance on a surface normal to the air, esTi is the saturated vapour pressure of air at sun's rays. the leaf temperature (T^, e is the water vapour pressure of free air, y is the psychrometric constant (66 Pa °C _ 1 ) , and rs, ra>H2o and rai H are are used to achieve good cosine response, but a stomatal and boundary layer resistances , common method is to use a raised white perspex (reciprocals of conductance). However, a more diffuser on top of the sensor. convenient expression can be derived from this Most radiation sensors give a millivolt output equation; this calculates leaf-air temperature and they can be connected directly to recorders, difference from the sum of terms that depend on data loggers or integrators. If connected to net radiation and the vapour pressure of air8. integrators they can give a total of received Plant temperature is therefore determined by radiation during a given period of time. This is the large number of factors which influence the useful in determining the total radiation magnitude of rad, E and C. Units of temperature interception by a canopy for energy conversion are degrees Celsius (°C) or Kelvin (K = 273 + °C) calculations. Daily integrals of irradiance or Q and it is generally held that a sensitivity of ± 1 °C is give energy or quantum flux density over a period sufficient for analysis of plant growth and of time, i.e. J m - 2 d _1 or mol m - 2 d _1. development while a sensitivity of ± 0.1 °C is required for calculations of transpiration or heat transfer determinations11. Physiological processes such as seed 3.3 Temperature germination, photosynthesis, respiration and leaf growth all respond to temperature but it is 3.3.1 Introduction important to be able to measure the temperature which is most relevant to the process being studied. For example, when measuring leaf The temperature of the aerial parts of plants is expansion in grasses the temperature in the determined by the balance between energy gain by meristematic region at the base of the leaf is the interception of radiation (abs) and the energy most relevant measurement; this may be closer to losses by re-radiation (rad), convection or soil temperature than air temperature because of ''sensible'' heat loss (C) and transpiration ( AE, the location of the meristems in vegetative where λ is the latent heat of vaporisation), so that grasses12. The problem is made more difficult by the fact that plant temperatures can often be tabs = t rad + AE + C several degrees different from air temperature and  PLA«NT MICROCLIMATE 33 there is also a spatial variation in temperature, Liquid (normally mercury)-in-glass thermo­ often as a result of solar radiation interception at meters are the most common instruments used for the top of the canopy. measuring temperature. They are widely used as In recent years there has been renewed interest accurate devices in meteorological stations, but in the concept of degree-days or thermal time in they have no facility for recording. However the controlling plant growth and development. Plant less accurate bimetallic strips used in development is assumed to show a linear response thermographs do register on a dial or strip chart to temperature from a threshold (Tb) to an through a series of levers. optimum (T0), and the time taken to reach a given Many temperature sensors depend on the fact phenological stage is related to thermal time, that a change in temperature can alter the defined as the integral of temperature with time. electrical properties of certain materials. These Units of thermal time (t) are degree-days, electrical temperature transducers are either calculated as the sum of the differences between thermocouples which generate a flow of electrons daily mean temperature ( T ) and the base between two junctions of dissimilar metals if their temperature for each day beyond a given starting temperatures are different, or resistance date: thermometers and thermistors where resistance n changes with temperature. t = Σ ( f - T„) for T > Tb Thermocouples are widely used for temperature 0 measurements in biology because they are small, In many natural environments, it is difficult to easy to construct and cheap. A number of types of uncouple developmental response to temperature thermocouple can be purchased or made from from other factors such as irradiance and combinations of different metals. They have saturation deficit. This problem can be overcome different electrical and physical properties which by calculating a thermal rate (p): influence their sensitivity and suitability for p = ζ / ( Τ - Tb) different uses. Characteristics of the more common thermocouples are shown in Table 3.2. where ζ is the rate of response (e.g. leaf extension When two thermocouple junctions