Principles of water uptake and use

C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 33

3. Principles of water uptake and use

The resource capture principles pioneered by JLM may also be applied to water by breaking its utilisa- tion down into ‘capture’ and ‘conversion efficiency’ components. As for light, the quantity of dry matter produced W depends on the quantity of water cap- tured and the ‘efficiency’ with which it is used to pro- duce dry matter. The ratio of dry matter production to water transpired, expressed on a unit leaf area or land area basis, is known as the water use ratio e w , a term equivalent to the conversion coefficient for radiation e. Using this analogy, dry matter production may be expressed as W=e w 6 E W , where 6E W represents cu- mulative transpiration; as for light, W is often linearly related to the quantity of water transpired, indicating that e w is conservative de Wit, 1958; Azam-Ali, 1983; Connor et al., 1985; Cooper et al., 1987. This rela- tionship depends on the close linkage between CO 2 and water vapour fluxes which results from the role of stomata in regulating the exchange of both gases. However, atmospheric saturation deficit D may exert a strong modifying influence on e w , as is considered further below. Actively growing vegetation which is well supplied with water exerts little control over water use and transpires at rates determined by the prevailing evap- orative demand. The maximum rate of water use for prescribed environmental conditions has been termed potential evaporation E o , which defines the upper limit to the actual evapotranspiration E t , a term rarely used by JLM; E t is smaller than E o when ground cover is incomplete, the soil is dry or stress factors enforce stomatal closure. Although E o is determined primar- ily by atmospheric conditions, E t is influenced by attributes of the vegetation which limit transpiration. These may be subdivided into plant characteristics which influence transpiration and rooting characteris- tics which influence absorption, processes integrated by feed-forward and feed-back linkages. Control of water use at the canopy level involves both short- and long-term regulatory mechanisms, i.e. those which protect against transient or more prolonged periods of stress. Short-term responses operating over periods of minutes or hours include stomatal closure to limit transpiration and leaf movements to decrease radia- tion interception, leaf temperature and the leaf-to-air vapour pressure gradient. Longer term adjustments in the transpiring area are achieved over periods of days or weeks through a combination of premature senescence and reduced production andor expansion of new leaves and shoots. Irrespective of how they are achieved, reductions in transpiration are generally accompanied by decreased assimilation and growth. Transpiration is influenced by the canopy and aero- dynamic conductances g c and g a or their reciprocal resistances r s and r a . g c takes account of the phys- iological and morphological attributes of the canopy since it is usually calculated as the product of leaf area index L and leaf conductance g l for the various lay- ers within the canopy Azam-Ali, 1983; Black et al., 1985. Leaf sheaths, pods, panicles and other green or- gans must be included in such calculations since they contribute up to 30 of g c and canopy transpiration, and 15 of all water transpired during the cropping cycle Batchelor and Roberts, 1983. Water use is fre- quently controlled by regulation of canopy size rather than leaf conductance during sustained drought Ong et al., 1996; thus g c commonly determines transpira- tion from open, stressed or senescent canopies, while g a may be limiting in dense canopies, particularly at low windspeeds. The balance between transpiration and absorption depends on both soil and atmospheric conditions. Well-watered vegetation generally transpires at rates close to the prevailing potential evaporation. Studies at ICRISAT by members of JLM’s group showed that in monsoon climates with E o values of ca. 5 mm per day this requires mean absorption rates of 1.7–2.5 g H 2 O m − 1 root per day for groundnut or millet stands with rooting densities of 2–3 km m − 2 of land area Gregory and Reddy, 1982. In this and similar studies of rice Oriza sativa; Yoshida and Hasegawa, 1982 and cassava Manihot esculenta; Aresta and Fukai, 1984, absorption was probably limited by E o rather than rooting or soil characteristics, whereas the con- verse applies in dry soil where absorption depends more closely on the size and distribution of the root system and soil water content than on atmospheric conditions. For example, Squire 1990 reported that transpiration from groundnut and millet grow- ing on drying soil was only 2–4 mm per day, even though total root lengths were greater than under well watered conditions. Thus, millet growing on stored water in Niger required a mean absorption rate of 0.65 g H 2 O m − 1 root per day over a total root length of 34 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 5.9 km m − 2 to support a transpiration rate of 3.8 mm per day; mean absorption rates were therefore up to four times lower than in moist soil. In some instances, it may be difficult to establish whether transpiration is limited primarily by above- or below-ground factors. For example, the