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Measured and simulated water relations in a Douglas-Fir forest during the development of drought in the Apennines, Central Italy
Forest Ecology and Management, 25 (1988) 181-194 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
181
Measured and Simulated Water Relations in a Douglas-Fir Forest During the Development of Drought in the Apennines, Central Italy P.J. KOWALIK 1'3, M. BORGHETTI 2'4, E. BUSONI 3, G. SANESI 3 and G.G. VENDRAMIN 2 1Centro di Studio per la Genesi, Classificazione e Cartografia del Suolo, Consiglio Nazionale delle Ricerche, Piazzale delle Cascine, 15 - 50144 Firenze (Italy) 2Istituto Miglioramento Genetico Piante Forestali, Consiglio Nazionale delle Ricerche, Via S. Bonaventura, 13 - 50145 Firenze (Italy) (Accepted 15 October 1987)
ABSTRACT Kowalik, P.J., Borghetti, M., Busoni, E., Sanesi, G. and Vendramin, G.G., 1988. Measured and simulated water relations in a Douglas-fir forest during the development of drought in the Apennines, central Italy. For. Ecol. Manage., 25: 181-194. A model of soil water dynamics (SWATRE) and of the soil-plant-atmosphere continuum (SPAC), including the plant water-retention component, were applied independently to describe the seasonal and diurnal water balance of a forest in Italy. The stand under consideration was a 26-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation with a density of 2044 trees h a - ' and average height of 14.2 m, located in the Apennines in Central Italy, 900 m a.s.1, in a mediterranean climate. Soil water potential was measured with tensiometers and psychrometers 2 or 3 times per week during the summer of 1985. The xylem water potential and leaf stomatal conductance were measured on 4 days in the period June-September 1985, at about 2-3-h intervals from sunrise to sunset. Global and net solar radiation, air temperature and air relative humidity above the canopy were measured. Soil water potentials simulated by the SWATRE model agreed well with the field measurements. The diurnal pattern of bulk stomatal resistance and xylem water potential simulated by the SPAC model also agreed with measurements. The results show that Douglas-fir is surprisingly well adapted to the drought conditions that occurred in Tuscany during the dry year 1985, and even with dry soil, shallow rooting depth and high evaporative demand, trees did not show evident water stress.
INTRODUCTION Several studies on water relations of Douglas-fir (Pseudotsuga menziesii ( M i r b . ) F r a n c o ) h a v e b e e n c a r r i e d o u t i n n o r t h e r n e c o s y s t e m s {see r e v i e w b y 3On leave from Technical University of Gdansk, PL-80952-Gdansk-Wrzeszcz (Poland) 4Correspondence address.
0378-1127/88/$03.50
© 1988 Elsevier Science Publishers B.V.
182
Lassoie, 1981). The present research is a contribution to this subject performed in mediterranean conditions. To evaluate the ecophysiological adjustments of trees to changing environments, and as an aid to watershed management, we require mathematical models based on carefully designed field experiments. Such experiments have recently been completed in a selected Douglas-fir plantation in the Apennines of Central Italy (Borghetti and Vendramin, 1987) and the present work concerns the development of a modelling approach. Two existing computer simulation models were used. The first is a model of soil water dynamics (SWATRE), developed by Feddes et al. (1978) and Belmans et al. (1983). The second is a model of the soil-plant-atmosphere continuum (SPAC), with a plant water-retention component, developed by Kowalik and Turner (1983) and Kowalik and Eckersten (1984). One objective was to calculate the transpiration of the forest during periods without rainfall. Another objective was to evaluate the contribution to transpiration of water flux from the lower regions of the soil profile. It was also important to know how Douglas-fir is adapted to the conditions of Tuscany, Italy, with its dry soil, shallow rooting depth and high evaporative demand and, in particular, to model the decline in stomatal conductance and xylem water potential during the development of drought in a dry summer. EXPERIMENTAL DETAILS
Experimental site The experiment was carried out in a Pseudotsuga menziesii plantation during the summer of 1985. The site is NW-facing on a slope of about 10%, at Passo della Consuma near Florence .(Tuscany, Italy), 43°46'43"N, 11 ° 44'13" E, elevation 900 m a.s.1. The 26-year-old plantation had a density of 2044 trees ha-1, whose average height was 14.2 m, basal area of 41.7 m 2 ha-1, and a projected leaf area index of 9.8. There was very sparse undergrowth at the experimental site. The mean annual temperature (period 1951-1980) is 9.4 ° C and the mean annual rainfall 1248 mm. The soil is an acid brown, about 110 cm in depth, classified as a Typic Distrochrepts in accordance with the USDA's Soil Taxonomy. Very few roots were found below 75-cm depth. Further description of the site can be found in Borghetti et al. (1983). The warm, dry summer of 1985 provided good drying cycles for soil and plant water studies. Temperature and rainfall during the period studied are given in Table 1. Most of the data used in this paper were obtained in the periods 25 May-13 June, 18 July-24 July, and 17 August-22 August, 1985.
