Maize canopies under two soil water regimes

Agricultural and Forest Meteorology 89 Ž1998. 201–213

Maize canopies under two soil water regimes III. Variation in coupling with the atmosphere and the role of leaf area index Pasquale Steduto a

a,b

, Theodore C. Hsiao

a,)

Department of Land Air and Water Resources, UniÕersity of California, DaÕis, CA 95616, USA b Istituto Agronomico Mediterraneo, Bari, 70010, Italy

Received 30 October 1996; received in revised form 1 September 1997; accepted 7 October 1997

Abstract The degree of coupling between the plant canopy and the atmosphere is indicative of the ability of the two systems to exchange momentum, energy, and mass. In terms of water vapor and CO 2 exchange, it characterizes the extent to which stomatal and canopy conductance may control transpiration and CO 2 assimilation. In the present work, the degree of coupling of maize fields under contrasting soil water regimes ŽWET and DRY. was investigated over daily time courses and over the season at Davis, CA. Water vapor fluxes were determined by the Bowen-ratiorenergy-balance technique ŽBREB.. From wind velocity measurements above the canopy, and using the Penman–Monteith big leaf model, aerodynamic Ž g a . and canopy Ž g c . conductances for water vapor were computed. Coupling for water vapor exchange was quantified by calculating the decoupling factor Ž V . from g a and g c . During the daily cycles, with wind typically being calm in the morning and increasing at midday and in the afternoon, V followed skewed patterns somewhat similar to diurnal g c , peaking in the morning or before noon and falling to zero at sunrise and sunset. After the closed canopies reached their full height of 3–3.2 m and before senescence, midday values of V ranged between 0.4 Žmoderately coupled. and 0.8 Žpoorly coupled. for both the WET and DRY treatments and went barely below 0.4 even on the most windy day Ž4 to 5 m sy1 .. After the area of green leaves Žleaf area index, LAI. decreased substantially due to senescence, midday V began to drop below 0.4, reaching the range of 0.1 at the end of the crop cycle. The drop in V was largely the result of reductions in g c , which in turn was correlated with LAI wSteduto, P., Hsiao, T.C., 1998b. Maize canopies under two soil water regimes: II. Seasonal trends of evapotranspiration, carbon dioxide assimilation and canopy conductance, and as related to leaf area index. Agric. For. Meteorol. 89, 189–203x. Before senescence, calculated equilibrium evaporation was below actual evapotranspiration, which in turn was below the calculated imposed evaporation. With senescence, g c decreased and imposed evaporation fell below equilibrium evaporation at the point when g c fell below 12 mm sy1. The point of crossover between imposed and equilibrium evaporation curves appears to define more clearly a critical point. Above the value of g c corresponding to the critical point, actual evaporation is hardly affected by changes in g c ; below that value, g c exerts a stronger and stronger modulating effect. The finding of a low degree of coupling between the maize crop Žof moderate to high LAI. and the atmosphere in spite of the tall stature of the crop Ž3–3.2 m. was somewhat unexpected, as earlier analyses had related V mostly to plant height. Additional investigations are needed to characterize further the degree of coupling of different crops,

)

Corresponding author. Department of Land, Air and Water Resources, Veihmeyer Hall, University of California, One Shields Avenue, Davis, CA 95616, USA. Tel.: q1-530-752-0691; fax: q1-530-752-5262; e-mail: [email protected] 0168-1923r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 1 9 2 3 Ž 9 7 . 0 0 0 8 3 - X

202

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

especially as related to plant height, leaf area index, severity of water stress, and crossover points between imposed and equilibrium evaporation. q 1998 Elsevier Science B.V. Keywords: Maize canopies; Soil water regime; Atmosphere

1. Introduction The ability to exchange momentum, energy, and mass between a leaf and the atmosphere is an expression of the coupling between the two systems ŽMonteith, 1981.. Conceptually, a leaf is perfectly coupled with the ambient air when its boundary layer resistance is zero, so that air temperature, saturation deficit of water vapor, and CO 2 concentration of its surface are the same as ambient values. At the other extreme, a leaf is perfectly uncoupled from ambient air when an infinite boundary layer resistance exists so that no mass exchange may occur. While the boundary layer resistance determines the coupling for individual leaves, the aerodynamic resistance determines the coupling for crop fields. The degree of coupling, by influencing the surface conditions of the leaves, will characterize the extent to which stomata may control transpiration and assimilation. Penman and Schofield Ž1951. and Bange Ž1953. observed early that in some circumstances the feed-back between plants and their immediate aerial environment may diminish the dependence of transpiration on stomatal conductance because of the smaller vapor gradient between the intercellular air space and the epidermal surface of leaves occurring under a low degree of plant–atmosphere coupling. As pointed out by Jarvis Ž1985a,b., the degree of coupling assumes special relevance when comparing different kinds of vegetation Žshort vs. tall, smooth vs. rough., when extrapolating from one situation to another Žspaced plants in pots vs. dense populations, small plots vs. large plantings, glass houses vs. open fields., and when using results from gas exchange cuvettes and chambers to draw inferences for open field conditions, especially when scaling up scalar transport from leaves to canopies. The concept of plant–atmosphere coupling was systematically developed by McNaughton and Jarvis Ž1983. and Jarvis and McNaughton Ž1986., who quantified the degree of coupling for water vapor exchange by calculating a decoupling factor V which varies between zero Žextreme case of totally coupled