are joined in millimetres). The thermal rate is expressed in the voltage (V) generated is proportional to the units of response per unit thermal time (e.g. mm difference in temperature between the measuring (°C.h) _1 ); it is now possible to correlate this with junction (sensor) and a reference junction: other environmental variables, although application of this technique to field measurement V = k(T - T0) needs some caution13. It is now possible to carry out temperature integrations to determine degree- where T is the sensor temperature, T0 the reference days using commercially available transducers and temperature and k the temperature coefficient (the millivolt integrators (Delta-T Devices, change in e.m.f. per unit change in temperature at Cambridge). the reference temperature). It is common practice to assume a linear relationship between 3.3.2 Temperature measurements thermocouple e.m.f. and temperature, but for more accurate work the relationship is more Temperature is measured by transducers which precisely described by a quadratic regression are based upon temperature effects on expansion, equation 7. Normally thermocouples are used with electrical or radiative responses. The two most the reference junction maintained at a constant important sources of error in temperature temperature; this is most conveniently an ice- measurement are the effects of incoming radiation water mixture contained in a Dewar (vacuum) and the effect of the thermal mass of the sensor. flask, which has a temperature of 0°C. Calculated Both these effects are more important in air than values of e.m.f. for a range of temperatures with in water or when measurements are made within the reference junction at 0°C are given in Table the plant tissue. 3.2. Alternatively, soil temperature at a depth of 34 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS Table 3.2. Typical electrical properties and characteristics of thermocouples. (Adapted from Woodward and Sheeny7). mni is the smallest practicable thermocouple diameter. Temperature °C Thermocouple Type Uniformity min(mm) 0° 10° 20° 30° 40° 50° Copper-constantan T Low 0.2 0 0.39* 0.79 1.19 1.61 2.03 Chromel-alumel K Low 0.1 0 0.40 0.80 1.20 1.61 2.02 Chromel-constantan E Medium 0.05 0 0.61 1.23 1.85 2.48 3.08 Iron-constantan J Low 0.05 0 0.52 1.05 1.58 2.12 2.66 Plantinum-platinum/10% S High 0.025 0 0.06 0.11 0.17 0.24 0.30 rhodium one metre is quite stable; this can be used as with temperature but with about ten times the reference if its temperature is measured with a sensitivity of resistance thermometers. They are thermometer. More convenient than either of available in a range of sizes down to miniature these references are the electronic references now bead types of 0.2 mm diameter, and the circuitry available on many meters; when used in this way required to give a readout is relatively simple and only one thermocouple is required to measure robust. temperature. The only non-contact method of measuring Thermocouples can be easily constructed, temperature is by using the infra-red taking care to ensure a good junction between the thermometer, which is based upon the principle two metals. Tin or silver can be used to solder the that all surfaces emit energy11. The flux of junctions, but silver gives the smallest junctions radiation follows the Stefan-Boltzmann law and is using borax as a flux. After soldering, the proportional to the fourth power of the absolute junctions can be cut with a blade under a temperature of the object (Section 3.2.1). As the binocular microscope to make them as small as temperature of vegetation is about 290 K it emits possible. Ideally, all thermocouples should be long wave radiation with a peak of emission at individually calibrated because of small variations about 10 μιτι. Infra-red thermometers are typically in characteristics of the wires and junctions. fitted with filters which allow only radiation in the Wire resistance thermometers are most often range 8 - 13 μιη to pass to the detector. They are constructed from platinum, nickel or copper. expensive, difficult to calibrate and prone to Usually the commercially available platinum errors if reflected long wave radiation is detected. resistance thermometers are quite bulky, being When used correctly, errors are between 0.1 and typically 20 mm long and 3 mm in diameter. They 0.5°C. They are intrinsically preferable to contact are therefore only useful for measuring methods because the latter can alter surface temperatures of large volumes but they are often temperature during measurement by simultaneous favoured for long term use because of high conduction between the thermometer, surface and stability, resistance to weathering and an almost air, possibly resulting in large