183 TABLE 1 Mean temperature (T) and total precipitation (P) in the study area during the sampling period in 1985 and for the period 1951-1980 Month
1985
T (°C) January February March April May June July August September
- 1.4 3.5 2.8 7.8 12.0 15.5 20.0 19.0 17.0
1951-1980
P (mm)
T (°C)
P (mm)
110 50 150 35 95 55 35 100 5
1.9 2.4 4.5 7.5 11.5 15.0 17.8 17.9 14.9
109 113 104 110 105 82 56 70 96
Modelling The model SWATRE is used to calculate the flux of water into the soil system with water uptake by plants. The flux of soil water is calculated by applying Darcy's equation. Water content as a function of time is calculated by combining Darcy's equation with the continuity equation. The soil is divided into a number of compartments, and fluxes calculated according to hydraulic head gradients and conductivities between compartments. In a cropped soil, the spatial and temporal variation in water content depends upon water redistribution in the soil profile and water uptake by plants. In SWATRE, water uptake by plants is calculated from each soil compartment within the root zone as a fraction of the maximum potential water uptake, i.e. potential transpiration. Water uptake by the roots is represented by a volumetric sink term (see Feddes et al., 1978) which simply is added to the continuity equation. Soil water is considered to be potentially available to the plants in equal amounts within the root zone. It means that roots can extract from each soil compartment an amount of water equal to the potential transpiration divided by the number of compartments within the root zone. In the model, the relationship between water uptake by the roots and soil matric potential comes from experimental determinations. SWATRE is applied here to simulate long-term daily water uptake by the roots during drying. Upper boundary conditions are daily potential transpiration rates (calculated according to Penman-Monteith) and potential soil evaporation (here set to zero because of the complete canopy cover and the thick litter layer); lower boundary conditions are daily measured soil matric potentials at 1.1-m depth. Parameters like sink term, soil water-retention curves,
184
and hydraulic conductivity were derived from measurements. The simulation runs with a 1-day time-step. The model SPAC describes interrelations of water potentials and fluxes in the soil-plant-atmosphere continuum, and takes into consideration the water stored in the plants (a capacitance term). The assumptions of SPAC are: (i) the water status of the canopy is governed by the water lost by transpiration and the water supplied by the roots; (ii) the water status of the canopy controls the rate of water uptake and water loss by the plant - - the opening of the stomata is regulated at least in part by the water status of the leaves while at the same time the rate of water uptake is governed by the difference in water status between the leaf and soil; (iii) the root system is treated as a single unit; and (iv) the model assumes that there is a certain amount of exchangeable water stored in the plant which allows, within limits, transpiration to exceed, to equal, or to be less than water uptake by the roots. Driving input variables (time-dependent data) are measured values of air temperature, relative humidity and solar radiation (30-min time-step data) and the daily value of soil water potential in the rooting zone (here at 25-cm depth). Time-invariant data include: (i) the relationships between bulk stomatal resistance and water potential, between bulk stomatal resistance and solar radiation and between leaf water potential and plant water content; (ii) aerodynamic resistance; (iii) plant hydraulic resistance; and (iv) the soil-toroot hydraulic resistance as a function of soil water potential. The simulation runs with a fixed time interval of 1 min, and results are printed out for every hour. The model describes the changes in the diurnal course of transpiration, bulk stomatal resistance and xylem water potential. For a more detailed description of the SPAC model see Kowalik and Turner (1983) and Kowalik and Eckersten (1984).
Measurement of meteorological and soil water variables Air temperature and humidity were measured using ventilated psychrometers with platinum resistance thermometers. Global and net radiation were measured using a Moll-Gorczynski-type pyranometer (type CM 5/6, Kipp and Zonen, The Netherlands) and a Funk-type net radiometer (Middleton, Australia), respectively. All the sensors were placed horizontally at a height of about 1 m above the canopy. The data were recorded every 30 min (each datum being the average of 60 measurements taken every 30 s) onto a magnetic tape cassette (type 4008, Facit, Sweden) by a data logger (type PM 4001, Philips, The Netherlands). The soil water potential (~s) was measured throughout the growing season (2-3 times per week), by tensiometers in the range 0 to - 0 . 0 8 MPa, and by a Wescor HR-35 dew-point microvoltmeter and PCT-55 psychrometers (Wescor Inc., Logan, UT, U.S.A.), for values less than - 0 . 2 MPa. Psychrometers
185
were calibrated using salt solutions of known osmotic potential in the range - 0 . 2 to - 2 . 8 MPa. Soil water potentials were measured at three different places between trees, from the soil surface to a depth of 100 cm. The soil water potential was measured at depths of about 10.0 and 30.0 cm (horizon A), and 50.0, 70.0, 80.0 and 90.0 cm (horizon B ). Additional measurements at a depth of 110 cm were taken as the lower boundary condition. These tensiometer or psychrometer measurements were taken in three replicates; the arithmetic average of these readings is applied here.
Measurements of ecophysiological variables The ecophysiological measurements, described in detail by Borghetti and Vendramin (1987), were, briefly: the leaf stomatal conductance (gs) and the xylem water potential (g/t) were measured at about 2-3-h intervals from sunrise to sunset on 12 and 27 June, 18 July and 4 September. Five trees of different D B H ' s (11, 14, 19, 25 and 28 cm) were sampled. Trees were located near the tensiometer-psychrometer groups. Measurements were performed at two levels (13.5 and 11.5 m above the ground) in the crown of each tree, on three twigs from the same branch on each level. Stomatal conductance, gs, was measured by a steady-state diffusion porometer (type 1600, Li-Cor, U.S.A. ). Projected needle area was determined by a leaf-area meter (type 3000, Li-Cor, U.S.A. ) at the end of each measurement day, and g~ computed on the basis of the single-side surface area. Xylem water potential, ~t, was measured on 1-year-old twigs, using a portable pressure chamber (Scholander et al., 1965). Measurements were made within a few min of excision. The relative water content (RWC) was determined on samples of needles taken from the same twigs used for the determination of ~t, using the method described by Waring et al. (1979).
Laboratory measurement of soil hydrologic parameters Water retention curves in the range -0.01 MPa to - 1 . 5 MPa were determined on undisturbed samples from various soil horizons by the sand-box and pressure-plate extraction technique. The water retention data and soil water potentials were used to obtain volumetric soil water contents in the soil profile. The unsaturated hydraulic conductivity (K) was measured in the laboratory by the transient-outflow method, using a procedure similar to that of Arya et al. (1975). Bulk soil samples were taken from the site using a coring device.
186 RESULTS AND DISCUSSION
Time-invariant data for SWA TRE and SPAC Time-invariant data for SWATRE are water retention curves (Fig. 1 ), soil water conductivity, K(~u~; Fig. 2 ) and the volumetric sink term. The sink term was obtained from experimental data. Water uptake by the roots was calculated from the rate of change of soil water potential and the water flux divergence at each compartment (see Nnyamah and Black, 1977). Water uptake was then plotted against soil matric potential to evaluate the sink term. According to our relations, Douglas-fir seems to optimally take up water from -0.002 MPa to - 0 . 0 2 MPa, then water uptake (expressed as a fraction of the potential transpiration) was about 0.55 and 0.2 when the soil matric potential was 0.1 and 1 MPa, respectively. For SPAC, the values of K(~u~) can be approximated using the relationship found by Lang and Gardner (1970) for a number of soils: K(~s)=a(l~'~l)-"
(1)
where n is normally between 2 and 3 for fine-textured soils and about 5 for sands, and a is a coefficient related to the hydraulic conductivity of saturated soil. In the range we measured, it was found that the coefficient n = l . 7 was appropriate to both A and B horizons, with a= 1.58-108 for the A horizon, and a=4.74.106 for the B2.1 and B2.2 horizons, if ~'s is in MPa and K in cm day -1 (Fig. 2). Soil-root hydraulic resistance, rr, can be calculated from: 1 l=Au ,O-Scm 2=A1~, 10-15 cm 3=Bz1,40-50 cm 4 = 8 ~ , 7 0 - 8 0 cm
-10-1
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-10-"-
- 1 0 -5
o
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0'.2
0'.3
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04
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® , ( m ~ m -~)
Fig. i. Water retention curves for various layers of the soilprofile; ¥. = soilwater potential; ~9= soil water content.