simultaneous senescence of leaves and roots during grain filling in determinate crops affects both leaf area index and root length at a time when the functional efficiency of these organs is also declining; in such cases it is difficult to disentangle the causes and effects underlying observed changes in water use. Partitioning of water use between the components of intercropping and agroforestry systems has posed a continuing challenge. Three broad approaches may be used to determine water use and e w for the compo- nents of multispecies communities: 1 transpiration by each component is measured separately; 2 total community water use and transpiration by one of the components are measured, leaving transpiration by the other component to be calculated as the difference; 3 transpiration may be estimated using transpiration models based on radiation interception by each com- ponent. Approach 1 is preferable since the values for each component are determined separately and so are subject only to the errors inherent in the techniques involved. In approach 2, the estimates for each com- ponent are not statistically independent and the values for the component derived by difference are influenced by two sets of errors, those for community water use and those for the component whose transpiration was actually determined. However, until recently, option 2 was widely regarded as the only realistic approach because the available methods were technically too demanding, labour-intensive or expensive for routine season-long measurements. Approach 3 assumes that the Penman–Monteith equation see below may be modified to calculate water use by each component provided its radiation interception can be measured or estimated; transpiration may then be calculated using single or dual source transpiration models. Substantial progress has recently been made in this area. For ex- ample, Shuttleworth and Wallace 1985 adapted the Penman–Monteith equation to allow for the interac- tive energy fluxes between the soil and sparse crops, while Wallace 1995 proposed further modifications based on fractional interception by the components of two-species agroforestry systems which offer promise. Several studies have attempted to partition wa- ter use in agroforestry systems by determining total community water use using the soil water balance approach, despite the practical limitations outlined below, and measuring transpiration by one of its components e.g. the leucaenamillet and perennial pigeonpeagroundnut systems described by Corlett et al., 1992a,b and Ong et al. 1996. In the water bal- ance approach Wallace 1996 and Ong et al. 1996 for reviews, total community water use is determined indirectly from the balance of all other components of the water balance, namely precipitation, inter- ception losses from the tree and crop canopies, soil evaporation, deep drainage and changes in soil wa- ter content within the rooting zone. Several of these terms are difficult or laborious to determine since they exhibit extensive spatial and temporal variation within multispecies communities, introducing sub- stantial uncertainties into estimates of the combined transpiration of trees and crops. Soil water content is generally determined using the neutron probe or time domain reflectometry TDR approaches, while tree or crop transpiration may be measured using diffusion porometry, chamber systems, deuterium labelling or sap flow techniques. Porometry and chamber systems allow daily or seasonal time courses of water use to be constructed, but the measurements are discontin- uous and labour-intensive; these approaches are also unusable when the foliage is wet, a major problem in monsoon climates. Deuterium labelling avoids these problems and permits transpiration to be measured over periods of several days, but is relatively ex- pensive and requires specialised analytical facilities. However, the advent of inexpensive and reliable sap flow techniques suitable for field use has allowed con- tinuous, non-destructive measurements to be made for both trees and established crops over extended periods, as is discussed below. A major problem with the water balance approach is that it may be difficult to monitor soil water con- tent throughout the rooting zone due to the deep root- ing habit of trees and technical difficulties resulting from the presence of stony or compacted horizons. This was the case for the leucaenamillet and grevil- leamaize systems examined at ICRISAT and ICRAF, with the result that the exploitation of deep water C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 35 reserves by the trees could not be fully quantified, causing community water use to be underestimated to an unknown extent. However, an excellent exam- ple of the successful application of the water balance approach was reported by Eastham et al. 1988, who installed access tubes to a depth of 5.6 m at various distances from Eucalyptus grandis trees in a silvopas- toral system in Queensland, Australia. Measurements of soil water content and concurrent information on rainfall and deep drainage allowed community water use to be calculated on both a short term and seasonal basis. Relationships between transpiration from the pasture, open-pan evaporation and soil water content established using lysimeters enabled transpiration by the pasture to be calculated