errors. However, linear change in resistance with temperature. infra-red thermometers cannot be used for However, the change in resistance with measuring "sky" temperature. temperature is relatively small, so a circuit to read voltage output must be designed with care to avoid 3.3.3 Use of thermometers large error resistances2,4. Thermistors are semiconductors, composed of Before use, thermometers should be calibrated sintered mixtures of metallic oxides. The over the expected range of temperature. The resistance of thermistors decreases exponentially simplest method is to immerse the sensors in a  PLANT MICROCLIMATE 35 water bath whose temperature is controlled and 3.4.2 Definitions compare temperatures with an accurate mercury- in-glass thermometer. The infra-red thermometer Because water vapour is a gas, its pressure cannot be calibrated in this way, but it can be set contributes to the total measured atmospheric up to receive radiation from the inside of a pressure and its potential pressure is called vapour blackened sphere immersed in a water bath whose pressure (e). When air above water has no extra temperature is known. capacity for holding water vapour the partial Temperatures measured are usually of air, pressure of the water vapour is the saturated surface, soil and tissue. The latter two are less vapour pressure (es) measured in kPa and its prone to difficulties because the thermometer is density the saturation density (g m~ 3 ). The immersed in the material it is sensing. For air saturation vapour pressure increases with temperatures to be measured accurately the temperature (Figure 3.7). If air is cooled without absorption of solar and long wave radiation change in water content, condensation occurs at should be prevented by use of a radiation shield its dewpoint temperature (Td), when e = es. and possibly also ventilation. The ideal shield When water evaporates into less than saturated should have a high reflectivity for solar radiaton air then the temperature of the air decreases up to and a high emissivity for long wave radiation. a point. This is the wetbulb temperature (Τ'), the Aluminised "Mylar" and clear matt white paint temperature to which the wet bulb falls in a have been found to be the most suitable shield psychrometer (Section 3.4.3). Its value is given by coverings. Surface temperature measurements are the intercept of a line of slope - y (where y is the the most difficult to make accurately because they psychrometric constant), passing through the depend on a good thermal contact between the vapour pressure of the air at the dry bulb sensor and surface being measured. Clips, springs temperature, with the curve of saturated vapour or tapes are often used to make contact but their pressure against temperature (Figure 3.7). The presence can lead to errors. Further details on the slope of the line differs according to whether the use of thermocouples and an analysis of the errors wet bulb is ventilated or not. which might be experienced can be found in Relative humidity is the ratio of the actual Perder 11. vapour pressure (e) to the saturated vapour pressure (es) at the dry bulb temperature (T). It is usually expressed as a percentage. However the 3.4 Humidity use of this term should be discouraged as plants do not respond directly to relative humidity. 3.4.1 Introduction Saturation deficit or vapour pressure deficit (òe) is the difference between the saturation vapour The water content of air is known as the pressure and the actual vapour pressure at the absolute humidity (χ) and is the density of water same temperature (Figure 3.7). It is an index of the vapour in the air in g m 3. The importance of drying power of the air; the higher the deficit the humidity to a plant's functioning is twofold. greater the evaporation rate. Firstly, it determines the rate of water lost in transpiration (E) because: 3.4.3 Measurements E = g (Xair - Xleaf) Many different devices can be used to measure where g is the conductance for water vapour the humidity of the air. They are based on several transfer between the evaporating surfaces within principles including the electrical properties the leaf and the air. Secondly, humidity has a of sulphonated polystyrene or thin-film solid direct effect on the stornata of many plants, so state semiconductors; wet-bulb depression; that stornata tend to close in dry air restricting condensation of water vapour on a surface cooled water loss but also reducing C 0 2 assimilation. to the dew-point; and infra-red absorption. Some 36 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS of the instruments more widely used in the field y is the psychometric constant (equal to 66 Pa are considered here. ° C - 1 at sea level in a ventilated psychrometer). A