187 1
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Fig. 2. Unsaturated hydraulic conductivity (K) as a function of soil water potential (~,s) for various layers of the soil profile; for Lang and Gardner, (1970) equation.
rr =b/K(~&)
(2)
where b is the root density-resistance factor that takes into account the length and geometry of the root system. Equation (2) was used by Gardner and Ehling (1963) and applied to Douglas-fir roots by Nnyamah et al. (1978). From the theory of Gardner (1960) and of Feddes and Rijtema (1972) the coefficient b is:
b=ln(Le/R)/2~LrL
(3)
where: L is the rooting depth (cm); L,, the root density (cm root cm -3 soil); R, the root radius (cm); and Le, half the distance (cm) between neighbouring roots (Le= (~L,)-°5). According to Nnyamah and Black (1978), the mean root diameter and the average root density were respectively 0.08 cm and 0.66 cm root cm -3 soil, for Douglas-fir forest. Because L=75 cm in our experiments, the coefficient of root density-resistance factor is taken to be 0.0069 cm. A sensitivity analysis showed that the effects of different values of b on the simulated output variables were small. The relationship between needle relative water content (RWC) and xylem water potential, assumed to be in equilibrium with needle water potential (Kelliher et al., 1984), is given in Fig. 3. The linear relation ~gt (RWC) can be applied, where: RWC = 99.56-- 11.76~t
r2=0.69
(4)
188 10
100
90
'E 8O
o
•
5C 70
O0
60 0.50
-0.75
1.00
1.25
1.50
1.75
-2.00
'~t (MPa)
Fig. 3. Relationship between the needle relative water content (RWC) and the twig water potential (~t); V = amount of water readily exchangeable in the plant per unit area of soil.
From the definition of RWC, Equation (4) can be written as:
~t = ~ t m ( 1 - V/Vo )
(5)
where: V (g m - 2 ) is the amount of easily exchangeable water in the plant per unit area of the soil at a certain time; Vo, the maximum value for V (RWC=100%); and ~tm ( M P a ) , the minimum value of ~gt when V=O. The maximum storage of easily exchangeable water w h e n no water stress is present was estimated as V-- 1000 g m -2. If V--500 g m -2, then RWC=80% and if V--0, t h e n RWC = 60% - - arbitrary but in agreement with the measured tendency. For V= 0, ~t is equal to - 3.6 MPa. We assumed that only a fraction of the water stored in the plant was readily available for transpiration. Within the short period used by the SPAC model for the simulation, it is to be expected t h a t most water will move to the liquidair interface from the nearest sources of most-readily available water, such as cells in the leaves. In fact, the assumed value of V has the same order of the amount of water estimated to be available in the leaves at saturation (see Borghetti and Vendramin, 1987). This assumption is in accordance with experimental results. Low values of water depletion from storage sources were found by Borghetti and Vendramin (1987), while N n y a m a h and Black (1977) also reported that the a m o u n t of water t h a t is withdrawn from the storage sources on a given day is relatively small, The following equation loggs = - 1.56-- 3.478/x//Q
(6)
was found to fit the relationship between the stomatal conductance (g~) and the photon flux density (Q). The threshold for stomata opening can be taken as about 80 z E m -2 s -1.
189
The relationship between the xylem potential (~gt) and the leaf stomatal conductance (gs) for the plant can be determined, but Landsberg and McMurtrie (1984) note that there is generally little effect of ~t on the stomatal conductance until potentials have fallen to rather low levels. Running (1976) found in Douglas-fir a threshold ~'t for stomatal closure close to - 2.0 MPa. In our measurements no such threshold was reached, and we assumed no effect of ~/]ton g~ over the range of ~/]tfrom 0 to - 2.0 MPa. In our experiments the bulk stomatal resistance rs was calculated from the equation: rs = 1 / g s ' L A I
(7)
where LAI is the leaf area index of the needle layer; the values of LAI were taken from profile measurements in Borghetti et al. (1986). The boundary layer resistance ( ra ) was assumed to be 5 s m - 1 ( Gash and Stewart, 1975 ), irrespective of wind speed. Plant resistance to liquid water flow from the roots to the mesophyll of the leaves was taken as r = 0 . 1 6 , l0 s MPa s m -1, in accordance with the value reported for willow (Kowalik and Eckersten, 1984). This value is considerably smaller with respect to the values reported for conifers (see the data tabulated by Hellkvist et al., 1974 ). However, working on the same Douglas-fir stand as we are now considering, Borghetti and Vendramin (1987) recently calculated a sapwood conductivity much greater t h a n values reported in the literature (Waring and Running, 1978; Waring et al., 1979).
Soil profile water depletion patterns Three major drying cycles were observed during 1985, but only the period 25 May-13 June was dry enough to enable comparisons of measured and simulated soil water potentials over a wide range. As the season progressed the soil profile became drier, not only at the top but at the bottom as well, due to intense root water uptake in the 75-cm soil layer. Daily values of soil water potential, soil water content, soil water flux (between layers) and root uptake rate through the soil profile were calculated using the SWATRE program. The lines in Fig. 4 are simulated values of ~ for the depth of 25 and 75 cm, for the period 25 May-13 June. Points are averages of measurements at different locations. For the three periods (25 May-13 June; 18-23 July; 17-21 August) the relationship between the measured and simulated values is shown in Fig. 5. The rates and patterns of the root water uptake during a 3-week drying period are shown in Fig. 6. The vertical distribution of the matric soil water po-
190
10
02
o'71 I69
I -2-
Q82 069
10
02
18,5
QI2
071 3
O_
@
069
-4
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11'911 oeg 21
-6
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ioo
AI.2 300 30~ 3g
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day number,[1985)
May 25
June13
Precipitation,(mm) 2r
0 t-
•
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,
.
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.
.
.
.
.
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.
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°
Fig. 4. Matrie soil water potential ~. simulated for depths 25 and 75 em (lines) and measured at similar depth (points with numbers indicating depth). tential ~ and uptake of water by roots, rz, are given for days 145, 14"7, 155 and 164. On the first days of the drying period water uptake occurred in the total root zone, but the water in the surface horizons was soon depleted, and an increasing fraction of the total water uptake was extracted from deeper in the root zone. An important point to check was whether the water uptake calculated by S W A T R E is of the same magnitude as the transpiration calculated from meteorological data in the SPAC model. Another problem to consider was whether the upward flux of water from below the root zone could be an important fraction of the water taken up as the soil dried out. Forests grown on slopes with shallow soil on rock material can be supplied by lateral movement of water (sub-surface runoff). The simulated pattern of the root water uptake, shown in Fig. 6 for the end of the drying period, indicates that the maximum uptake is always at the bottom of the rooting layer. Below 75 cm, dense soil material (horizon B2.2) limited root pene-
191
/J
i
-1-
-2-
3--3
-~
;
o
-log ~,(MPa] measured
Fig. 5. Relationship between simulated and measured values of the soil water potential (~,,). The equation holds; y = 0.0399 + 1.0192x w h e r e y = log ~& simulated and x = log ~ measured; r = 0.99.
