42: Transpiration

JOHN ROBERTS Centre for Ecology and Hydrology, Wallingford, Oxfordshire, UK

The transpiration process is the uptake of water by plant roots, transport through the plant and evaporation from the leaf through pores called stomata. Evaporation of water from the leaf is determined by atmospheric conditions such as radiation, temperature and humidity deficit but the plant can limit transpiration by partial or complete stomatal closure. Generally stomata open in response to increasing radiation but tend to close with increasing air humidity deficit and reduced availability of soil moisture. Because the stomata have to be open in daylight for the entry of carbon dioxide into the leaf for the photosynthesis process, water loss is an inevitable consequence. Nevertheless transpiration itself has important roles. Nutrients are brought into the plant when water is taken up from the soil and evaporation of transpired water prevents leaf temperature reaching supra-optimal levels. There are numerous ways that transpiration might be measured. These include measurements of soil water changes below vegetation or changes in atmospheric humidity above vegetation. Alternatively measurements can be made on individual plants or leaves. In most circumstances the source of water for transpiration is the soil and principally the surface soil layers where most roots are found. In the future, increased levels of atmospheric CO2 are expected to reduce transpiration through reduction in stomatal aperture.

WHAT IS TRANSPIRATION? There is a continuous stream of water from within the leaves of plants down through the plant to the roots and soil. Water molecules bind together and these bonds have Transpiration is the process by which water is evaporated substantial strength. There is a continuous column of water from within a plant. Essentially, water is evaporated through from within the leaf drawing up water from the soil. small holes (known as stomata) in the leaves, and this The cohesive forces of water molecules mean that water draws water up through the plant (in microscopic tubes columns down through the plant can be maintained under termed xylem) from the soil. This “transpiration stream” significant tensions. In very tall trees, these tensions can be brings water to the plant to be used in photosynthesis, to considerable. Suctions equivalent to 5 MPa (50 bars) have produce carbohydrates, and to maintain turgidity (rigidness) commonly been reported for actively transpiring tall trees. in the cells and tissues. However, very little of the water is Transpiration from vegetation is usually reported as a actually used in photosynthesis and to maintain turgidity. depth of water (mm), in the same way that rainfall and Most of the water sucked up from the soil is evaporated evaporation are reported. Transpiration might range from through the stomata. A primary purpose of the stomata is to very low or zero in completely water-stressed vegetation, exchange carbon dioxide and oxygen with the atmosphere sparse crops, or vegetation in winter. The highest values in addition to regulating the loss of water from the leaves. of transpiration might be up to rates estimated as the The movement of water up from the soil through the plant potential transpiration rate (see Chapter 41, Evaporation plays a key role in bringing minerals from the soil into and Modeling: Potential, Volume 1). In this case, it would through the plant. In situations where leaves experience a be expected that the vegetation completely covered the high radiation loading, leaf temperatures can be critically ground, there was no shortage of soil water and climatic high. Cooling of the leaf by the dissipation of heat during conditions are optimal, for example, high summer or the evaporation of transpired water is another important role tropical conditions. Transpiration can exceed the potential for transpiration. rate if extra energy is available as advection.

Encyclopedia of Hydrological Sciences. Edited by M G Anderson.  2005 John Wiley & Sons, Ltd. 2 HYDROMETEOROLOGY

Table 1 Annual transpiration of global forest types Annual Vegetation Location transpiration (mm) Reference Tropical rainforest Manaus, Brazil (2◦57’S: 59◦ 57’W) 1030 Shuttleworth (1988) Southern European Evora, Portugal (38◦ 32’N: 8◦ 01’W 207 David et al. (2004) evergreen oak Temperate coniferous Thetford, UK(52◦ 25’N: 0◦ 39’E) 352 Gash and Stewart (1977) forest Boreal coniferous Saskatchewan, Canada (53◦ 55’N: 104◦ 41’W 204 Saugier et al. (1997) forest