conditions. and one Žextreme case of totally decoupled conditions.. Unfortunately, it is not yet feasible to derive a similar factor to quantify the degree of coupling for the exchange of CO 2 . A criterion suggested by Jarvis Ž1985b. is the comparison of the o . CO 2 concentration at the crop surface Ž PCO with 2 a . Ž that in the bulk air PCO 2 . Recently, McNaughton and Jarvis Ž1991. revised the theoretical context of the coupling concept, using control theory ŽDi Stefano et al., 1988; Aubinet et al., 1989. as a framework. Notwithstanding the importance of the concept, up to now coupling has been quantified experimentally for only very few types of vegetation Že.g., McNaughton and Jarvis, 1983; Miranda et al., 1984; Meinzer and Grantz, 1989; Kostner et al., 1992; Lee ¨ and Black, 1993; San Jose´ et al., 1995; Granier and Breda, 1996.. Some estimates of V have been indicated for crops of different height and leaves of different sizes ŽJarvis, 1985a., and more generally for different types of vegetation ŽMeinzer, 1993., with attempts to use V for the parameterization of vegetation on a landscape scale ŽPinty et al., 1992.. The objective of this study is to analyze the diurnal and seasonal variation of coupling for water vapor exchange of a maize crop grown under two soil–water regimes, with particular attention to the maturation and senescing phase. The original treatment of coupling as described by McNaughton and Jarvis Ž1983. was used in the analysis. To avoid some potential pitfalls pointed out by Paw U and Gao Ž1988., net radiation was measured. 2. Theory The starting point for the coupling analysis is the Penman–Monteith ŽP–M. combination equation ŽMonteith, 1973.,

lEs

s Ž yR n y G . q ra c p g a Ž e ) y ea . ga sqg 1q gc

ž

/

Ž 1.

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

where s is the slope of the function relating saturation vapor pressure to temperature ŽkPa Ky1 ., R n is net radiation flux ŽW my2 ., G is soil heat flux ŽW my2 ., ra is air density Žmol my3 ., c p is air specific heat capacity ŽJ moly1 Ky1 ., e ) is saturation vapor pressure at air temperature ŽkPa., ea is actual saturation vapor pressure of the air ŽkPa., g is the psychrometric constant ŽkPa Ky1 ., g a is aerodynamic conductance Žm sy1 ., and g c is canopy conductance Žm sy1 .. By taking limits of Eq. Ž1. for g a tending to zero, and to infinity, one obtains, respectively s lim l E s Ž yR n y G . Ž 2. sqg g a™0 and lim l E s

ra c p g c Ž e ) y ea .

Ž 3.

g

g a™`

The limit for l E as g a ™ 0 is known as equilibrium evaporation Ž l Eeq ., which depends only on the available radiative and storage energy. The limit for l E as g a ™ ` is known as imposed evaporation Ž l Eimp ., which depends on the saturation deficit and g c only. McNaughton and Jarvis Ž1983. defined a decoupling factor Ž V . as

Vs



eq1 eq1q

ga gc

0

Ž 4.

with ´ s srg , so that lim V s 1 and lim V s 0. g a™0

g a™`

The P–M equation then becomes

l E s Vl Eeq .q Ž 1 y V . l Eimp .

Ž 5.

V , with its value varying between 0 and 1, indicates the relative importance of the l Eeq vs. l Eimp in determining actual l E. It also indicates the extent canopy conductance controls transpiration, in as much as the value of V reflects the importance of g a relative to g c . Once V is derived, the relative change in transpiration of a crop stand Ž l E ., for a given level of available energy ŽyR n y G ., is related to the relative change in canopy conductance Ž g c . through the imposed evaporation Ž l Eimp .. In their analysis, McNaughton and Jarvis Ž1983. and Jarvis and McNaughton Ž1986. implicitly used the simplifying assumption that ŽyR n y G . is inde-

203

pendent of surface temperature as g a varies. Because longwave radiation from the surface increases with increases in surface temperature, and changes in g a affect latent and sensible heat exchange and thus the energy balance and temperature of the surface, this assumption may lead to an underestimation of the degree of coupling ŽPaw U and Gao, 1988.. The underestimation becomes more severe as g a becomes smaller, and temperature differences between the surface and the atmosphere gets larger. One solution to the problem is to measure R n under the conditions of altered g a . The other is to extend the formulation of V to account for the temperature changes. Martin Ž1989. allowed for temperature changes with changes in g a by working with isothermal net radiation and radiative conductances, two parameters introduced by Monteith Ž1973.. Paw U and Gao Ž1988. recognized that the inaccuracy due to changes in R n as the result of changes in surface temperature would be minimized or eliminated if instead of the P–M equation, which contains the linearized form of the saturation deficit vs. temperature function, a more accurate approximation is used. In fact, they showed that the use of the quadratic or quartic equations in place of the linearized form of the saturation deficit yields the limit of l E as g a ™ 0 lim l E s Ž yR n y G .

g a™0

Ž 6.

while the limit of l E as g a ™ ` ŽEq. Ž3.. remains unchanged. Thus, the difference between the linear and non-linear limits is the term w srŽ s q g .x, which can be considered as an error term and can be relatively large. The error starts to be significant at very low aerodynamic conductances Ž g a F 10 mm sy1 , or when V ) 0.8.. These, however, are situations hard to find in open fields except for very limited periods of time. In parallel with the coupling concept for latent heat transport, Jarvis Ž1985b. described a concept for o a the coupling between PCO and PCO by deriving 2 2 analogous expressions of ‘imposed’ and ‘equilibrium’ assimilation rates. A leaf perfectly coupled to the atmosphere would have the CO 2 concentration of the ambient air at its surface. In this case, assimilation would proceed at the imposed rate. A leaf completely decoupled from the atmosphere would