Oi
204
20
404
~40
E
604
U~60
804
80
~o~4
100
-0.2
-o.4
-0.8
Ws,(lo-' MPa)
-os
-4
2'o
40
60
8'o ~60
G.10-',(day-~}
Fig. 6. Depth distribution of matric soil water potential ~'s and the rate of root water uptake rz d u r i n g t h e drying period from 25 M a y - 13 J u n e ; z = depth.
tration, but apparently it did not prevent the upward movement of water into the root zone. The cumulative transpiration from the SWATRE model during the period 25 May-13 June was 61.3 mm, and cumulative upward movement of water at the bottom of the soil profile was 7.5 mm, i.e. 16%. For 20 days the upward flux was equivalent to 0.37 mm day-1, which is in agreement with data of Nnyamah and Black (1977). They found a maximum upward flux of 0.20.3 mm day- 1 in Douglas-fir forest on soil overlying a compacted basal till; the water flux into the bottom of the root zone became an increasingly more important fraction of the water taken up as the soil dried out. Our measurements and simulation show that, as the soil dried, the deeper
192
soil layers which were wetter than the surface layers contributed an increasing fraction to the total water uptake. Water flux in the soil was predominantly upward, becoming an important fraction of the total water uptake as the soil dried, but this problem needs a more detailed study. Water in the trees
The results of simulation by the SPAC model and field measurements are given in Fig. 7, where the solar radiation, air temperature, xylem water potential and bulk stomatal resistance are shown for the days 12 June, 27 June, 18 July and 4 September 1985; values of soil water potentials, ~s, are also given. Figure 7 indicates that plants do not show great diurnal variations in water status; for example, the amplitude of daily fluctuations of the xylem water potential is not dramatic and the stomata are open even at midday, with the high evaporative demand of the atmosphere. 1985-06-12
1000
Ro'{Wm-2)
1985-06-27
Ta'(°C)
201000 IF10
.10 bOO 10
0
j lO ta
0
&+ra,(S cn'T11
p~ /
2
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6
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1985-07-18
1985-09-04
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186 10 14 18 Time,ihours) nz= 3,77mmdayfl ~U= 4,57mmday-I
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186 10 Time ,(hours]
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nz- 240 mmday-1 ~U= 2,74mmday-fl
Fig. 7. Example of diurnal changes of simulated variables by SPAC model. Output variables are: the twig water potential ~'t (points are measured) and the bulk stomatal resistance rs (points are measured values obtained from g8 and LAI registration); r8= 0.05 s c m - 1;R~ = global solar radiation; T = air temperature; ~8 = soil water potential; U = water uptake; Rz = transpiration.
193 Comparison can be made between the water uptake simulated by the SWATRE model and by the SPAC model. In SWATRE it is a sink of soil water, in SPAC it is a supply of water fortranspiration; taking the period 25 May-13 June, the cumulative simulated values of transpiration are 66.5 and 62.7 mm, respectively. Cumulative values show good agreement, but large deviations have been observed on comparisons of daily values. CONCLUSION Transpiration rates from a Douglas-fir forest simulated by the SWATRE and SPAC model were 3.5 and 3.3 m m day -1, respectively, between 25 May and 13 June 1985. Root water uptake, simulated by both models, gave similar cumulative results. Simulated and measured values of soil water potentials agreed well for the 20-day period. The agreement between measured and simulated values was satisfactory, comparing the bulk stomatal resistance and the xylem water potential for the dates 12 and 27 June, 18 July and 4 September. The experimental results indicate that Douglas-fir trees did not show evident water stress during the dry year of 1985, even for dry soil, shallow rooting depth and high evaporative demand. Stomata were still fully open even when the soil water supply was limited. Xylem water potential did not drop to very low values, even during hot days with dry soils. Simulation shows quite large participation of upward water flux from parent rock into lower regions of the soil profile. For the period 25 May-13 June 1985 this upward flux was equal to 16% of the transpiration rate, and for later dry periods it can be even more important. This problem is not yet solved and remains an open question for future research. ACKNOWLEDGEMENTS Dr. J. Grace (University of Edinburgh) and two anonymous referees are gratefully acknowledged for the critical reading of the manuscript. The research was supported by Italian CNR, Spe.cial Grant IPRA-SubProject No. 1, Paper No. 1199.
REFERENCES Arya, L.M., Farrel, D.A. and Blake, G.R., 1975. A field study of soil water depletion patterns in presence of growingsoybean roots: 1. Determination of hydraulic properties of the soil. Soil Sci. Soc. Am. Proc., 39: 424-430. Belmans, C., Wesseling,J.G. and Feddes, R.A., 1983. Simulation model of the water balance of a cropped soil: SWATRE.J. Hydrol.,63: 271-286. Borghetti, M. and Vendramin, G.G., 1987. Seasonal changes of soil and plant water relations in Douglas-firforest. Acta Oecol.Oecol.Plant, 8."113-126.