In summer conditions in the United Kingdom, maxi- ) 400 1

−1 − Reserva Ducke mum transpiration rates of up to 4 mm day have been s 2 measured routinely in forests although much higher rates − Thetford forest ∼8mmday−1 have been observed in fast-growing short 300 rotation coppice plantations. In forests with 750 trees ha−1, a transpiration rate of 4 mm day−1 would mean that 50 kg 200 of water per day would be lost on average from each tree. In a wheat or barley field with around 250 stems m−2,tran- spiration loss through each stem would be of the order of 100 150 gm. An insight into the range of annual forest transpiration that might be encountered can be achieved by comparing Stomatal conductance (mmol m 0 0 5 10 15 20 values from various studies carried out in a range of Specific humidity deficit (g kg−1) forest types occurring from boreal to tropical regions. The annual transpiration (with associated information) of forest Figure 1 The decline in leaf stomatal conductance with air types occurring in different global regions are given in humidity deficit in Piptadenia suaveolens, an upper canopy Table 1. As expected the highest annual total is found tree species at the Reserva Florestal Ducke, Manaus, Brazil in tropical rainforest in Brazil. The high annual total (unpublished data from John Roberts) and the upper canopy of Scots pine (Pinus sylvestris L.) at Thetford is largely a consequence of the evergreen canopy and Forest, UK (Redrawn from Beadle et al., 1985, Journal of therefore year-round transpiration with no limitations of Applied Ecology 22, 557–571, by permission of British solar radiation, air temperature, or available soil moisture. Ecological Society) Potential evaporation rates in this area of the Amazon basin would enable much higher transpiration, but the reduction Transpiration from the temperate coniferous forest is of canopy conductance in response to an increased air vapor higher than both the southern European and boreal forests. pressure deficit (see Figure 1) means that daily transpiration Although not constrained by water stress, daily transpiration is often around 3.5 mm, barely different from transpiration is likely to be limited by a strong decline in stomatal con- of Scots pine (Pinus sylvestris) measured at Thetford Forest ductance with increasing vapor pressure deficit (Figure 1). on summer days. The annual transpiration of around 325–350 mm year−1 Annual transpiration from the evergreen oak (Quercus shown for Thetford Forest was shown to be very similar for rotundifolia) woodland in Portugal is 207 mm. Although many woodlands (both broadleaf and coniferous) in Europe this estimate does not include losses from ground vegetation by Roberts (1983). Roberts identified a number of fac- beneath the trees, the low transpiration rate is largely a tors that might contribute to this similar transpiration. Few consequence of the sparse open canopy of the woodland. forests are limited by water stress, and daily transpiration Shortage of water is probably not an issue as the trees had is constrained by probable links between stomatal/surface access to groundwater. The annual transpiration from the conductance and air humidity deficit. Furthermore, the pres- boreal forest is also low. Although the growing season is ence of understory vegetation below an open tree cover will short (∼140 days), a major constraint on transpiration is have a significant role in eliminating tree transpiration dif- probably soil temperature, which will still be cold enough to ferences between dense and open forests. One factor that limit root water uptake, and probably also mineral nutrients has been shown to be important in determining transpiration throughout the growing season. This was considered as a from forests and woodlands is the age of the trees. There major factor in producing low stomatal conductances in the is now substantial evidence from studies both on trees and jack pine (Pinus banksiana) in the boreal forest. catchments that as trees age their transpiration declines. TRANSPIRATION 3

MesophyII Intercellular cell spaces

rw

Substomatal + + ri = rc(rc ri rw) cavity rl + + rc(rc ri rw)

rs rc

Cuticle Guard cell ra Cuticular Stomatal evaporation evaporation

Figure 2 Pathways for water loss from one surface of a leaf showing the boundary layer (ra), cuticular (rc), variable stomatal (rs), intercellular space (ri ), wall (rw ), and leaf (rl ) resistances. The total leaf resistance is the parallel sum rl for upper and lower surfaces (Redrawn from Jones (1992), Plants and Microclimate, Second Edition, Cambridge University Press)

Practically all of the water lost from vegetation will have trees, and herbaceous plants. Figure 2 shows a cross-section been taken from the soil. The amount of water taken up through a stomatal apparatus, with the stomatal opening from the soil and used in metabolic processes, for example occurring on the underside of the leaf. Figure 2 shows photosynthesis, is trivial. Some water is lost from plants as the resistances met as water vapor is transpired from one transpiration does not come directly from the soil but from surface of the leaf. Evaporation of water from cell walls is storage in the body of the plant. Generally, this stored water regarded by some to be the site of the first resistance (rw) is a smaller fraction of the total daily transpiration than in the water loss pathway. The transfer resistance within water coming directly from the soil. In any case, water lost the intercellular spaces (ri) is followed by the stomatal from storage in the plant tissues will be replaced with water resistance (rs), and the boundary-layer resistance (rb). The from the soil, and this may occur in the following night, or pathway of water transfer through the cuticle is very high in the case of some forests, in the following winter. and is in parallel with the stomatal resistance (rs ). All of these resistances may be expressed as a conductance which is the reciprocal value of the resistance. The stomatal CONTROLS OF TRANSPIRATION resistance or conductance is variable, depending on the degree of stomatal opening. Figure 2 shows that the whole The evaporation of water from leaf surfaces is influenced leaf resistance for one surface of the leaf is r , with the by all the factors involved in evaporation from free water l surfaces; evaporation is increased as energy inputs increase, total leaf resistance being the parallel sum for the upper and and as the vapor pressure gradient from the leaf surface lower surfaces. The dominance of the stomata as the major into the atmosphere increases. Evaporation can also be sites for the transfer of water vapor from the inside of leaves enhanced by increases in wind speed. Nevertheless, there to the outside is reinforced, because the rest of the leaf is is a particularly important control in the leaf surface. The covered with the cuticle which has a very high resistance apertures (stomata) in the leaf surface are able to regulate to vapor transfer, unless the leaf surface (and therefore the the loss of water vapor as transpiration. Closure of the cuticle) is damaged in some way. The low permeability of stomata also means that the entry of carbon dioxide into the cuticle is reflected in a very high cuticular resistance. the leaf for fixation by photosynthesis will be limited. The boundary layer of a leaf refers to the air layer next A comprehensive discussion of stomata, their structure, to the leaf surface, where the surface friction reduces the occurrence, function, and factors in the control of stomatal wind speed in comparison to the bulk airflow. The air layer opening will be found in Willmer and Fricker (1996). immediately adjacent to the leaf surface effectively remains Stomata may occur in small numbers on stems and leaf static, and the transfer of molecules such as water vapor, stalks but are much more common on one or more leaf occurs by diffusion only. In still air, the low boundary- surface. Stomata occur in more or less equal numbers over layer conductance will have a major influence on the flux the surfaces of grass and cereal leaves, and also in some of water vapor out of a leaf unless the stomata are practi- conifers for example. The most common situation, however, cally closed. In moving air, on the other hand, the exit of is for stomata to be found on the undersides of leaves, and water vapor from the vicinity of the leaf is regulated more this is the situation for tropical and temperate broad-leaved by the stomata (Figure 3). 4 HYDROMETEOROLOGY