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

204

have its surface CO 2 concentration falling to the CO 2 compensation point, and net CO 2 flux through the stomata would decline to zero. In that case assimilation proceeds at an ‘equilibrium rate’, depending only on temperature and photosynthetic active radiation, using the CO 2 evolved through respiration. o a will depend not only on PCO , representing PCO 2 2 the ‘supply’, but also on the ‘demand’ for CO 2 represented by the CO 2 concentration at the interceli . set jointly by the carboxylation lular space Ž PCO 2 activity and epidermal Žmostly stomatal. conductance of the leaf. Furthermore, while there is a close parallel between coupling of surface CO 2 and water vapor to an air reference point above the canopy, there is less influence of vegetation on ambient CO 2 in comparison to the influence of vegetation on water vapor ŽJarvis, 1985b.. Anyway, while for water vapor transport the degree of coupling can be expressed by the V factor, in the case of CO 2 transport there is no equivalent of V , and the meao a itself, by comparison with PCO , could sure of PCO 2 2 provide a good indication of the degree of coupling. Taking the canopy as a big leaf ŽMonteith, 1973. o can be estiand assuming steady-state flux, PCO 2 a mated from PCO 2 and the canopy CO 2 flux Ž A cf . as o a PCO s PCO y 2 2

A cf ra

ž / Ma

planting ŽDAP. in 1989 and 54 DAP in 1990, while continued on the other half ŽWET treatment.. Additional information on soil status, seasonal weather Žsolar radiation, air temperature, vapor pressure deficit, wind, precipitation, and reference evapotranspiration., crop characteristics Žarea of green leaves per unit land area s LAI, leaf water potential, and net canopy assimilation., are reported in the second paper of this series ŽSteduto and Hsiao, 1998b.. Water vapor fluxes were measured by the Bowen-ratiorenergy-balance technique ŽBREB. at 5-min intervals. The BREB measurement has been described in some detail in the subsequent paper ŽSteduto and Hsiao, 1998c., along with evidence supporting the short signal averaging time of 5 min. Aerodynamic conductance was calculated from wind data and estimated zero plane displacement and roughness parameter of the crop, as detailed in Steduto and Hsiao Ž1998a.. The excessive resistance to the water vapor flux relative to the momentum flux was accounted for ŽThom, 1972, 1975. by adding a term in the denominator of the equation for calculating momentum conductance, so that aerodynamic conductance for water vapor Ž g aw . is uz k2

g aw s

Ž 7. g ac

where g ac is aerodynamic conductance for CO 2 , which is smaller than that for water due to the ‘excess resistance’ for the CO 2 scalar ŽThom, 1972., and Ma is the molar mass of air.

3. Materials and methods The experiment was carried out at Davis, CA. Maize Ž Zea mays L.. was planted in large fields on a deep and relatively uniform soil of high water-holding capacity ŽYolo silty clay loam., well fertilized and wetted to field capacity by prior irrigation. Planting was on May 25 in 1989, and on May 14 in 1990. Irrigation was carried out regularly on the whole field only up to before tasselling, but was cut off on one half of the field ŽDRY treatment. 51 days after

ln

zyd

ž / zo

2

q Ž 1.35u) 1r3 . k ln

zyd

ž / zo

Ž 8. where z is height above the ground, u z is wind velocity at height z, d is zero plane displacement, z o is roughness length, k is the von Karman constant Ž0.42 used., and u) is friction velocity. The added term is Ž1.35u) 1r3 . k lnwŽ z y d .rz 0 x, which reduced g aw to about two thirds of the size of conductance for momentum Ž g am .. For the calculation of CO 2 flux, the same equation was used, except that the term Ž1.35u) 1r3 . in the denominator was replaced with Ž2.18u) 1r3 . in accordance with Thom Ž1972.. Actual midday differences between g am and g aw are reported in Fig. 3 of Steduto and Hsiao Ž1998b.. Canopy conductance was calculated by inverting the P–M equation ŽEq. Ž1.. with g aw taken as g a . The results reported here are based on data obtained in 1990, when plant density was 11.9 my2 .

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

4. Results and discussion 4.1. Diurnal and seasonal trend in coupling quantified as V In the two preceding papers ŽSteduto and Hsiao, 1998a,b., the component of energy balance and the associated parameters, including air temperature, g a , and g c , were either measured or calculated from the primary data. Using these results, V , the decoupling factor, was calculated according to Eq. Ž4. as given by McNaughton and Jarvis Ž1983.. The diurnal trends of V and wind velocity are shown for some days of the 1990 season in Fig. 1. Early in the life cycle of the crop, on 37 DAP Ž6r20r90., before there were differential irrigations, canopy cover was incomplete ŽLAI f 0.58. and the crop was about 30 cm tall ŽFig. 1a.. With an average wind velocity of about 1.53 m sy1 over the day, V

205

ranged between 0.65 and 0.45 for much of the day. This indicates a somewhat decoupled condition, in spite of the low LAI, probably because the wind velocity was low to moderate that day and the short plant stature. On 51 DAP, just before differential irrigation was imposed, the canopy was fairly well developed ŽLAI s 3.4. and the plants had attained one half of the final height Ž1.56 m.. For the morning with wind velocities similar to those on 37 DAP, V ranged between 0.7 to 0.5 ŽFig. 1d., only slightly higher than on 37 DAP. In the afternoon V decreased to about 0.4, partly the result of higher wind velocities Žhigher g a . and partly the result of lower g c ŽSteduto and Hsiao, 1998a.. After the canopy was fully developed, on one of the days Ž102 DAP. with the highest wind velocity Ž) 4 m sy1 ., V remained mostly above 0.4 from early morning to shortly after noon ŽFig. 1b. for the WET treatment with a LAI of 5.5 and a crop height of 3–3.2 m. On 119 DAP ŽFig.

Fig. 1. Diurnal trends in the decoupling factor V for maize of the WET Ž Ø . and DRY Ž(. treatment and wind velocity on selected days of the 1990 season. Planting was on May 25. Height of the crop Ž h c . and days after planting ŽDAP. are indicated in the figures.