194 Borghetti, M., Busoni, E., Calamini, G., Cantiani, M., Giannini, R. Grazi, S., Paganucci, L., Sanesi, G. and Vendramin, G.G., 1983. I1 test-site al Passo della Consuma. Progetto Agricoltura Nuova, Universit~ di Firenze, 73 pp. Borghetti, M., Vendramin, G.G. and Giannini, R., 1986. Specific leaf area and leaf area index distribution in a young Douglas-fir stand. Can. J. For. Res., 16: 1283-1288. Feddes, R.A. and Rijtema, P.E., 1972. Water withdrawal by plant roots. J. Hydrol., 17: 33-50. Feddes, R.A., Kowalik, P.J. and Zaradny, H., 1978. Simulation of field water use and crop yield. Simulation Monographs, 90-220-0676-X, Pudoc, Wageningen, 188 pp. Gardner, W.R., 1960. Dynamic aspects of water availability to plants. Soil Sci., 89: 63-73. Gardner, W.R. and Ehling, C.F., 1963. The influence of soil water on transpiration of plants. J. Geophys. Res., 68: 5719-5724. Gash, J.H.C. and Stewart, J.B., 1975. The average surface resistance of a pine forest derived from Bowen ratio measurements. Boundary:layer Meteorol., 8: 453-464. HeUkvist, J., Richards, G.P. and Jarvis, P.G., 1974. Vertical gradients of water potential and tissue water relations in Sitka Spruce trees measured with the pressure chamber. J. Appl. Ecol., 11: 637-667. Kelliher, F.M., Black, T.A. and Barr, A.G., 1984. Estimation of twig xylem water potential in young Douglas-fir trees. Can. J. For. Res., 14: 481-487. Kowalik, P.J. and Eckersten, H., 1984. Water transfer from soil through plants to the atmosphere in willow energy forest. Ecol. Model., 26: 251-284. Kowalik, P.J. and Turner, N.C., 1983. Diurnal changes in the water relations and tranpiration of a soybean crop simulated during the development of water deficits. Irrig. Sci., 4: 225-238. Landsberg, J.J. and McMurtrie, R., 1984. Water use by isolated trees. In: M.L. Sharma (Editor), Evapotranspiration from Plant Communities. Developments in Agricultural and ManagedForest Ecology, 13. Elsevier, Amsterdam. Reprinted from Agric. Water Manage., 8: 223-242. Lang, A.R.G. and Garner, W.R., 1970. Limitation to water flux from soils to plants. Agron. J., 62: 693-695. Lassoie, J.P., 1981. Physiological activity in Douglas-fir. In: R.L. Edmonds (Editors), Analysis of Coniferous Forest Ecosystems in the Western United States. US/IPB Synthesis Series N. 14. Hutchinson Ross, Stroudsburg, PA, pp. 126-185. Mohren, G.M.J., Van Gerwen, C.P. and Spitters, C.J.T., 1984. Simulation of primary production in even-aged stands of Douglas-fir. For. Ecol. Manage., 9: 27-49. Nnyamah, J.U. and Black, T.A., 1977. Rates and patterns of water uptake in a Douglas-fir forest. Soil Sci. Soc. Am. J., 41: 972-979. Nnyamah, J.U., Black, T.A. and Tan, C.S., 1978. Resistance to water uptake in a Douglas-fir forest. Soil Sci., 126: 63-76. Running, S.W., 1976. Environmental control of leaf water conductance in conifers. Can. J. For. Res., 6: 104-112. Scholander, P.F., Hammel, H.T. and Hemmingsen, E.H., 1965. Sap pressure in vascular plants. Science, 148: 339-346. Tan, C.S., Black, T.A. and Nnyamah, J.U., 1977. Characteristics of stomatal diffusion resistance in a Douglas-fir forest exposed to soil water deficits. Can. J. For. Res., 7: 595-604. Waring, R.H. and Running, S.,W., 1978. Sapwood water storage: its contribution to transpiration and effect upon water conductance through the stems of old-growth Douglas-fir. Plant Cell Environ., 1: 131-140. Waring, R.H., Whitehead, D. and Jarvis, P.J., 1979. The contribution of stored water to transpiration of Scots pine. Plant Cell Environ., 2: 309-317.
181
Measured and Simulated Water Relations in a Douglas-Fir Forest During the Development of Drought in the Apennines, Central Italy P.J. KOWALIK 1'3, M. BORGHETTI 2'4, E. BUSONI 3, G. SANESI 3 and G.G. VENDRAMIN 2 1Centro di Studio per la Genesi, Classificazione e Cartografia del Suolo, Consiglio Nazionale delle Ricerche, Piazzale delle Cascine, 15 - 50144 Firenze (Italy) 2Istituto Miglioramento Genetico Piante Forestali, Consiglio Nazionale delle Ricerche, Via S. Bonaventura, 13 - 50145 Firenze (Italy) (Accepted 15 October 1987)
ABSTRACT Kowalik, P.J., Borghetti, M., Busoni, E., Sanesi, G. and Vendramin, G.G., 1988. Measured and simulated water relations in a Douglas-fir forest during the development of drought in the Apennines, central Italy. For. Ecol. Manage., 25: 181-194. A model of soil water dynamics (SWATRE) and of the soil-plant-atmosphere continuum (SPAC), including the plant water-retention component, were applied independently to describe the seasonal and diurnal water balance of a forest in Italy. The stand under consideration was a 26-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation with a density of 2044 trees h a - ' and average height of 14.2 m, located in the Apennines in Central Italy, 900 m a.s.1, in a mediterranean climate. Soil water potential was measured with tensiometers and psychrometers 2 or 3 times per week during the summer of 1985. The xylem water potential and leaf stomatal conductance were measured on 4 days in the period June-September 1985, at about 2-3-h intervals from sunrise to sunset. Global and net solar radiation, air temperature and air relative humidity above the canopy were measured. Soil water potentials simulated by the SWATRE model agreed well with the field measurements. The diurnal pattern of bulk stomatal resistance and xylem water potential simulated by the SPAC model also agreed with measurements. The results show that Douglas-fir is surprisingly well adapted to the drought conditions that occurred in Tuscany during the dry year 1985, and even with dry soil, shallow rooting depth and high evaporative demand, trees did not show evident water stress.
INTRODUCTION Several studies on water relations of Douglas-fir (Pseudotsuga menziesii ( M i r b . ) F r a n c o ) h a v e b e e n c a r r i e d o u t i n n o r t h e r n e c o s y s t e m s {see r e v i e w b y 3On leave from Technical University of Gdansk, PL-80952-Gdansk-Wrzeszcz (Poland) 4Correspondence address.
0378-1127/88/$03.50
© 1988 Elsevier Science Publishers B.V.
182
Lassoie, 1981). The present research is a contribution to this subject performed in mediterranean conditions. To evaluate the ecophysiological adjustments of trees to changing environments, and as an aid to watershed management, we require mathematical models based on carefully designed field experiments. Such experiments have recently been completed in a selected Douglas-fir plantation in the Apennines of Central Italy (Borghetti and Vendramin, 1987) and the present work concerns the development of a modelling approach. Two existing computer simulation models were used. The first is a model of soil water dynamics (SWATRE), developed by Feddes et al. (1978) and Belmans et al. (1983). The second is a model of the soil-plant-atmosphere continuum (SPAC), with a plant water-retention component, developed by Kowalik and Turner (1983) and Kowalik and Eckersten (1984). One objective was to calculate the transpiration of the forest during periods without rainfall. Another objective was to evaluate the contribution to transpiration of water flux from the lower regions of the soil profile. It was also important to know how Douglas-fir is adapted to the conditions of Tuscany, Italy, with its dry soil, shallow rooting depth and high evaporative demand and, in particular, to model the decline in stomatal conductance and xylem water potential during the development of drought in a dry summer. EXPERIMENTAL DETAILS
Experimental site The experiment was carried out in a Pseudotsuga menziesii plantation during the summer of 1985. The site is NW-facing on a slope of about 10%, at Passo della Consuma near Florence .(Tuscany, Italy), 43°46'43"N, 11 ° 44'13" E, elevation 900 m a.s.1. The 26-year-old plantation had a density of 2044 trees ha-1, whose average height was 14.2 m, basal area of 41.7 m 2 ha-1, and a projected leaf area index of 9.8. There was very sparse undergrowth at the experimental site. The mean annual temperature (period 1951-1980) is 9.4 ° C and the mean annual rainfall 1248 mm. The soil is an acid brown, about 110 cm in depth, classified as a Typic Distrochrepts in accordance with the USDA's Soil Taxonomy. Very few roots were found below 75-cm depth. Further description of the site can be found in Borghetti et al. (1983). The warm, dry summer of 1985 provided good drying cycles for soil and plant water studies. Temperature and rainfall during the period studied are given in Table 1. Most of the data used in this paper were obtained in the periods 25 May-13 June, 18 July-24 July, and 17 August-22 August, 1985.