psychrometer is a pair of identically shaped Several types of psychrometers are available as thermometers, one of which is covered with a wet commercial units, the best of which ensure sleeve. Evaporation cools the wetted sensor to the efficient radiation shielding of the thermometers wet-bulb temperature, and the vapour pressure (e) and minimise heat conduction along the stem of is calculated as: the thermometer 2. The Assman psychrometer is a ventilated psychrometer containing matched e = es -y(T - Τ') thermometers; it is used for standard humidity measurements. Smaller ventilated psychrometers where T ' and T are the wet- and dry-bulb are now available for use above and within plant temperatures respectively, e s / r is the saturated canopies (DeltaT Devices, Cambridge). The hand­ vapour pressure at the wet-bulb temperature, and held whirling or sling psychrometers are the 3.0r 130 Temperature (°C) Fig.3.7. The influence of temperature on the saturated water vapour pressure of water. The point X represents air at 18°C and 1.0 KPa vapour pressure (e). The line Y-X-Z, with a slope of - y , gives the wet-bulb temperature (Τ') where it intercepts the curve at Y (12°C). The water vapour pressure deficit (de) is the difference between X and W (the saturated vapour pressure at 18°C). The dew-point (Td) is the point at which the saturated vapour pressure is equal to X.  PLANT MICROCLIMATE 37 simplest and cheapest ventilated units. In order to always employed to measure concentration achieve an aspiration rate of 3 m s"1 they have to differences, making it very suitable for profile be rotated at about two revolutions per second. studies. Many materials show a change of physical dimensions when they absorb water, and this 3.5 Wind property can be used to make instruments that measure humidity. For example, the length of Wind is the large-scale transport of air masses animal hair increases as the air becomes wetter resulting from differences in air pressure. It is and decreases as the air dries; this property is used directly involved in heat and mass transfer by in simple hygrometers. Provided an allowance is forced convection, so it is very important in made for the effect of temperature, hair influencing heat and gas exchange across the hygrometers are usually accurate to within 5% boundary layers of plants. Increase in wind speed over most of the humidity range. decreases the boundary layer resistance over leaves The change in electrical properties of materials (see Figure 3.1); this tends to increase evaporation as they absorb water is used in several humidity and bring leaf temperature closer to air sensors. Until recently the lithium chloride sensor temperature. Wind is also important because it was the most common type of electrical sensor. causes mechanical deformation of plants (due to Lithium chloride is hygroscopic and the moisture the frictional drag of moving air) and because it content of the air determines how much water is disperses pollen, seeds and aerial pollutants. absorbed, which in turn influences the AC However, of all the elements of the microclimate, resistance of the sensor. This type of sensor is wind is the most spasmodic. Short term variations susceptible to contamination by dust and other in wind speed are described by the intensity of hygroscopic particles, and it suffers from a certain turbulence; this value represents the standard amount of hysteresis when wetting or drying. deviation of the instantaneous values, divided by More recently, capacitance hygrometers, which the mean wind speed14. Turbulent air moves in measure the change in electrical capacitance packets or eddies; these are important for the caused by water-absorption into a dielectric, have movement of C0 2 , H 2 0 and other gases in and become commercially available (Humicap, above plant canopies. The meteorologists' manufactured by Vaisala, Helsinki, Finland) and measurements of wind speed are normally made are less temperature sensitive and show less 10 m above the ground, but wind speed decreases hysteresis than other electric sensors. rapidly as the plant surface or ground is Dewpoint meters measure the temperature at approached. Wind speeds near or within which dew forms on a cooled surface. Dewpoint is vegetation can therefore be much lower than at 10 usually determined by cooling a surface to below m. The analysis of the profile of mean wind speed the point of saturation, allowing water to above the canopy can be used to derive condense onto it, and then gradually raising the coefficients for calculating the flux of C 0 2 and temperature until the film of condensation starts H 2 0 between the canopy and the atmosphere5,7. to evaporate. The temperature at which this However, the principles of environmental