300 can also be transmitted. Stressed roots produce substances )

1 that are transported to the guard cells of the stomata via −

s 250 the transpiration stream. There is good evidence the plant 2 − Wind hormone, abscisic acid (ABA), is an important hormonal 200 factor which is transported from the roots (Davies and Zhang, 1991). ABA has a strong inhibitory influence on 150 stomatal opening. There is still a need to research particular aspects of ABA and the control of stomata, and therefore 100 Still air transpiration. For example, up until now there has not been 50 a convincing demonstration of the transport of ABA from Transpiration rate (ng cm rate Transpiration the roots of mature trees into their canopies, during the 0 development of soil water stress. 0246810121416182022 Much understanding about the controls of stomatal open- Stomatal aperture classes (µm) ing by environmental factors came from detailed studies involving individual plants in controlled conditions. In these Figure 3 The regulation of transpiration by stomatal studies, the convention was to modify one environmental aperture in Zebrina pendula leaves under moving and still air conditions (Redrawn from Bange, 1953) factor whilst all others were held constant. In real field situ- ations, especially in a complex canopy, the degree to which stomata are open depends on the interplay of a range of A number of factors, both environmental and within the environmental conditions. Transpiration from an individ- plant, can influence stomatal aperture. Stomata open in ual leaf will depend on (1) the degree of stomatal opening response to increases in solar radiation and show an asymp- (stomatal conductance) determined by environmental con- totic relationship with radiation up to full midday illumina- ditions, (2) the leaf-boundary conductance which is usually tion conditions. There is an increase in stomatal opening strongly influenced by the size and shape of the leaf, and in response to temperature. Stomatal opening increases (3) environmental conditions that force transpiration partic- up to an optimum value of temperature which is species ularly temperature, available energy, and air vapor pressure dependent but can be as much as 30 ◦C. In many species, deficit. Wind speed around the leaf will have an influence particularly trees, stomata close in response to increases in on boundary-layer conductance. the air atmospheric humidity deficit. The exact mechanism At any one time during the day, the transpiration of by which plants might sense humidity and reduce stom- a vegetation canopy reflects the environmental influences atal opening remains to be revealed. Nevertheless, strong on the stomata of individual leaves, depending on their negative relationships have been observed between stomatal position in the canopy. In plant canopies, particularly the conductance (gs) and air humidity deficit (D) in a wide vari- more complex ones, for example temperate and tropical ety of vegetation types, particularly those including woody forests, there is usually significant systematic variation in plants (see Figure 1). An important consequence of the neg- leaf stomatal and leaf-boundary conductance down through ative relationship between gs and D is that from day to day the canopy. This variation in the conductances coupled transpiration remains fairly similar, and never reaches the with systematic microclimatic variation also down towards high values expected from considerations of radiation input. the base of the canopy means a major influence on levels There is a high covariance between D, solar radiation, and of transpiration from different zones in the canopy. Light air temperature. Therefore on bright, hot days with high D, levels decline with depth in plant canopies. These reduced stomata may be no more open than on a duller, cooler day light levels mean that stomatal opening is reduced, and with smaller D. also that less energy is available to force transpiration. It could be postulated that the relationship between Humidity levels are also usually higher within canopies. gs and D in plants acts as a form of daily rationing This means that the leaf to atmosphere vapor pressure of water, and might delay the development of severe gradient is reduced which limits transpiration. Wind speeds soil water deficits which might lead to a more long-term are also lower within canopies. This will mean that leaf state of stomatal closure. The reduction in the availability boundary-layer conductances might become critically low of soil moisture has a fundamental control of stomatal in canopies where the air is less agitated. opening, usually operating through the leaf water status and The modification of microclimate within complex cano- turgidity of the leaf. Plants operate as integrated systems pies means that although the leaf area index (leaf area per with signals to a leaf coming from all parts of the plant unit