183 TABLE 1 Mean temperature (T) and total precipitation (P) in the study area during the sampling period in 1985 and for the period 1951-1980 Month
1985
T (°C) January February March April May June July August September
- 1.4 3.5 2.8 7.8 12.0 15.5 20.0 19.0 17.0
1951-1980
P (mm)
T (°C)
P (mm)
110 50 150 35 95 55 35 100 5
1.9 2.4 4.5 7.5 11.5 15.0 17.8 17.9 14.9
109 113 104 110 105 82 56 70 96
Modelling The model SWATRE is used to calculate the flux of water into the soil system with water uptake by plants. The flux of soil water is calculated by applying Darcy's equation. Water content as a function of time is calculated by combining Darcy's equation with the continuity equation. The soil is divided into a number of compartments, and fluxes calculated according to hydraulic head gradients and conductivities between compartments. In a cropped soil, the spatial and temporal variation in water content depends upon water redistribution in the soil profile and water uptake by plants. In SWATRE, water uptake by plants is calculated from each soil compartment within the root zone as a fraction of the maximum potential water uptake, i.e. potential transpiration. Water uptake by the roots is represented by a volumetric sink term (see Feddes et al., 1978) which simply is added to the continuity equation. Soil water is considered to be potentially available to the plants in equal amounts within the root zone. It means that roots can extract from each soil compartment an amount of water equal to the potential transpiration divided by the number of compartments within the root zone. In the model, the relationship between water uptake by the roots and soil matric potential comes from experimental determinations. SWATRE is applied here to simulate long-term daily water uptake by the roots during drying. Upper boundary conditions are daily potential transpiration rates (calculated according to Penman-Monteith) and potential soil evaporation (here set to zero because of the complete canopy cover and the thick litter layer); lower boundary conditions are daily measured soil matric potentials at 1.1-m depth. Parameters like sink term, soil water-retention curves,
184
and hydraulic conductivity were derived from measurements. The simulation runs with a 1-day time-step. The model SPAC describes interrelations of water potentials and fluxes in the soil-plant-atmosphere continuum, and takes into consideration the water stored in the plants (a capacitance term). The assumptions of SPAC are: (i) the water status of the canopy is governed by the water lost by transpiration and the water supplied by the roots; (ii) the water status of the canopy controls the rate of water uptake and water loss by the plant - - the opening of the stomata is regulated at least in part by the water status of the leaves while at the same time the rate of water uptake is governed by the difference in water status between the leaf and soil; (iii) the root system is treated as a single unit; and (iv) the model assumes that there is a certain amount of exchangeable water stored in the plant which allows, within limits, transpiration to exceed, to equal, or to be less than water uptake by the roots. Driving input variables (time-dependent data) are measured values of air temperature, relative humidity and solar radiation (30-min time-step data) and the daily value of soil water potential in the rooting zone (here at 25-cm depth). Time-invariant data include: (i) the relationships between bulk stomatal resistance and water potential, between bulk stomatal resistance and solar radiation and between leaf water potential and plant water content; (ii) aerodynamic resistance; (iii) plant hydraulic resistance; and (iv) the soil-toroot hydraulic resistance as a function of soil water potential. The simulation runs with a fixed time interval of 1 min, and results are printed out for every hour. The model describes the changes in the diurnal course of transpiration, bulk stomatal resistance and xylem water potential. For a more detailed description of the SPAC model see Kowalik and Turner (1983) and Kowalik and Eckersten (1984).
Measurement of meteorological and soil water variables Air temperature and humidity were measured using ventilated psychrometers with platinum resistance thermometers. Global and net radiation were measured using a Moll-Gorczynski-type pyranometer (type CM 5/6, Kipp and Zonen, The Netherlands) and a Funk-type net radiometer (Middleton, Australia), respectively. All the sensors were placed horizontally at a height of about 1 m above the canopy. The data were recorded every 30 min (each datum being the average of 60 measurements taken every 30 s) onto a magnetic tape cassette (type 4008, Facit, Sweden) by a data logger (type PM 4001, Philips, The Netherlands). The soil water potential (~s) was measured throughout the growing season (2-3 times per week), by tensiometers in the range 0 to - 0 . 0 8 MPa, and by a Wescor HR-35 dew-point microvoltmeter and PCT-55 psychrometers (Wescor Inc., Logan, UT, U.S.A.), for values less than - 0 . 2 MPa. Psychrometers
185
were calibrated using salt solutions of known osmotic potential in the range - 0 . 2 to - 2 . 8 MPa. Soil water potentials were measured at three different places between trees, from the soil surface to a depth of 100 cm. The soil water potential was measured at depths of about 10.0 and 30.0 cm (horizon A), and 50.0, 70.0, 80.0 and 90.0 cm (horizon B ). Additional measurements at a depth of 110 cm were taken as the lower boundary condition. These tensiometer or psychrometer measurements were taken in three replicates; the arithmetic average of these readings is applied here.
Measurements of ecophysiological variables The ecophysiological measurements, described in detail by Borghetti and Vendramin (1987), were, briefly: the leaf stomatal conductance (gs) and the xylem water potential (g/t) were measured at about 2-3-h intervals from sunrise to sunset on 12 and 27 June, 18 July and 4 September. Five trees of different D B H ' s (11, 14, 19, 25 and 28 cm) were sampled. Trees were located near the tensiometer-psychrometer groups. Measurements were performed at two levels (13.5 and 11.5 m above the ground) in the crown of each tree, on three twigs from the same branch on each level. Stomatal conductance, gs, was measured by a steady-state diffusion porometer (type 1600, Li-Cor, U.S.A. ). Projected needle area was determined by a leaf-area meter (type 3000, Li-Cor, U.S.A. ) at the end of each measurement day, and g~ computed on the basis of the single-side surface area. Xylem water potential, ~t, was measured on 1-year-old twigs, using a portable pressure chamber (Scholander et al., 1965). Measurements were made within a few min of excision. The relative water content (RWC) was determined on samples of needles taken from the same twigs used for the determination of ~t, using the method described by Waring et al. (1979).