routinely, and the values obtained were subtracted from community water use to estimate tree transpiration. This approach was used to show that transpiration was dominated by the pas- ture component when the trees were widely spaced but the converse applied at higher densities, when the trees formed a closed canopy. Water abstraction at depth increased with tree density and cumulative seasonal transpiration greatly exceeded rainfall at the highest tree density in both years examined, suggesting that the system was not sustainable Eastham and Rose, 1990a,b. The development of sap flow techniques which pro- vide direct, non-destructive measurements of transpi- ration by intact plants over extended periods has been an important factor in improving our understanding of resource partitioning in mixed communities. A range of relatively straightforward and inexpensive sap flow techniques now exist which permit continuous mea- surements of transpiration in species varying in size from small herbaceous plants such as rice Sakuratani, 1990 to large forest trees Granier et al., 1996, of- ten with a claimed accuracy of ±5–10. A detailed review of the principles and application of sap flow techniques is given by Smith and Allen 1996, while Granier et al. 1996 evaluated the merits of several heat-based approaches. The heat pulse technique de- veloped by Hüber and Schmidt 1937 is the oldest sap flow method, but is still widely used for both small plants and large trees, and has recently been adapted for use with tree roots Khan and Ong, 1996. Swanson and Whitfield 1981 developed a theoretical correc- tion for the disturbance caused by the presence of the sensor probes within the stem which avoids the need for calibration. The technique is particularly valuable for measuring sap flow in wood of differing age within the trunk, and integration over the entire sapwood area can be achieved using multipoint probes. Cermak et al. 1973 developed a method known as the trunk heat balance, which is suitable for large trunks, its main advantage being that sap flow is calculated from the energy balance of a sector of functional xylem. The heat balance approach initially devised by Viewig and Ziegler 1960 and later refined by Sakuratani 1981 and Baker and van Bavel 1987 uses an external jacket to supply heat at a known rate; the dissipation of this energy, or heat balance, is then resolved to estimate the convective flux resulting from heat transfer in the xylem sap. Heat balance gauges may be used reli- ably without direct calibration provided the effective thermal conductivity of the system can be established from measurements made under zero flux conditions, but calibration against absolute, usually gravimetric, measurements of transpiration has often been adopted for increased rigour. Heat balance and heat pulse systems have been used routinely for over a decade at ICRISAT and ICRAF in various agroforestry systems Marshall et al., 1994; Howard et al., 1995, 1997; Lott et al., 1996. The sap flow approach has the major benefit of allowing tran- spiration by each component of agroforestry systems to be followed continuously and reliably. For example, Fig. 3 shows typical diurnal time courses for transpira- tion from the line-planted and dispersed arrangements of perennial pigeonpea referred to previously. No dif- ference between treatments was apparent during the dry season Fig. 3a, shortly after the trees had been cut for fodder and their leaf area was small. However, during the rainy season Fig. 3b, transpiration was five times greater at midday in the dispersed pigeon- pea which, because of its wider spacing and larger canopy, was able to exploit the soil profile more fully. Transpiration tracked the diurnal time course for at- mospheric saturation deficit, although irradiance, soil water content and leaf area were also important in de- termining water use. The daily values obtained in this way were used to calculate cumulative transpiration over extended periods Fig. 3c, again emphasising the consistently greater water use of the dispersed pigeon- pea, which accounted for 60 of the annual rainfall as compared to 30 in the line-planted treatment; these values exclude water use by the understorey ground- 36 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 Fig. 3. Typical diurnal time courses of transpiration and leaf to air vapour pressure difference for perennial pigeonpea during a the dry and b the rainy seasons, and c the seasonal time courses for cumulative transpiration modified from Marshall et al., 1994. nut. Although cumulative transpiration was two-fold greater in the dispersed system, the water use ratio e w differed by 10, with the result that the greater productivity of the dispersed pigeonpea was directly linked to its greater water use Ong et al., 1996. In recent studies at ICRAF, the heat balance and heat pulse techniques were modified to measure water movement through the tap or major lateral roots of trees in an approach which has proved invaluable in establishing the quantity of water extracted from the crop rooting zone, and hence the below-ground com- petitive impact of trees on crop performance. The time courses for sap flow through the lateral roots of gre- villea presented in Fig. 4 Lott et al., 1996 show con- siderable diurnal variation and indicate that sap flow decreased with increasing