of ground area, m2 m−2) might be high, that is, six resulting in a net response. As well, the hydraulic signal or more, transpiration in the lower levels of the canopy to the foliage from the roots indicating the development of might be very much reduced. Roberts et al., (1996), work- reduced availability of water in the soil chemical signals ing in the tropical rainforest in the central Amazon, Brazil, TRANSPIRATION 5 examined the interaction of microclimate and leaf con- • in dry zones there is a prevalence of annual plants where ductances in controlling transpiration using a five layer the plant survives dry periods as a seed, or if a perennial, formulation canopy layer and total transpiration estima- the plant survives as a subterranean or otherwise much- tion routine (CLATTER) of the Monteith–Penman equa- reduced structure. tion (see Chapter 45, Actual Evaporation, Volume 1). They showed that because of reduced light levels, leaf conductances fall systematically down through the for- WHAT ARE THE ROLES FOR est canopy. Leaf boundary-layer conductances fell because TRANSPIRATION ? of the reduced wind speeds towards the forest floor, and The foliage of plants with a large surface area relative to its humidity deficits also declined systematically towards the volume, constantly exposed to sun and wind, will lose water forest floor. CLATTER showed that although there is a sub- very rapidly. This does not occur with leaves, because the ∼ stantial fraction of the total leaf area index (forest LAI 6) entire surface is covered with a thin, waterproof layer, the in the lower layers of the canopy, the fraction of the total cuticle. Although there may be some microscopic cracks forest transpiration from the lower layers is much less than and fissures in the cuticle, largely the cuticle provides layers in the upper canopy that contain a lower foliage den- a very effective means of restricting the loss of water sity (Figure 4). from the leaf. However, the leaf must be able to take up As well as the possession of stomata which can regulate the gas, carbon dioxide (CO2), for photosynthesis, and to water loss, land plants living in regions of water shortage lose the excess oxygen (O2) not required for respiration. avoid desiccation, and may possess a large number of The leaf must also take up oxygen for respiration when structural and life history adaptations that are considered to photosynthesis is not taking place, such as during the contribute to conserving water. Some of these adaptations night and in low sunlight. The stomata in the leaf surfaces are as follows: control both the entry of CO2, and the exit of O2.The • highly reflective leaf surfaces brought about by wax diffusion outwards of water vapor is regarded by some as deposits or reflective hairs; an unnecessary consequence of the stomata being open for • stomata sunk into pits in the leaf surfaces; the uptake of CO2. Nevertheless, there are important roles • reductions in leaf area per mass of plant; for transpiration and the need for stomata to function as • leaves reduced in size; they do in the leaf surface. • deep roots, or a large mass of root per mass of shoot; • modifications of stem or root to form water stor- Transpiration and Leaf Temperature age organs; • ability to shed leaves during the driest periods to avoid A key feature of transpiration for plants is the cooling water deficits; effect that prevents leaf temperatures from exceeding lethal limits and to be maintained close to the optimum for the functioning of a range of physiological processes which 50 occur inside leaves. Particularly in regions such as deserts, with high radiation inputs, the need for leaves to be cooled Leaf area 40 is of paramount importance. Figure 5 (Lange, 1959) shows Transpiration that the temperatures of freely transpiring leaves of the 30 desert cucumber (Citrullus colocynthis) are maintained at around or below air temperature, and well below lethal limits. When transpiration is interrupted, leaf temperature 20 rises to beyond critical limits within the next hour. Evaporation, and especially transpiration, from plant and Canopy height (m) Canopy 10 vegetation surfaces, is particularly important to the land surface energy balance. Therefore, changes in vegetation 0 cover which modify levels of radiant energy used to drive transpiration flux have a large influence on temperatures at 0 102030405060 the vegetation surface. % Contribution

Figure 4 Leaf area in each of five canopy layers as Transpiration and Nutrient Uptake a percentage of total leaf area and transpiration from each layer as a percentage of total transpiration. Reserva A crucial aspect of transpiration is the associated uptake Florestal Ducke, Manaus (After Roberts et al., 1996. of certain nutrient ions and their transport and distribution Reproduced with permission from CEH) through the plant, to support plant growth and development. 6 HYDROMETEOROLOGY