Laboratory measurement of soil hydrologic parameters Water retention curves in the range -0.01 MPa to - 1 . 5 MPa were determined on undisturbed samples from various soil horizons by the sand-box and pressure-plate extraction technique. The water retention data and soil water potentials were used to obtain volumetric soil water contents in the soil profile. The unsaturated hydraulic conductivity (K) was measured in the laboratory by the transient-outflow method, using a procedure similar to that of Arya et al. (1975). Bulk soil samples were taken from the site using a coring device.
186 RESULTS AND DISCUSSION
Time-invariant data for SWA TRE and SPAC Time-invariant data for SWATRE are water retention curves (Fig. 1 ), soil water conductivity, K(~u~; Fig. 2 ) and the volumetric sink term. The sink term was obtained from experimental data. Water uptake by the roots was calculated from the rate of change of soil water potential and the water flux divergence at each compartment (see Nnyamah and Black, 1977). Water uptake was then plotted against soil matric potential to evaluate the sink term. According to our relations, Douglas-fir seems to optimally take up water from -0.002 MPa to - 0 . 0 2 MPa, then water uptake (expressed as a fraction of the potential transpiration) was about 0.55 and 0.2 when the soil matric potential was 0.1 and 1 MPa, respectively. For SPAC, the values of K(~u~) can be approximated using the relationship found by Lang and Gardner (1970) for a number of soils: K(~s)=a(l~'~l)-"
(1)
where n is normally between 2 and 3 for fine-textured soils and about 5 for sands, and a is a coefficient related to the hydraulic conductivity of saturated soil. In the range we measured, it was found that the coefficient n = l . 7 was appropriate to both A and B horizons, with a= 1.58-108 for the A horizon, and a=4.74.106 for the B2.1 and B2.2 horizons, if ~'s is in MPa and K in cm day -1 (Fig. 2). Soil-root hydraulic resistance, rr, can be calculated from: 1 l=Au ,O-Scm 2=A1~, 10-15 cm 3=Bz1,40-50 cm 4 = 8 ~ , 7 0 - 8 0 cm
-10-1
!
_10-~
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-10-"-
- 1 0 -5
o
o'.1
0'.2
0'.3
,
,
04
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o.6
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Fig. i. Water retention curves for various layers of the soilprofile; ¥. = soilwater potential; ~9= soil water content.
187 1
10-1 • ~ | o \ 10-~ 10_~
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Fig. 2. Unsaturated hydraulic conductivity (K) as a function of soil water potential (~,s) for various layers of the soil profile; for Lang and Gardner, (1970) equation.
rr =b/K(~&)
(2)
where b is the root density-resistance factor that takes into account the length and geometry of the root system. Equation (2) was used by Gardner and Ehling (1963) and applied to Douglas-fir roots by Nnyamah et al. (1978). From the theory of Gardner (1960) and of Feddes and Rijtema (1972) the coefficient b is:
b=ln(Le/R)/2~LrL
(3)
where: L is the rooting depth (cm); L,, the root density (cm root cm -3 soil); R, the root radius (cm); and Le, half the distance (cm) between neighbouring roots (Le= (~L,)-°5). According to Nnyamah and Black (1978), the mean root diameter and the average root density were respectively 0.08 cm and 0.66 cm root cm -3 soil, for Douglas-fir forest. Because L=75 cm in our experiments, the coefficient of root density-resistance factor is taken to be 0.0069 cm. A sensitivity analysis showed that the effects of different values of b on the simulated output variables were small. The relationship between needle relative water content (RWC) and xylem water potential, assumed to be in equilibrium with needle water potential (Kelliher et al., 1984), is given in Fig. 3. The linear relation ~gt (RWC) can be applied, where: RWC = 99.56-- 11.76~t
r2=0.69
(4)
188 10
100
90
'E 8O
o
•
5C 70
O0
60 0.50
-0.75
1.00
1.25
1.50
1.75
-2.00
'~t (MPa)
Fig. 3. Relationship between the needle relative water content (RWC) and the twig water potential (~t); V = amount of water readily exchangeable in the plant per unit area of soil.
From the definition of RWC, Equation (4) can be written as:
~t = ~ t m ( 1 - V/Vo )
(5)
where: V (g m - 2 ) is the amount of easily exchangeable water in the plant per unit area of the soil at a certain time; Vo, the maximum value for V (RWC=100%); and ~tm ( M P a ) , the minimum value of ~gt when V=O. The maximum storage of easily exchangeable water w h e n no water stress is present was estimated as V-- 1000 g m -2. If V--500 g m -2, then RWC=80% and if V--0, t h e n RWC = 60% - - arbitrary but in agreement with the measured tendency. For V= 0, ~t is equal to - 3.6 MPa. We assumed that only a fraction of the water stored in the plant was readily available for transpiration. Within the short period used by the SPAC model for the simulation, it is to be expected t h a t most water will move to the liquidair interface from the nearest sources of most-readily available water, such as cells in the leaves. In fact, the assumed value of V has the same order of the amount of water estimated to be available in the leaves at saturation (see Borghetti and Vendramin, 1987). This assumption is in accordance with experimental results. Low values of water depletion from storage sources were found by Borghetti and Vendramin (1987), while N n y a m a h and Black (1977) also reported that the a m o u n t of water t h a t is withdrawn from the storage sources on a given day is relatively small, The following equation loggs = - 1.56-- 3.478/x//Q
(6)
was found to fit the relationship between the stomatal conductance (g~) and the photon flux density (Q). The threshold for stomata opening can be taken as about 80 z E m -2 s -1.
189
The relationship between the xylem potential (~gt) and the leaf stomatal conductance (gs) for the plant can be determined, but Landsberg and McMurtrie (1984) note that there is generally little effect of ~t on the stomatal conductance until potentials have fallen to rather low levels. Running (1976) found in Douglas-fir a threshold ~'t for stomatal closure close to - 2.0 MPa. In our measurements no such threshold was reached, and we assumed no effect of ~/]ton g~ over the range of ~/]tfrom 0 to - 2.0 MPa. In our experiments the bulk stomatal resistance rs was calculated from the equation: rs = 1 / g s ' L A I
(7)
where LAI is the leaf area index of the needle layer; the values of LAI were taken from profile measurements in Borghetti et al. (1986). The boundary layer resistance ( ra ) was assumed to be 5 s m - 1 ( Gash and Stewart, 1975 ), irrespective of wind speed. Plant resistance to liquid water flow from the roots to the mesophyll of the leaves was taken as r = 0 . 1 6 , l0 s MPa s m -1, in accordance with the value reported for willow (Kowalik and Eckersten, 1984). This value is considerably smaller with respect to the values reported for conifers (see the data tabulated by Hellkvist et al., 1974 ). However, working on the same Douglas-fir stand as we are now considering, Borghetti and Vendramin (1987) recently calculated a sapwood conductivity much greater t h a n values reported in the literature (Waring and Running, 1978; Waring et al., 1979).