distance from the trunk; the C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 37 Fig. 4. Diurnal time courses of sap flow in lateral roots at distances of 50 r , 115 j and 190 cm m from the trunks of Grevillea robusta at Machakos, Kenya modified from Lott et al., 1996. trees nevertheless extracted significant quantities of water from the crop rooting zone up to 190 cm from the trunk. Experiments in which the lateral roots were severed indicated that these 3 year old trees were capable of extracting up to 80 of their water require- ments from beneath the crop rooting zone, in agree- ment with studies of the same species in which the soil around the trees was excavated to a depth of 60 cm Howard et al., 1997. Studies of this type to establish the influence of soil and microclimatic factors on wa- ter use contribute significantly to our understanding of the success or failure of specific agroforestry sys- tems, and also provide valuable information for the development and validation of improved resource capture and tree and crop growth models. The development of the Penman–Monteith equa- tion Monteith, 1963 marked a major landmark in our ability to model evapotranspiration from vege- tated surfaces Campbell, 2000. In 1948, Penman Penman, 1948 proposed a model for determining potential evaporation E o from open water surfaces based on physical variables which were either readily measurable or available from meteorological records. He later modified the model to estimate evapotrans- piration E t by including a term describing the ratio of E t from well-watered turf to evaporation from an open water surface E t E o , and establishing a dry- ing curve to account for the influence of drought on E t Although the Penman model proved suitable for determining E o , estimates of E t were less satisfac- tory, particularly during drought. A major advance in the Penman–Monteith model was the inclusion of boundary layer and canopy resistance terms r a and r c , respectively to take account of the environmental and physiological factors which influence transpi- ration, thereby removing the principal limitation to the effectiveness of the earlier Penman model. Infor- mation on net radiation R n , atmospheric saturation deficit D, air temperature T a and windspeed U are also required. The Penman–Monteith equation has subsequently been widely adopted to estimate evapo- transpiration from open water surfaces and uniform full-cover canopies in a range of agricultural and nat- ural systems. However, the equation is less useful at a regional scale where surfaces are characterised by a patchy combination of vegetation and soil, partic- ularly in arid and semi-arid regions. Difficulty may also be experienced when applying the approach to mixed or discontinuous vegetation due to the difficulty of obtaining suitable values for r a and r c . However, Leuning and Foster 1990 successfully estimated transpiration from single trees using a ventilated cham- ber, leaf energy budgets and the Penman–Monteith equation, but suggested that multiple net radiometers 38 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 and stratified sampling of the boundary layer and stomatal resistances should be adopted. The Penman–Monteith model was first linked to re- motely sensed measurements by Jackson et al. 1981, who combined the model with a one-dimensional energy balance equation to obtain estimates of maxi- mum and minimum canopy temperatures; by compar- ing these with measured foliage temperatures, they were able to estimate the ratio of actual to potential evapotranspiration and infer the severity of water stress. This approach was used to develop a crop wa- ter stress index CWSI which has subsequently been adopted for practical applications such as irrigation scheduling and yield prediction. Moran et al. 1994 suggested that CWSI could be modified for partly vegetated surfaces by includ- ing measurements of surface reflectance as well as surface temperature. Based on the Penman–Monteith equation, they calculated the maximum and minimum soil temperatures associated with maximum and min- imum evaporation rates and coupled these values to a spectral vegetation index which was linearly corre- lated with ground cover to derive a water deficit index WDI covering all possible combinations of water availability and ground cover. They also demonstrated the value of the WDI approach for agricultural ap- plications at the field scale, with an emphasis on irrigation scheduling. Moran et al. 1996 extended this approach by using remotely sensed measurements of surface reflectance and temperature to enable the Penman–Monteith theory to be applied to partly vegetated semi-arid grassland at the local or regional scale without a pri- ori knowledge of percentage groundcover or canopy resistance. They linked the Penman–Monteith equa- tion to the energy balance equation to estimate the surface temperatures associated with four states: bare soil or full cover vegetation with evaporation rates set at zero or potential. Linear extrapolations between the soil temperatures for full ground-cover and bare soil conditions were used to provide information for the intermediate states based on measurements of surface reflectance and temperature. They concluded that this approach has potential for mapping evapo- ration rates from heterogeneous landscapes