physics change occurs is taken as the dewpoint required to make these calculations are beyond the temperature, and the presence of the film can be scope of this discussion. Furthermore, the number detected optically or electrically. Dewpoint of environmental sensors and the size of data temperatures must be corrected for changes in recording and processing facilities required are atmospheric pressure if they are converted into beyond the budget of most research groups. vapour pressure. Infra red gas analysis can measure water vapour 3.5.1 Measurement concentration of air as well as C 0 2 (Chapter 6). The instruments are expensive but they are A complete picture of air movements requires accurate and respond quickly. With suitable continuous recording of instantaneous wind speed switching systems this type of instrument is almost measured in three directions; the vertical, the 38 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS horizontal lateral and the horizontal measurements of the weather including solar perpendicular. However, these are difficult to radiation, net radiation, wind run, wind direction, measure, so it is generally sufficient to determine air temperature, wet-bulb temperature and the mean value in one direction over the rainfall. The output from these instruments is measurement period. recorded on standard cassette tapes, used in The most commonly used instrument is the cup conjunction with battery operated data loggers. anemometer. This normally consists of three Modern data loggers often incorporate many hemispherical or conical cups mounted on arms channels, enabling the recording of additional and attached to a central vertical spindle, so that data such as soil temperature. they are free to rotate in the wind. The number of rotations of the cup assembly is usually measured in metres (the "run of the wind"), and can be 3.7 Recording divided by the elapsed time to give the mean wind speed. An alternative mechanical anemometer is the vane or propellor type. The vane anemometer The simplest method of recording the output is simply a miniature windmill, consisting of a from micrometeorological instruments in the field number of light vanes radially mounted on a is using a pencil and note-pad; and in many cases horizontal spindle. The main sources of error with this is all that is necessary. There are, of course, mechanical anemometers are firstly, that they many advantages in adopting automatic recording have a threshold below which the friction of the but the methods used must be considered in system prevents rotation, and secondly, that their relation to the use to which the measurements will inertia makes them over-run when the wind speed be put. For example, detailed measurements of drops. Additionally, the vane anemometer is vertical profiles of temperature, wind speed, directional; it must be aligned to the wind humidity, radiation and C 0 2 concentration can be direction. used to estimate canopy évapotranspiration and A third type of instrument is the hot-wire C 0 2 exchange, but they require complex recording anemometer, which estimates wind speed by facilities; these techniques are normally only measuring the rate of cooling of a heated wire in possible when resources of equipment and moving air. These can be made very small, and are manpower are extremely good. However, less useful for measuring the rate of air flow around intensive measurements using limited recording leaves14, but they are often delicate and easily facilities can still tell us a lot about the relationship broken. For this reason they are not used routinely between the plant and its environment. Micro- for field measurements. Further details of meteorological measurements are not an end in instruments for wind measurement can be found themselves and usually we need to relate them to in Grace14. plant physiological responses such as photosynthesis, stomatal movement, water potential and leaf expansion. These responses have different time scales (e.g. photosynthesis and 3.6 Automatic weather stations leaf expansion) and measurements should be recorded accordingly. In many studies, a rather broad description of The simplest automatic method of the climate experienced by the plants is sufficient accumulating output from instruments is using to begin untangling plant/climate relationships. analogue recorders. The most suitable of these are For these purposes, information obtained from galvanometer recorders which can be multi­ standard meteorological sites located close to the channel, and either use pen and ink as tracer or vegetation under investigation is useful. However, record on pressure-sensitive paper using a chopper in recent years, automatic weather stations have bar. Analogue integrators