206

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

1c., after senescence had reduced LAI Žgreen leaves only. to approximately 3 but the total leaf area remained near 5.5, V ranged between 0.5 and 0.8 in the first half of the day when there was little wind Žabout 0.8 m sy1 . and declined markedly as the wind increased in the afternoon. In the DRY treatments, on 88 DAP there was some indication of very mild water stress ŽSteduto and Hsiao, 1998b.. The daily pattern of V ŽFig. 1e., however, did not appear to be substantially different from that of the WET treatments given the wind velocity and LAI Ž5.8.. On 113 DAP, when water stress was quite severe ŽSteduto and Hsiao, 1998b. and had reduced LAI to 2.1, V decreased to the range of 0.3 by 1000 h and to 0.2 by early afternoon ŽFig. 1f., as the result of a marked reduction in g c relative to g a ŽSteduto and Hsiao, 1998a.. For different days in general, very early in the morning and late in the afternoon, the canopy was better coupled Žlow V values. to the atmosphere because of low values of g c ŽSteduto and Hsiao, 1998a., presumably the result of low epidermal conductance due to reduced stomatal opening under low radiation. Under the summer conditions of Davis, V usually rose to a peak value early in the morning as the stomata opened while the air was still relatively calm. From mid-morning to early afternoon, values of V tended to be dominated by aerodynamic conductance, a function of wind velocity, because canopy conductance remained high and relatively constant during that period. Late in the afternoon, canopy conductance declines and V decreases in spite of the usually windy conditions. Our diurnal patterns are similar in shape to that obtained by Kostner et al. ¨ Ž1992. on a Nothofagus Žred beech. forest varying from 26 to 31 m in height with a LAI of 7 in New Zealand, one of the few examples of diurnal trends of V in the literature. The difference between their data and ours is in the degree of coupling. Their peak V value was around 0.4, whereas ours ranged between 0.55 and 0.8, except on the day of the highest wind ŽFig. 1b. or when LAI was reduced substantially by senescence near the end of the crop life cycle. On the other hand, our diurnal patterns appear to be in contrast with that obtained by Baldocchi Ž1994. for a maize canopy about 0.45 m high with a LAI f 1.5. For the only date shown, V ranged between 0.25 and 0.4 for much of the time with

morning values no higher than afternoon values. On the same day for a wheat canopy with a LAI f 2.7, Baldocchi calculated V to be higher, in the range of 0.37 to 0.55, in spite of the fact that the wheat Žabout 0.8 m high. was taller than the maize. The seasonal trends in midday Žmean of 1200 to 1400 h. V are shown in Fig. 2a. The midday values are chosen because the relative error in g c should be the least that time of the day ŽHeld et al., 1990.. Most of the midday V fell within the range of 0.5 to 0.85 until 85 DAP for both treatments, indicating a relatively low degree of coupling of the canopy with the atmosphere. Later, V decreased, first for the DRY treatment after 100 DAP, then also for the WET treatment after about 110 DAP. At the end of the season, V of the DRY treatment declined to the range of 0.1. Again, the decrease in V as the crop matured was the result of the decreases in g c shown in the preceding paper ŽSteduto and Hsiao, 1998b.. The decrease in g c , in turn, was mainly the result of decreases in green leaf area; only a minor part was attributable to stomatal closure for the DRY treatment ŽSteduto and Hsiao, 1998b.. The link between the reductions in V and the decline in LAI is easily

Fig. 2. Seasonal trend of Ža. midday Žmean from 1200 to 1400 h. decoupling factor V and Žb. leaf area index ŽLAI. for maize of the WET Ž Ø . and DRY Ž(. treatment in the 1990 season.

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

seen by comparing the seasonal trend in the two parameters ŽFig. 2a vs. b.. At present, analysis of coupling based on values of V are limited mainly to forest or moorland type of vegetations ŽMcNaughton and Jarvis, 1983; Miranda et al., 1984; Verma et al., 1986; Kostner et al., ¨ 1992; Meinzer et al., 1993; Lee and Black, 1993; San Jose´ et al., 1995; Granier and Breda, 1996.. Studies on field crops are rare Že.g., Baldocchi, 1994., and there is a dearth of information on the seasonal changes in coupling of crops with the atmosphere. Jarvis Ž1985a. indicated some likely values of V for crops of different heights in open fields, based on literature values of aerodynamic and canopy conductances. These varied from 0.9 for strawberry Ž0.2 m high., to 0.5 for grape vine Ž1.0 m high., to 0.3 for raspberry Ž1.5 m high., and to 0.1 for cherry orchard Ž15 m high.. These estimates suggest an inverse relationship with height of the crop. The present data do not appear to be in accordance, as the 3-m high maize exhibited only modest coupling Ž V f 0.4. even under very windy conditions Žvelocity 4–5 m sy1 , Fig. 1b. and the midday V ranged between 0.45 and 0.8 for most of the season before senescence. These findings also indicate that the maximum V value of 0.3 estimated for herbaceous crops by Pinty et al. Ž1992. in their large scale modeling analysis is too low. The way canopy and aerodynamic conductances are obtained can cause significant variation in the value of V . For sugarcane, with the same height Ž3 m. and similar LAI Ž5.6. as our maize crop, Meinzer and Grantz Ž1989. estimated average V to be 0.91 to 0.92, indicating extremely uncoupled conditions. They used neither the aerodynamic approach to parameterize g a nor the P–M equation to derive g c , but based their calculations on a mixture of canopy and single leaf data. It is difficult to see how their unconventional approach can yield quantitatively meaningful data in terms of V as defined by McNaughton and Jarvis Ž1983.. In the work on V of the red beech forest ŽKostner et al., 1992. mentioned ¨ above, g a was calculated from wind measurements without accounting for the excess resistance, and g c was derived from evapotranspiration rate, air saturation deficit and g a , not from the P–M equation as we did. Lee and Black Ž1993. did use the aerodynamic log-law of wind profile for deriving g a and