Soil profile water depletion patterns Three major drying cycles were observed during 1985, but only the period 25 May-13 June was dry enough to enable comparisons of measured and simulated soil water potentials over a wide range. As the season progressed the soil profile became drier, not only at the top but at the bottom as well, due to intense root water uptake in the 75-cm soil layer. Daily values of soil water potential, soil water content, soil water flux (between layers) and root uptake rate through the soil profile were calculated using the SWATRE program. The lines in Fig. 4 are simulated values of ~ for the depth of 25 and 75 cm, for the period 25 May-13 June. Points are averages of measurements at different locations. For the three periods (25 May-13 June; 18-23 July; 17-21 August) the relationship between the measured and simulated values is shown in Fig. 5. The rates and patterns of the root water uptake during a 3-week drying period are shown in Fig. 6. The vertical distribution of the matric soil water po-
190
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02
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day number,[1985)
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Precipitation,(mm) 2r
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,
,
.
.
.
.
.
.
.
.
.
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Fig. 4. Matrie soil water potential ~. simulated for depths 25 and 75 em (lines) and measured at similar depth (points with numbers indicating depth). tential ~ and uptake of water by roots, rz, are given for days 145, 14"7, 155 and 164. On the first days of the drying period water uptake occurred in the total root zone, but the water in the surface horizons was soon depleted, and an increasing fraction of the total water uptake was extracted from deeper in the root zone. An important point to check was whether the water uptake calculated by S W A T R E is of the same magnitude as the transpiration calculated from meteorological data in the SPAC model. Another problem to consider was whether the upward flux of water from below the root zone could be an important fraction of the water taken up as the soil dried out. Forests grown on slopes with shallow soil on rock material can be supplied by lateral movement of water (sub-surface runoff). The simulated pattern of the root water uptake, shown in Fig. 6 for the end of the drying period, indicates that the maximum uptake is always at the bottom of the rooting layer. Below 75 cm, dense soil material (horizon B2.2) limited root pene-
191
/J
i
-1-
-2-
3--3
-~
;
o
-log ~,(MPa] measured
Fig. 5. Relationship between simulated and measured values of the soil water potential (~,,). The equation holds; y = 0.0399 + 1.0192x w h e r e y = log ~& simulated and x = log ~ measured; r = 0.99.
Oi
204
20
404
~40
E
604
U~60
804
80
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100
-0.2
-o.4
-0.8
Ws,(lo-' MPa)
-os
-4
2'o
40
60
8'o ~60
G.10-',(day-~}
Fig. 6. Depth distribution of matric soil water potential ~'s and the rate of root water uptake rz d u r i n g t h e drying period from 25 M a y - 13 J u n e ; z = depth.
tration, but apparently it did not prevent the upward movement of water into the root zone. The cumulative transpiration from the SWATRE model during the period 25 May-13 June was 61.3 mm, and cumulative upward movement of water at the bottom of the soil profile was 7.5 mm, i.e. 16%. For 20 days the upward flux was equivalent to 0.37 mm day-1, which is in agreement with data of Nnyamah and Black (1977). They found a maximum upward flux of 0.20.3 mm day- 1 in Douglas-fir forest on soil overlying a compacted basal till; the water flux into the bottom of the root zone became an increasingly more important fraction of the water taken up as the soil dried out. Our measurements and simulation show that, as the soil dried, the deeper
192
soil layers which were wetter than the surface layers contributed an increasing fraction to the total water uptake. Water flux in the soil was predominantly upward, becoming an important fraction of the total water uptake as the soil dried, but this problem needs a more detailed study. Water in the trees
The results of simulation by the SPAC model and field measurements are given in Fig. 7, where the solar radiation, air temperature, xylem water potential and bulk stomatal resistance are shown for the days 12 June, 27 June, 18 July and 4 September 1985; values of soil water potentials, ~s, are also given. Figure 7 indicates that plants do not show great diurnal variations in water status; for example, the amplitude of daily fluctuations of the xylem water potential is not dramatic and the stomata are open even at midday, with the high evaporative demand of the atmosphere. 1985-06-12
1000
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1985-06-27
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Fig. 7. Example of diurnal changes of simulated variables by SPAC model. Output variables are: the twig water potential ~'t (points are measured) and the bulk stomatal resistance rs (points are measured values obtained from g8 and LAI registration); r8= 0.05 s c m - 1;R~ = global solar radiation; T = air temperature; ~8 = soil water potential; U = water uptake; Rz = transpiration.
193 Comparison can be made between the water uptake simulated by the SWATRE model and by the SPAC model. In SWATRE it is a sink of soil water, in SPAC it is a supply of water fortranspiration; taking the period 25 May-13 June, the cumulative simulated values of transpiration are 66.5 and 62.7 mm, respectively. Cumulative values show good agreement, but large deviations have been observed on comparisons of daily values. CONCLUSION Transpiration rates from a Douglas-fir forest simulated by the SWATRE and SPAC model were 3.5 and 3.3 m m day -1, respectively, between 25 May and 13 June 1985. Root water uptake, simulated by both models, gave similar cumulative results. Simulated and measured values of soil water potentials agreed well for the 20-day period. The agreement between measured and simulated values was satisfactory, comparing the bulk stomatal resistance and the xylem water potential for the dates 12 and 27 June, 18 July and 4 September. The experimental results indicate that Douglas-fir trees did not show evident water stress during the dry year of 1985, even for dry soil, shallow rooting depth and high evaporative demand. Stomata were still fully open even when the soil water supply was limited. Xylem water potential did not drop to very low values, even during hot days with dry soils. Simulation shows quite large participation of upward water flux from parent rock into lower regions of the soil profile. For the period 25 May-13 June 1985 this upward flux was equal to 16% of the transpiration rate, and for later dry periods it can be even more important. This problem is not yet solved and remains an open question for future research. ACKNOWLEDGEMENTS Dr. J. Grace (University of Edinburgh) and two anonymous referees are gratefully acknowledged for the critical reading of the manuscript. The research was supported by Italian CNR, Spe.cial Grant IPRA-SubProject No. 1, Paper No. 1199.
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