and sug- gested that, provided the vegetation type is known, the required inputs are spatially distributed measure- ments of the meteorological variables required for the Penman–Monteith equation, remotely sensed mea- surements of surface temperature and the soil-adjusted vegetation index, and estimates of surface roughness. Alternative approaches have been used success- fully to model the impact of seasonal water shortage on water use and photosynthesis. For example, Sala and Tenhunen 1996 used a mechanistically-based C3 photosynthesis model coupled with an empirical stomatal model and a canopy model of light intercep- tion and microclimate to simulate transpiration and net photosynthesis by an oak forest in northern Spain. Effective predictions of the diurnal and seasonal time courses were achieved by altering a single variable within the model, a dimensionless factor describing the relationship between stomatal conductance and as- similation rate, relative humidity and CO 2 partial pres- sure at the leaf surface. This factor was linearly related to predawn xylem water potential, a feature which proved useful in allowing canopy-level assessments of seasonal water use and CO 2 fluxes. The model predic- tions of water use agreed to within 10 with exper- imentally determined values, even though the former excluded transpiration by the understorey vegetation. The modelling approach therefore provided a realistic description of the diurnal and seasonal patterns of leaf and canopy responses as affected by water availability. Tournebize et al. 1996 also used a mod- elling approach to estimate water losses from a hedgerowpasture system containing Giricidium sepium and the C4 grass, Dichantium aristatum. Wa- ter losses were estimated by determining the energy balance of each component using a detailed model of radiation transfer capable of estimating partitioning between species Sinoquet and Bonhomme, 1992 and a simplified model of heat and mass transfer. The energy balances were calculated from micrometeo- rological data recorded within the canopy, while the stomatal conductances required to calculate transpi- ration were modelled as a function of photosynthet- ically active radiation. This approach allowed water losses to be estimated separately for the soil, grass and trees; good agreement was obtained between the modelled and measured time courses of transpiration from the monocrop grass and Giricidium irrespective of climatic conditions. Soil evaporation contributed up to 76 of evapotranspiration, but was lower in the agroforestry system because the trees reduced the quantity of radiation reaching the soil. C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 39 The importance for crop production in water-limited environments of maximising both the quantity of water available and the proportion used for transpiration has already been alluded to, but the ‘efficiency’ with which absorbed water is used for dry matter production is also critical. As for the radiation conversion coefficient e, the water use ratio e w may be calculated over time scales ranging from instantaneous measurements of the ratio of net CO 2 and water vapour fluxes to sea- sonal estimates based on dry matter accumulation and water use. Long-term estimates are invariably lower than short-term values determined under favourable conditions because of respiration, which may consume up to 50 of the photosynthate produced Ong et al., 1996, and the impact of adverse environmental e.g. drought, nutrient availability and biological factors e.g. pests, diseases. Below-ground biomass is rarely included in calculations of e w , while above-ground biomass often declines as crops approach maturity due to senescence, leading to underestimation of e w . As for e, e w may be expressed as the ratio of biomass, yield or energy equivalents to the quantity of water consumed; the first two options are often preferred because of their relative simplicity, but the latter is important when comparing species with contrasting chemical composition, such as grain and oil crops. A further factor is that e w should be calculated on the ba- sis of transpired water rather than evapotranspiration, since water evaporated from the soil has no direct role in dry matter production although it may influence the atmospheric saturation deficit experienced by the crop and hence e w . Soil evaporation dominates evapotran- spiration during the early part of the growing season for annual crops and may comprise 30–60 of sea- sonal water use depending on the rate of canopy de- velopment and maximum leaf area Wallace, 1991. The e w values for tropical C4 cereals are often a little more than double those for C3 species under equivalent conditions. For example, experiments in In- dia under similar mean atmospheric saturation deficits 2.0–2.5 kPa provided season-long values of 3.9 and 4.6 g kg − 1 for millet Squire et al., 1984, compared to 1.5–2.0 g kg − 1 for groundnut Ong et al., 1987; Matthews et al., 1988; Azam-Ali et al., 1989. How- ever, e w is not invariably higher in C4 species, since similar values have been reported for drought tolerant C3 species such as cowpea and cotton Gossypium barbadense and relatively drought-sensitive cultivars of the C4 species, sorghum and maize Rees, 1986. As for e, direct comparisons between studies may be complicated by variation