207

the P–M equation to obtain g c . They calculated V to be in the range of 0.1 to 0.2 for a Douglas fir forest, which is in agreement with the values estimated by Jarvis Ž1985b. for forests. They did not, however, account for the excess resistance. The same approach was followed by San Jose´ et al. Ž1995. on a forest grove and by Granier and Breda Ž1996. on an oak forest. The impact of excess resistance on the calculation of V , in fact, is not negligible as is evident from Eq. Ž8. and shown by Verma et al. Ž1986. for a deciduous forest between 17 and 26 m high. Those authors calculated midday V to be in the range of 0.35 to 0.65 when excess resistance was accounted for, but to be in the range of 0.2 to 0.35, when excess resistance was not accounted for. Only when excess resistance was not accounted for was the range of V values similar to those suggested by McNaughton and Jarvis Ž1983. and Jarvis Ž1985b.. When excess resistance was not considered, our V values averaged about 30% lower at low g a , and about 50% lower at high g a . In this study, the reference climatic variables were measured with the upper unit of the BREBq sensors located between 1.6 and 2.3 m above the crop, and were not a strict representation of the bulk atmosphere. An underestimation of V is expected but should be insignificant ŽMcNaughton and Jarvis, 1983.. 4.2. Diurnal and seasonal trends in eÕapotranspiration compared to equilibrium and imposed eÕaporation Jarvis Ž1985a,b. evaluated the degree of coupling by comparing actual evapotranspiration Ž E . with equilibrium evaporation and imposed evaporation. The diurnal time course of the absolute values of actual l E Ž l Eact ., l Eeq and l Eimp are shown for our WET and DRY treatment on several days in Fig. 3, along with the corresponding V . For simplicity, the latent heat fluxes, almost always negative by our sign convention, are presented and discussed as absolute values, but omitting the conventional sign. For the WET treatment, either at the very beginning of the season Ž37 DAP. or later in the season but before marked senescence Ž102 DAP., l Eeq was nearly always lower than l Eact , which in turn was less than l Eimp . After senescence became marked, on 120

208

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

. and equilibrium Ž l Eeq P PPP . latent heat flux Žabsolute values. Fig. 3. Diurnal trends of imposed Ž l Eimp —-., actual Ž l Eact with the corresponding decoupling factor V for maize of the WET Ž Ø . and DRY Ž(. treatment on selected days of the 1990 season. Height of the crop Ž h c . did not change after 101–102 DAP.

DAP, the crop was near maturity and there was a major reduction in LAI and the associated reduction in g cw ŽSteduto and Hsiao, 1998b., to the point that the relative magnitudes were reversed, with l Eact falling below l Eeq and l Eimp falling below l Eact , from the mid-morning to the afternoon. For the DRY treatment, on 101 DAP and the subsequent days shown in Fig. 3 Ž105, and 113 DAP., water stress, developing late in the season, had reduced g cw largely by reducing LAI, but also through apparent reduction in epidermal conductance ŽSteduto and Hsiao, 1998b. so that l Eeq ) l Eact ) l Eimp from early morning to afternoon on all the three days. The seasonal trends are shown as midday values of the three latent heat fluxes in Fig. 4, along with the midday V . In the early and middle part of the season, the line for l Eact always fell between the lines for l Eimp and l Eeq , which formed respectively

the upper and lower limit. Toward the end of the crop life cycle, the relative position of the three lines switched, from l Eimp ) l Eact ) l Eeq to l Eimp l Eact - l Eeq . For the DRY treatment, the switch occurred early, at 100 DAP Žinset of Fig. 4.. For the WET treatment, the switch occurred at 119 DAP Žinset of Fig. 4.. In the inset of Fig. 4 for the DRY treatment, the anomalously low l Eact on 100 DAP, lower than both l Eeq and l Eimp , is an artifact of interpolation. At the time of switching for the DRY treatment, g cw fell below 13 mm sy1 for the first time, to 8 mm sy1 . For the WET treatment, the switching occurred when g cw fell below 13 mm sy1 for the first time, to 9 mm sy1 . This narrow range of g cw values corresponds well to the threshold range estimated earlier from plots of ratios of actual E to potential E vs. g cw ŽSteduto and Hsiao, 1998b., which makes E and

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

209

Fig. 4. Seasonal trend of midday Žmean from 1200 to 1400 h. imposed Ž l Eimp - - -., actual Ž l Eact — ., and equilibrium Ž l Eeq PPPP . latent heat flux Žabsolute values. with the corresponding decoupling factor V and canopy conductance g cw for maize of the WET Ž Ø . and DRY Ž(. treatment in the 1990 season. Insets show enlargement of the portions of the curves where the lines for l Eeq cross the lines for l Eimp and l Eact . In the inset for the DRY treatment, the anomalously low l Eact on 100 DAP, lower than both l Eeq and l Eimp , is an artifact of interpolation.