65 Shoot

60 Soil 55 3 2 C) ° 50 Lethal point 45

40 Temperature ( Temperature 35 Root 30 1 25 8 101214161820 Figure 6 Schematic presentation of the movement of Time of day (h) mineral elements to the root of a tree. (1) Root interception: soil volume displaced by root volume. (2) Mass flow: Figure 5 Transpirational cooling in the desert plant transport of bulk soil solution along the water potential (Citrullus colocynthis). Graph shows air temperature ( ), gradient, driven by transpiration. (3) Diffusion nutrient leaf temperature (-··--··) and temperature of another leaf transport along the concentration gradient. •=available which was cut off (arrow shows time of detachment) to nutrients. (After Marschner, 1995) stop transpiration (- - - - ). (After Lange, 1959. Reproduced with permission from Elsevier GmbH) concentrations in the soil from where the water is taken up. Barber (1995) discusses three mechanisms by which plants Transpiration by the plants has a fundamental role in the obtain nutrients from soil and the influence of soil moisture mass flow of nutrients through and from the soil, and into conditions and transpiration in these mechanisms. The three the plants via the roots. Mass flow is the process by which methods are shown schematically in Figure 6. All three plants usually acquire at least three of their most important methods may be in use at the same time but the uptake nutrient requirements, nitrogen, calcium, and magnesium, of certain nutrients may be exclusively by one method. and considerable nutrient quantities enter the vegetation Root growth can mean that roots encounter unexploited as part of the water uptake process. It was estimated that zones of available nutrients, this is regarded as nutrient 80 kg ha−1 year−1 of nitrogen is taken up by the broadleaf uptake by interception. Diffusion is the movement of woodland on the Hubbard Brook catchment, USA (Likens certain nutrient ions along a concentration gradient, usually and Bormann, 1995). Nutrient uptake will fluctuate with towards the root surface where concentration is lowest, soil water availability. In many circumstances, nutrient from the bulk soil where the concentration is highest. availability in the topsoil declines steeply during the Root interception is used to describe the acquisition of growing season because low soil moisture becomes a soil nutrients at the interface of the soil and the root. These limiting factor for nutrient delivery to the root surface. The nutrients are acquired by virtue of the growth of roots in the second process by which nutrients move through soil, is soil. Because soil moisture status will determine the amount by diffusion along a concentration gradient. Rate of ion and distribution of root growth, the level of soil moisture diffusion is directly related to the water content of the soil will influence nutrient uptake by root interception. Nutrients (θ). Increases in θ reduce the tortuosity of the diffusion move in soil in two ways. First, nutrients are transported to path, and increases the diffusion flux. There is often a linear the roots by mass flow along with the water taken up as part relation between θ and the diffusion coefficient. Diffusion of transpiration. Therefore, the uptake of nutrients will be is particularly important for nutrients such as potassium directly influenced by the transpiration rate and the nutrient and phosphorus. Table 2 shows the relative importance of

Table 2 Nutrient demand of a maize crop and estimates of nutrient supply from the soil by root interception, mass flow, and diffusion (After Barber, 1995 by permission of John Wiley & Sons Inc.) Estimates of amounts (kg ha−1) supplied by