in factors such as atmo- spheric saturation deficit D. For example, Squire 1990 reported that the seasonal mean e w value for groundnut decreased from 5.2 to 1.5 g kg − 1 as mean daytime D increased from 1 to 2 kPa. Similarly, e w for millet decreased from 6.4 to 2.1 g kg − 1 as D in- creased from 1.4 to 4.0 kPa Azam-Ali et al., 1984; Squire et al., 1984. Squire 1990 concluded that D is one of the most important factors limiting produc- tivity in dryland areas since dry matter production decreased at least twofold as D increased from 1 to 4 kPa. Monteith 1986 suggested that the product of e w and D e w D is often conservative, while Loomis and Connor 1992 used an analogous approach to adjust e w for seasonal differences in relative humidity to show that the adjusted values for C4 cereals were at least double those for C3 species; the adjusted val- ues showed little benefit of intensive 50-year breeding programmes when calculated on the basis of total biomass, but did when computed against economic yield because of increased partitioning to the grain. Thus e w values depend on the time scale over which they are calculated, whether they are corrected for seasonal or site-specific differences in D, and whether they are based on above-ground biomass, final yield or energy content; the wide range of reported values em- phasises the need for consistency and care in the cal- culation and interpretation of data for e w . For instance, Vijaya Kumar et al. 1996 showed that e w decreased with lateness of planting and exhibited substantial inter-annual variation when castor beans were grown under semi-arid conditions in India; the values ranged from 0.88–1.31 g kg − 1 and were negatively correlated with D and air temperature. They concluded that accu- rate prediction of crop growth would require the effects of weather to be incorporated into simulation models. As for light, intercropping and agroforestry offer substantial scope for spatial and temporal complemen- tarity of water use resulting from improved exploita- tion of available supplies. However, the opportunity for significant complementarity is likely to be limited unless the species involved differ appreciably in their rooting patterns or duration; the former permits at least partial exploitation of different soil volumes, while the latter allows continued abstraction of moisture following harvest of the shorter duration component. 40 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 Total water use by intercrops is frequently little differ- ent from monocrops, particularly when water losses from land left bare after harvesting the shorter duration component are taken into account Morris and Garrity, 1993. These workers reviewed several experiments in which seasonal rainfall varied between 84 and 575 mm and concluded that water use by intercrops was gen- erally within ±7 of equivalent monocrops, although larger benefits were occasionally observed. For exam- ple, Natarajan and Willey 1980a,b found no differ- ence in water use between monocrops and intercrops of pigeonpea and sorghum up to the point when the shorter duration sorghum was harvested. However, the longer duration pigeonpea extracted a further 170 mm prior to harvest 10 weeks later, utilising residual water and late-season rains that would otherwise have been lost. Nevertheless, although total water use by the in- tercrop greatly exceeded that of monocrop sorghum, there was no advantage over monocrop pigeonpea. In a similar study, Reddy and Willey 1981 showed that water use by milletgroundnut intercrops exceeded that of the monocrops, primarily because a larger leaf area index was maintained for longer. Substantial improve- ments in water use may also occur when species with complementary root distributions are used, as in the ricepigeonpea system described by Jena and Misra 1988 in which the deep rooting pigeonpea extracted substantial quantities of water from below the rooting zone of rice Oriza sativa. The beneficial effect of intercropping generally originates from improvements in e w rather than sea- sonal water use, to which several factors may con- tribute. Intercropping may increase the proportion of available water that is used for transpiration because the presence of a C4 cereal results in more rapid canopy development and reduced soil evaporation; however, this potential benefit must be offset against the risk of premature depletion of available water prior to maturity. Fast growing C4 species with inherently high e w values may also capture more of the avail- able water, thereby increasing the overall yield and e w value for the system. The dominant and subsidiary components of many intercrops are, respectively, C4 and C3 species with relatively high and low e w val- ues. Finally, intercropping may confer microclimatic benefits including partial shade and reduced evapora- tive demand on the shorter component, frequently a C3 species with a relatively low photosynthetic light saturation point; in such instances, partial shade may have little effect on assimilation, with the result that e w is improved by the concurrent reductions in tran- spiration. These comments are less applicable to agro- forestry, in which the overstorey trees are invariably C3 species and the understorey species may be C4 cereals which do not respond favourably to shading.

4. Thermal time