canopy assimilation sensitive to changes in g cw . A similar threshold g cw range was simulated by McNaughton and Spriggs Ž1986, 1989.. Hence, the crossing over between l Eeq and l Eimp may represent a deterministic way to identify the threshold value of g c at which it begins to exert significant effect on l E and A cf . Plots of l Eact vs. l Eeq or vs. l Eimp have been used to evaluate the degree of coupling ŽJarvis, 1985a,b; Meinzer and Grantz, 1989.. Variations in V diurnally ŽFig. 1. and seasonally ŽFig. 2., however, makes that approach uncertain. Jarvis Ž1985a. distinguished a canopy that is well coupled from one that is decoupled by examining whether l Eact is correlated with l Eimp or with l Eeq . Analyzing our data by the same correlation approach, however, did not yield a clear distinction in many cases. For example, when the diurnal data of l Eact was plotted in Fig. 5 against l Eeq or l Eimp for the WET treatment on 120 DAP, an excellent correlation was

found for either case, leading to the impossible conclusion that the crop was well coupled and uncoupled at the same time. On the other hand, if one examines the same data set ŽWET, 120 DAP. but presented as the diurnal time courses of the actual l E and the two theoretical latent heat fluxes ŽFig. 3., it becomes clear that l Eimp fell below l Eeq for much of the morning and early afternoon, the result of the senescence induced decrease in g cw . The accompanying time course of V in Fig. 3 shows that the crop was largely decoupled in the morning and then became more and more coupled as the day progressed. The correlation approach is even more problematic when applied to maize under water stress. For instance, the plots of l Eact vs. l Eeq and vs. l Eimp for the DRY treatment on 113 DAP ŽFig. 5. defy simple interpretation. Again, the same data set presented as diurnal time courses of three kinds of latent heat fluxes ŽFig. 3. is more informative, showing the canopy was fairly coupled, beginning in

210

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

Fig. 5. Correlation of actual latent heat flux Žabsolute values. with equilibrium Župper figures. and imposed Žlower figures. latent heat flux Žabsolute values. on two dates, one for the WET Žleft. and one for the DRY Žright. treatment of maize in 1990. All values are for a time interval of 5 min, the set averaging time of the instrument signals. Solid line represents the 1:1 relationship. Dashed lines Ž — – . represent linear regressions for the WET treatment, with r 2 s 0.976 for the upper figure and r 2 s 0.926 for the lower figure.

the morning. Similar problems were encountered when we applied the correlation approach to the seasonal data Žnot shown.. 4.3. Attempts at eÕaluating coupling for carbon dioxide transfer o . The CO 2 concentration at the crop surface Ž PCO 2 Ž . was calculated according to Eq. 7 from A cf , g ac , and an assumed bulk atmospheric CO 2 concentration a . Ž PCO of 360 m mol moly1 for the midday period. 2 a PCO 2 was assumed because we did not have an extra instrument to measure absolute CO 2 concentration while monitoring the CO 2 differential. The estimates, covering the periods from tasselling to the start of senescence in 1989, and during senescence in

o 1990, showed PCO to increase with increasing g ac , 2 as would be expected. Due to data scatter, however, o the regression between PCO and g ac was significant 2 for the 1989 WET and 1990 DRY treatment, but not for the 1990 WET treatment. A correlation between o V and PCO is also expected since both parameters 2 are indicative of the extent of coupling of the canopy with the atmosphere. Statistically, however, only the regression for the 1990 DRY treatment was significant, again because of the substantial data scatter. The main cause of the scatter was probably the use a of 360 m mol moly1 in the of the assumed PCO 2 o calculation of PCO 2 . Data obtained recently by Xu and Hsiao Žunpublished. showed that day-to-day variations in CO 2 concentration at the reference height above a maize canopy were approaching in

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

magnitude the calculated CO 2 depletion from the bulk atmosphere to the canopy surface. Another possible cause of the lack of a more consistent trend was the fact that efflux of CO 2 from the soil, which served as the other source of CO 2 for assimilation, was substantial and variable, especially for the DRY treatment ŽSteduto, 1993.. Because of these uncertainties, the data related to CO 2 coupling are not presented, but interested readers can find the data in Steduto Ž1993.. Our attempt shows that much additional work is necessary to obtain reliable data to evaluate coupling of the canopy with the atmosphere in terms of CO 2 transport.

5. Conclusion The degree of coupling has many agronomic implications and assumes particular relevance when interpreting results from different kinds of vegetation and different microclimatic conditions. Despite its significance, only limited experimental data on coupling for different conditions have been reported in the literature. In the present study, diurnal and seasonal trends of the decoupling factor V were characterized for maize, considered a tall crop Ž3–3.2 m.. The degree of coupling was found to be relatively low, with midday V ranging between 0.4 and 0.8 for most of the season before the start of senescence. Wind enhanced coupling. On the other hand, even under very windy conditions Ž4 to 5 m sy1 . midday V was observed to be G 0.4, as long as the canopy cover was substantial and not senescent. Also, a sparse and short canopy was found to be fairly uncoupled on a day of low to moderate wind ŽFig. 3, 37 DAP.. The relatively low degree of coupling reflected a lack of plant control over E. With the start of senescence, however, canopy conductance declined. Midday V declined below 0.4 when g c was reduced to below approximately 12 mm sy1 . At that point, the plant began to exert more influence over E under the prevailing Žmoderate. wind conditions, and further reductions in g c caused substantial reductions in E ŽSteduto and Hsiao, 1998b.. Differences in coupling between the WET and the DRY treatment were not substantial before 100 DAP, and became significant thereafter when early senescence and loss of transpiring green leaf area were induced