Demand (kg ha−1) Interception Mass flow Diffusion Potassium 195 4 35 156 Nitrogen 190 2 150 38 Phosphorus 40 1 2 37 Magnesium 45 15 100 0 TRANSPIRATION 7 the different mechanisms of nutrient uptake in the case of Lysimeters several key elements. A lysimeter is a device in which a volume of soil with associated vegetation is isolated hydrologically from the surrounding soil. Drainage is measured or is zero, and HOW IS TRANSPIRATION MEASURED? in the case of lysimeters that are weighed, changes in Because there are a wide range of questions relating to water storage are determined by weight difference. The transpiration from plants and vegetation, a wide range of application of weighing lysimeters to studies in mature or techniques have been used to measure the process. The semimature trees has been very limited. A single large most appropriate technique to be used depends largely on Douglas fir tree was installed in a weighing lysimeter the temporal and spatial scale over which estimates are by Fritschen et al. (1973). Reyenga et al. (1988) installed needed. There are approaches, because they are capable a lysimeter in a regenerating eucalyptus forest in New of measuring total transpiration from stands of vegetation, South Wales, Australia. Large potted trees capable of being for example, micrometeorological or soil depletion stud- weighed (up to 500 kg) have proved useful for calibrating ies, that are not able to provide detailed information about other techniques such as isotopic tracers (e.g. Dugas et al., the contribution of individuals that comprise the vegetation. 1993). Another form of lysimeter is the drainage lysimeter, They are not appropriate for isolated individual plants. Nor which has been used very commonly in short crops can the contribution of different vertical layers in complex but rarely in forests. Calder (1976) described a drainage vegetation be separated by techniques that measure total lysimeter constructed in a Norway spruce plantation in mid- vegetation transpiration. Sometimes detailed information is Wales, UK. The lysimeter was sealed at the base by a required about the contribution of fragments of the vegeta- soil layer of impermeable clay. Data from the lysimeter tion to total transpiration and the factors that control their associated with nearby net rainfall measurements enabled transpiration. In this case, approaches such as porometry or calculation of three separate years of transpiration loss sap flow will have to be used. (Calder, 1977). Soil Water Depletion The Cut-tree Technique Studies of rates of soil water depletion are usually used to estimate transpiration over timescales of at least a Excising the bases of plants, even large trees (Ladefoged, few days, and they cannot distinguish water taken up by 1963) under water and measuring water uptake, can be different species when they are growing in close proximity. used to estimate transpiration. However Roberts (1977, Sufficient measurements of soil water content need to be 1978), showed that removal of soil and root resistances made to account for spatial variation in water storage in can improve the leaf water status of cut-trees compared the soil. In addition, the amount of drainage needs to be to controls, leading to differences in stomatal conductance measured, or it must be insignificant. Provided drainage and transpiration between normal trees, and those with from and recharge to the soil is quantified or insignificant, excised roots. Nevertheless, the tree-cutting technique has changes in soil water storage allow evaporation to be proved useful in examining the water relations of mature calculated when rainfall is absent or measured separately. trees (Roberts, 1977), and the amount of water stored in Soil moisture depletion techniques require repeated in trees that can contribute to transpiration (Roberts, 1976). situ measurements of soil water content, which may be The technique has also proved particularly valuable as with a neutron probe (Bell, 1987; Dean et al., 1987), the a means of calibrating other techniques such as isotopic impedance “ThetaProbe” technique (Gaskin and Miller, tracers (Waring and Roberts, 1979). Because in the cut-tree 1996), or by using time domain reflectometry (Topp et al., technique water is drawn up through the tree in a natural 1980, 1984). way and conditions around the canopy are not modified, Micrometeorological Methods the method offers the best option for the calibration of sap flow techniques. A revealing approach to measuring evaporation, where it is possible to discriminate between evaporation from wet Sap Flow Techniques canopies and transpiration from dry canopies, exploits some of the micrometeorological methods that are available. Sap flow techniques provide a means of continuously These approaches are covered elsewhere in this volume monitoring rates of sap flow. Information about sapwood (see Chapter 40, Evaporation Measurement, Volume 1). cross-sectional area of sampled trees, or the leaf area of Following rain events, there will be a significant contribu- the sampled tree in relation to the leaf area of the forest, tion to the vapor flux from partially wetted leaves and damp enables transpiration to be estimated on a land area basis. soil and litter surfaces. The values from micrometeorolog- The range of techniques for measuring sap flow and the ical approaches to determine transpiration flux under these limitations of different approaches have been reviewed by circumstances need to be interpreted with care. Swanson (1994) and Smith and Allen (1996). 8 HYDROMETEOROLOGY