can be used where been increasingly used. These stations can be set detailed chart records are not necessary as they up on experimental sites to provide detailed integrate small currents and voltages and can be  PLANT MICROCLIMATE 39 used to determine characteristics such as degree- the temperature of the undersurface of a leaf and days and daily solar radiation integrals. of the air below the leaf. Repeat the measurements Digital data logging is perhaps the most at a number of heights between the top of the convenient way of collecting micrometeorological canopy and the soil and also measure soil data, especially where a large number of temperature by carefully pushing the measurements are involved which can thermocouple into the soil to a depth of 1 cm. subsequently be handled by a computer. Here the (d) Wet- and Dry-bulb temperature - using the analogue input from the instruments is converted Assman or Delta-T ventilated psychrometers to into a number (by an analogue-digital converter) make measurements at a number of heights in the and recorded on a magnetic tape. Usually a large plant canopy. Calculate the saturation deficit of number of inputs can be scanned in sequence to the air (kPa) from the wet- and dry-bulb give discontinuous but frequent records of a large temperatures using the tables or slide-rule number of measurements. For further discussion provided. of this topic see Woodward and Sheehy7. 3.8 Experimental work References The objective of this experiment is to measure 1. Monteith, J.L. (1981) Coupling of plants to the the vertical profiles of different environmental atmosphere. In: Plants and their Atmospheric factors in canopies of two crops of contrasting Environment, 21st Symposium of the British Ecological Society (Grace, J., Ford, E.D. and structure (e.g. maize and bean). These are carried Jarvis, P.G. eds.) pp. 1-29. Blackwell Scientific out in conjunction with measurements of stomatal Publications, Oxford. conductance and leaf water potential in order to 2. Szeich, G. (1975) Instruments and their Exposure. determine which factors control stomatal activity. In: Vegetation and the Atmosphere, Vol. 1: Measure the following parameters at five Principles (J.L. Monteith ed.) pp. 229-273. Academic Press, London. different positions in the crop. 3. Monteith, J.L. (1972) Survey of Instruments for (a) Photosynthetically Active Radiation (PAR) - Micrometeorology. IBP Handbook No. 22. using the Lambda linear quantum sensor. These Blackwell Scientific Publications, Oxford. measurements should be made at right angles to 4. Fritschen, L.J. and L.W. Gay, (1979) the rows. If the incident radiation is constant, Environmental Instrumentation. Springer-Verlag, New York. measure above the crop and then at progressively 5. Monteith, J.L. (1973) Principles of Environmental lesser heights down to the soil surface. However, Physics. Edward Arnold, London. if the incident radiation is changing, measure first 6. Campbell, G.S. (1977) An Introduction to above the canopy and then at a lower height, then Environmental Biophysics. Springer-Verlag, New above the canopy again before measuring the next York. 7. Woodward, F.I. and J.E. Sheehy, (1983) Principles lowest position. Express the value of quantum and Measurements in Environmental Biology. flux at a particular height as a percentage of the Butterworths, London. incident flux. 8. Jones, H.G. (1983) Plants and Microclimate. (b) Short- Wave and Visible Radiation - using Cambridge University Press. two tube solarimeters, one of which is fitted with a 9. McCree, K.J. (1976) A rational approach to light measurements in plant ecology. In: Commentaries filter to eliminate the visible wavelengths in Plant Science (H. Smith ed.). Pergamon Press, (400-700 nm) to measure the non-visible Oxford. component of short-wave radiation. The 10. Grace, J. (1983) Plant-Atmosphere Relationships. difference between the two gives the value for Outline Studies in Ecology. Chapman and Hall, visible radiation. Express the value at a given London. 11. Perrier, A. (1971) Leaf temperature measurement. height as a percentage of the incident radiation. In: Plant Photosynthetic Production, a Manual of (c) Leaf and air temperature - using a Methods, (ed. Z. Sestâk, J. Catsky and P.G. Jarvis) WESCOR thermocouple thermometer to measure pp. 632-671. Dr. W. Junk, The Hague. 40 TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS 12. Peacock, J.M. (1975) Temperature and leaf growth stand of pearl millet (Pennisetum typhoïdes) 1. in Lolium perenne. II. The site of temperature Vegetative development. J. Exp. Bot. 34, 322-336. perception. J. Appi. Ecol. 12, 115-123. 14. Grace, J. (1977) Plant Response to Wind. 13. Ong, C.K. (1983) Response to temperature in a Academic Press, London.