211

by water stress in the DRY treatment. The reduction in LAI appeared to account for much of the increases in coupling at the end of the crop cycle, although stomatal closure in the remaining green leaves apparently also played a role ŽSteduto and Hsiao, 1998b.. In assessing coupling, how V is calculated is of paramount importance. The excess resistance for water vapor and sensible heat over the resistance for momentum must be accounted for ŽThom, 1972, 1975., or else the degree of coupling will be significantly overestimated. In our experiment, V was reduced by 30% at low g a and by up to 50% at high g a when excess resistance was not considered. At the time when the canopy became better coupled with the atmosphere and g c fell to the threshold range where it exerted more control over E, l Eimp , initially higher than l Eeq , fell below l Eeq , a switch that appears to define the threshold for canopy conductance to have significant impact on E and canopy assimilation. A way of viewing at l Eimp and l Eeq is to consider the first as a potential l E flux for any g c , as if it were totally driven by VPD, and the second as a potential l E flux for any g c as if it were totally driven by net radiation and storage heat flux ŽyR n y G, so called ‘available energy’.. The aerodynamic conductance relative to canopy conductance would then establish how close the actual l E would be to l Eimp Žlow V . or to l Eeq Žhigh V .. Using the P–M model, and provided that stable condition is not dominating, the actual l E will fall within these two limits, with l Eimp as the upper and l Eeq as the lower boundary only if g c is not limiting evapotranspiration. Once g c is low enough to assume control, l Eeq will become the upper boundary and l Eimp the lower boundary. The switch between the two boundaries represents a reversal in the role of the crop and soil over the weather, or vice versa, in controlling the water loss. Compared to the critical value of V and the threshold value of g c , which appear to spread over a range, the switch between the two boundaries is a more definitive way of identifying the point of role reversal between the plant and the weather in controlling E. Analyzing l Eimp , l Eeq , and l Eact in relation to V and g c represents a key to the interpretation of many phenomena. Examples are: the lack of significant differences in l E over a certain range of g c ŽMcNaughton and Jarvis, 1991; Steduto and Hsiao,

212

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213

1998b.; the use of one single coefficient Ž a s 1.26. to approximate the reference crop E as in the equation of Priestley and Taylor Ž1972.; the similar values of crop coefficients ŽDoorenbos and Pruitt, 1984. for diverse crop species, regardless of whether C 3 or C 4 , when well watered and with a complete canopy cover; and the deviation between top–down and bottom–up values of the canopy conductance ŽBaldocchi et al., 1991.. One special example is that l Eact can either increase or decrease with increasing wind velocity Že.g., Grace, 1981.. Since with increasing wind velocity the coupling increases Ž V ™ 0. and l Eact gets closer to l Eimp ŽEq. Ž5.., l Eact can respectively increase or decrease with increasing wind velocity, depending on whether l Eimp ) l Eeq or l Eimp - l Eeq . Of course, to switch between these two conditions there will be a special case where l Eimp s l Eeq s l Eact . The value of canopy resistance which meets the requirement of the special case is sometimes referred to as ‘climatological resistance’ ŽMonteith, 1965. or ‘critical resistance’ ŽDaudet and Perrier, 1968.. Under the condition of l Eimp s l Eeq s l Eact , it is evident ŽEq. Ž5.. that V can assume any value between 0 and 1 without affecting l Eact , which means no wind effect on the water vapor flux. The outcome of this study shows that the coupling of crops to the atmosphere partly depends on plant density, LAI, as well as leaf epidermal conductances, in addition to climatic variables. Consequently, no straightforward relationships can be found with crop height and roughness alone. The suspicion is, in fact, that for any dense field crop well supplied with water, the degree of coupling might be similar regardless of crop height. More field measurements and the determinations of crossover points between l Eeq and l Eimp in relation to V are needed to better define the coupling behavior of different crops.

Acknowledgements This work was supported by U.S. Department of Agriculture Competitive Research Grant 88-371303849, U.S. Department of Energy Program for Ecosystem Research Grant DE-FG03-93-ER-6187, and Regional Research W-154. We are grateful to

Tony Matista for his excellent assistance in maintaining and improving the Bowen ratio and other instruments. We also thank the Director of CIHEAMIAM-Bari for encouragement and the financial support that made the stay of SP in Davis possible.

References Aubinet, M., Deltour, J., De Halleux, D., Nijskens, J., 1989. Stomatal regulation in greenhouse crops: analysis and simulation. Agric. For. Meteorol. 48, 21–44. Baldocchi, D.D., 1994. A comparative study of mass and energy exchange over a closed C 3 Žwheat. and an open C 4 Žcorn. canopy: I. The partitioning of available energy into latent and sensible heat exchange. Agric. Meteorol. 67, 191–220. Baldocchi, D.D., Luxmoore, R.J., Hatfield, J.L., 1991. Discerning the forest from the trees: an assay on scaling canopy stomatal conductance. Agric. For. Meteorol. 54, 197–226. Bange, G.G.J., 1953. On the quantitative explanation of stomatal transpiration. Acta Bot. Neerl. 2, 255–297. Daudet, F.A., Perrier, A., 1968. Etude de l’evaporation ou de la ´ condensation a` la surface d’un corps a` partir du bilan energe´ ´ tique. Rev. Gen. ´ Therm. 76, 353–364. Di Stefano, J.J., Stubberud, A.R., Williams, I.J., 1988. Feedback and Control Systems. McGraw-Hill, Singapore, 371 pp. Doorenbos, J., Pruitt, W.O., 1984. Crop water requirements. Irrigation and Drainage Paper no. 24. FAO, Rome, 179 pp. Grace, J., 1981. Some effects of wind on plants. In: Grace, J., Ford, E.D., Jarvis, P.G. ŽEds.., Plants and their Atmospheric Environment. Blackwell, Oxford, pp. 31–56. Granier, A., Breda, N., 1996. Modeling canopy conductance and transpiration of an oak forest from sap flow measurements. Ann. Sci. For. 53, 537–546. Held, A.A., Steduto, P., Orgaz, F., Matista, A., Hsiao, T.C., 1990. Bowen-ratio energy-balance technique for estimating crop net CO 2 assimilation and comparison with a canopy chamber. Theor. Appl. Climatol. 42, 203–213. Jarvis, P.G., 1985a. Coupling of transpiration to the atmosphere in horticultural crops: the omega factor. Acta Hort. 171, 187–205. Jarvis, P.G., 1985b. Transpiration and assimilation of tree and agricultural crops: The omega factor. In: Cannel, M.G.R., Jackson, J.E. ŽEds.., Attributes of Trees as Crop Plants. Institute of Terrestrial Ecology, England, pp. 460–480. Jarvis, P.G., McNaughton, K.G., 1986. Stomatal control of transpiration: scaling up from leaf to region. Adv. Ecol. Res. 15, 1–49. Kostner, B.M.M., Schulze, E.-D., Kelliher, F.M., Hollinger, D.Y., ¨ Beyers, J.N., Hunt, J.E., McSeveny, T.M., Meserth, R., Weir, P.L., 1992. Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements. Oecologia 91, 350– 359. Lee, X., Black, T.A., 1993. Atmospheric turbulence within and above a douglas-fir stand: Part II. Eddy fluxes of sensible heat and water vapor. Boundary-Layer Meteorol. 64, 369–389.