HEAT-PULSE VELOCITY THERMAL DISSIPATION The heat-pulse velocity (HPV) method determines rates of Granier (1985, 1987) proposed an alternative sap flow sap flow by determining the velocity of a short pulse of heat technique. Each probe consists of a pair of needles, which removed by the upward-moving sap stream. The technique are inserted into the sapwood. The upper needle contains is only really useful on woody stems, and the depth of a heating probe and a thermocouple, which is referenced sapwood must not be so deep that the sensor probe cannot to a second needle inserted in the sapwood lower down in sample it adequately. the stem. Continuous heating of the upper needle sets up a Each set of heat-pulse probes consists of one heater probe temperature difference (T ) between the two needles. T and two sensor probes containing miniature thermistors. is at a maximum when the sap flow is at a minimum and Typically, four sets of probes are installed at equal distances decreases as the sap flow increases. Granier (1985) found around the circumference of the stem. The heat-pulse that for two conifer species and oak, volumetric sap flux 3 −2 −1 technique is based on a compensation principle; the velocity density (uv, m m s ) is related to T by the following of sap ascending the stem is determined by correcting the relationship: 1.231 measured velocity of a heat-pulse for the dissipation of uv = 0.000119Z (3) heat by conduction through the wood matrix. In practice, this is achieved by installing the sensor probes at unequal where distances upstream and downstream of the heater probe. T − T Z = 0 (4) The upstream sensor is usually nearer the heater than the T downstream one. Heat-pulse velocity (vh) is calculated from when T0 is the value of T when there is no sap flow. − χd χu The mass sap flow rate (Fm)isthen vh = (1) 2t0 Fm = ρs uvAsw (5) where χd and χu are the distances between the heater and the upstream and downstream sensors, respectively, and t0 where ρs is the sap density and Asw is the sapwood is the time taken after the heat pulse for the temperature of cross-sectional area. Granier et al. (1990) suggest that the the two sensors to become equal again. parameters in the equation above are not dependent on wood properties or tree species and that the technique may possibly be used without calibration. However, this STEM HEAT BALANCE possibility needs testing for a wider range of species than has been the case thus far because Granier type gauges The stem heat balance (SHB) method can be used to measure sap flow in both woody and herbaceous stems, are commercially available and now very widely used. A and these can be very small in diameter. The approach has calibration rig in which heat dissipation can be measured in been used on branches, small trees, and even roots (Smith a large beech log while water is passed through at different et al., 1997). A full description of a SHB gauge is given rates is shown in Figure 7. by Smith and Allen (1996). Heat is applied to the outside Porometers and Infrared Gas Analyzers (IRGAs) of the segment of stem enclosed by the heater, and the sap flow derived from the fluxes of heat into and out of the Porometers enable measurements of stomatal conductance, heated section. Sap flow (F ) is related to the different heat gs , of individual leaves to be measured in situ. An Infrared losses from the stem section (Swanson, 1994) by Gas Analyzer (IRGA) can also be used to determine CO2 exchange from the leaf as well as gs. Additional useful Q − Q − Q − Q information that can be acquired or calculated are leaf F = h t v s gs−1 (2) Cs T transpiration rates, leaf temperatures, and the internal CO2 concentration of the leaves. gs determined with a porometer where Qh is heater power, Qv is vertical heat loss, Qr is or IRGA gives the most detailed information in both radial heat conduction, Qs is heat storage, Cs is the heat temporal or spatial scales about the environmental and capacity of the sap, and T is the temperature difference internal controls of gs , and hence transpiration. between the top and bottom of the heated section. The gs measured with porometers and IRGAs has also been stem sector heat balance method, as described by Cermak´ used to estimate transpiration from plant canopies. This et al. (1984), requires stainless steel electrode plates to be involves multiplying gs by the leaf area index to produce inserted in a tree stem. If there is substantial variation in a surface or canopy conductance (gc) · gc can be used sap flux around a large tree trunk, installations would be with a canopy boundary-layer conductance, ga, to esti- made at more than one point. mate transpiration using the Penman–Monteith equation TRANSPIRATION 9

upper layers of soil. Measurements of drying patterns in the few meters of soil below vegetation usually show the largest reduction in soil moisture in the soil surface layer with reduced changes in deeper zones. Jackson et al. (1996) reviewed the pattern of root mass and vertical distribution that had been previously reported for different terrestial biomes. The relationship proposed by Gale and Grigal (1987) was used by Jackson et al. to describe the vertical distribution of roots.

Y = 1 − βd (7)

Y is the cumulative fraction of root mass (0 < Y < 1) from the soil surface to a depth of d cm. β is a fitted Figure 7 Calibration of the output from a thermal dissi- parameter and has larger values for relatively deeply rooted pation probe in a large beech (Fagus sylvatica) log. In the vegetation. For example, β for temperate coniferous forests rig water can be passed through the log at different rates is 0.976, but 0.943 for temperate grassland. (Photo by John Roberts). A color version of this image is The concentration of roots in the soil surface is probably available at http://www.mrw.interscience.wiley.com/ehs related to a number of factors; the ready availability of plant nutrients from litter deposition and decomposition in (see Chapter 45, Actual Evaporation, Volume 1; Mon- the soil surface, and the ease with which surface roots can teith, 1965). This type of approach has been used suc- exploit small frequent inputs of rainfall from storms after cessfully even in very complex tropical rain forest canopy soil drying has progressed from a saturated condition. Roots (Roberts et al., 1993), although detailed information of in the soil surface offer the shortest pathway, that is, the the vertical distribution of leaf area density and the ver- lowest resistance for water movement into the plant, and tical changes in canopy microclimate are required, which the association of a large population of fine roots that are imposes a severe logistical constraint. associated with large structural roots that are in the soil surface for stability against wind blow. Radioactive and Stable Isotope Tracers Nevertheless, the small amount of deep roots (constitut- A number of tracers have been used to measure transpira- ing a relatively small fraction of total root biomass) have tion from branches and individual trees. These values can crucial roles in acquiring water for growth and survival dur- then be scaled up to give stand transpiration. Waring and ing drought. The tropical rainforest in the eastern part of 32 Roberts (1979) used P and tritium to measure the transpi- the Amazon basin in Brazil remains evergreen even though ration of Scots pine trees. Calder et al. (1992) described an the dry season may extend to 4 to 5 months. In this forest approach using deuterium oxide (D2O) which was injected Nepstad et al. (1994), founds roots to a depth of 18 m. The into eucalyptus trees in plantations. Transpired water was water stored below 2 m in the soil and available to the trees collected in polythene bags tied on to selected branches. provided more than 75% of the water extracted from the From the information of the D2Oinjected(M), and the con- entire profile during the dry season. centration of the transpirate produced over a known time interval (C dt), transpiration (F ) can be calculated.
∞ HOW WILL GLOBAL CHANGES INFLUENCE M = F C dt(6) TRANSPIRATION? 0 Vegetation affects the amount of evaporation from The time resolution for tracing techniques is relatively a catchment through transpiration and interception low. Transpiration values can only be resolved over a (see Chapter 43, Evaporation of Intercepted Rainfall, few days. It is, therefore, difficult to use the techniques Volume 1). Both vary with vegetation type, so a change to understand the influence of short-term environmental in vegetation due to a change in climate would have an fluctuations on transpiration. effect on (catchment) evaporation, with the effect of course depending on the extent of the change in vegetation. The SOURCES OF WATER FOR TRANSPIRATION rate of transpiration for a given plant, however, will also be influenced by climate change. Plants absorb CO2 from the In the vast majority of cases for land plants, the source of atmosphere as part of the process of photosynthesis, and water transpired by plants comes from water stored in the this CO2 is absorbed through stomata. 10 HYDROMETEOROLOGY