P. Steduto, T.C. Hsiaor Agricultural and Forest Meteorology 89 (1998) 201–213 Martin, P., 1989. The significance of radiative coupling between vegetation and the atmosphere. Agric. For. Meteorol. 49, 45–53. McNaughton, K.G., Jarvis, P.G., 1983. Predicting effects of vegetation changes on transpiration and evaporation. In: Kozlowski, T.T. ŽEd.., Water Deficit and Plant Growth, Vol. 7. Academic Press, New York, pp. 1–47. McNaughton, K.G., Jarvis, P.G., 1991. Effects of spatial scale on stomatal control of transpiration. Agric. For. Meteorol. 54, 279–301. McNaughton, K.G., Spriggs, T.W., 1986. A mixed-layer model for regional evaporation. Boundary-Layer Meteorol. 34, 243– 262. McNaughton, K.G., Spriggs, T.W., 1989. An evaluation of the Priestley and Taylor equation and the complementary relationship using results from a mixed-layer model of the convective boundary layer. In: Black, T.A., Spittlehouse, D.L., Novak, M.D., Price, D.T. ŽEds.., Estimation of Areal Evapotranspiration. IAHS Press Publication no. 177, Wallingford, pp. 89–104. Meinzer, F.C., 1993. Stomatal control of transpiration. Tree 8, 289–294. Meinzer, F.C., Grantz, D.A., 1989. Stomatal control of transpiration from a developing sugarcane canopy. Plant Cell Environ. 12, 636–642. Meinzer, F.C., Goldstein, G., Holbrook, N.M., Jackson, P., Cavelier, J., 1993. Stomatal and environmental control of transpiration in a lowland tropical forest tree. Plant Cell Environ. 16, 429–436. Miranda, A.C., Jarvis, P.G., Grace, J., 1984. Transpiration and evaporation from heather moorland. Boundary-Layer Meteorol. 28, 227–243. Monteith, J.L., 1965. Evaporation and environment. In: Fogg, G.E. ŽEd.., The State and Movement of Water in Living Organisms. Symposium of the Society for Experimental Biology, Vol. 19. Cambridge Univ. Press, New York, pp. 205–234. Monteith, J.L., 1973. Principles of Environmental Physics. Edward Arnold, London, 241 pp. Monteith, J.L., 1981. Coupling of plants to the atmosphere. In: Grace, J., Ford, E.D., Jarvis, P.G. ŽEds.., Plants and Their Atmospheric Environment. Blackwell, Oxford, pp. 1–29. Paw U, K.T., Gao, W., 1988. Applications of solutions to non-lin-

213

ear energy budget equations. Agric. For. Meteorol. 43, 121– 145. Penman, H.L., Schofield, R.K., 1951. Some physical aspects of assimilation and transpiration: Carbon dioxide fixation and photosynthesis. Symp. Soc. Exp. Biol. 5, 115–129. Pinty, J.P., Mascart, P., Bechtold, P., Rosset, R., 1992. An application of the vegetation–atmosphere coupling concept to the HAPEX-MOBILHY experiment. Agric. For. Meteorol. 61, 253–279. Priestley, C.H.B., Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev. 100, 81–92. San Jose’, J.J., Montes, R.A., Florentino, A., 1995. Water flux through a semi-deciduous forest grove of the Orinoco savannas. Oecologia 101, 141–150. Steduto, P., 1993. Water vapor and CO 2 : Fluxes of post-anthesis maize in the field under two soil–water regimes: leaf vs. canopy scale. PhD Dissertation, University of California, Davis, 217 pp. Steduto, P., Hsiao, T.C., 1998a. Maize canopies under two soil water regimes: I. Diurnal patterns of energy balance, carbon dioxide flux, and canopy conductance. Agric. For. Meteorol. 89, 173–188. Steduto, P., Hsiao, T.C., 1998b. Maize canopies under two soil water regimes: II. Seasonal trends of evapotranspiration, carbon dioxide assimilation and canopy conductance, and as related to leaf area index. Agric. For. Meteorol. 89, 189–203. Steduto, P., Hsiao, T.C., 1998c. Maize canopies under two soil water regimes: IV. Validity of the Bowen ratio–energy balance technique for measuring water vapor and carbon dioxide fluxes at 5-min intervals. Agric. For. Meteorol. 89, 219–232. Thom, A.S., 1972. Momentum, mass and heat exchange of vegetation. Q. J. R. Meteorol. Soc. 98, 124–134. Thom, A.S., 1975. Momentum, mass, and heat exchange of plant communities. In: Monteith, J.L. ŽEd.., Vegetation and the Atmosphere, Vol. 1. Academic Press, London, pp. 57–109. Verma, S.B., Baldocchi, D.D., Anderson, D.E., Matt, D.R., Clement, R.J., 1986. Eddy fluxes of CO 2 , water vapor, and sensible heat over a deciduous forest. Boundary-Layer Meteorol. 36, 71–91.