An increasing concentration of CO2 in the atmosphere Barber S.A. (1995) Soil Nutrient Bioavailability: A Mechanistic has two main (hydrologically relevant) effects on plants, Approach, Second Edition, John Wiley: New York, p. 413. which have been demonstrated through experiments at Beadle C.L., Neilson R.E., Talbot H. and Jarvis P.G. (1985) the leaf and plant scale. These influences are likely Stomatal conductance and photosynthesis in a mature Scots to have opposing effects on water use by vegetation. pine forest. I. Diurnal, seasonal and spatial variation in shoots. Journal of Applied Ecology, 22, 557–571. First, net photosynthesis increases with increasing car- Bell J.P. (1987) Neutron Probe Practice, IH Report No. 19, bon dioxide concentrations in the atmosphere. This is Institute of Hydrology: Wallingford. expected to increase plant growth and leaf area index. Betts R.A., Cox P.M. and Woodward F.I. (2000) Simulated This should be expected to increase vegetation transpiration responses of potential vegetation to doubled CO2 climate (e.g. Long, 1999). change and feedbacks on near-surface temperature. Global On the other hand, a second effect of CO2 enrichment Ecology and Biogeography, 9, 171–180. is to make stomata smaller, which has the effect of reduc- Calder I.R. (1976) The measurement of water losses from a ing the stomatal conductance to evaporation. The largest forested area using a ‘natural’ lysimeter. Journal of Hydrology, reduction in stomatal conductance, as much as 40%, has 30, 311–325. been found in grasses and herbaceous species, with a lower Calder I.R. (1977) A model of transpiration and interception loss reduction (∼20%) being more typical in woody vegetation from a spruce forest in plynlimon, central wales. Journal of Hydrology 33 (Saxe et al., 1998). The reductions in stomatal conduc- , , 247–275. Calder I.R., Kariyappa G.S., Srinivasalu N.V. and Srinivasa tance lowers the rate of transpiration for a given set of Murthy K.V. (1992) Deuterium tracing for the estimation meteorological inputs, and the water use efficiency (WUE: of transpiration from trees. 1. Field calibration. Journal of ratio of carbon uptake (i.e. biomass growth) to transpira- Hydrology, 130, 17–25. tion) of plants increases. This has frequently been taken Cermak´ J., Jen´ıkJ.,KuceraJ.andZ´ıdek V. (1984) Xylem water to mean that increased CO2 concentrations will lead to a flow in a crack willow tree (Salix fragilis L.) in relation to reduction in catchment scale evaporation, but this is not diurnal changes of environment. Oecologia, 64, 145–151. necessarily so. In fact, many of the few studies that have David T.S., Ferreira M.I., Cohen S., Pereira J.S. and David J.S. looked at scales larger than the plant have shown that (2004) Constraints on transpiration from an evergreen oak tree in southern Portugal. Agricultural and Forest Meteorology, 122, evaporation per unit area does not decrease as CO2 concen- trations rise, largely because the increased WUE is offset 193–205. by additional plant growth. Kruijt et al. (1999), for exam- Davies W.J. and Zhang J. (1991) Root signals and the regulation of growth and development of plants in drying soil. Annual ple, estimated that increasing CO2 concentrations would Review of Plant Physiology and Plant Molecular Biology, 42, have little net effect on transpiration over forest. In a study 55–76. using a dynamic vegetation model coupled with a climate Dean T.J., Bell J.P. and Baty A.J.B. (1987) Soil moisture model to simulate the effects of both CO2 increase and measurement with an improved capacitance technique. Part 1: associated climate change Betts et al. 2000, found that, at sensor design and performance. 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