Bioclimate of three varieties of oat

Agricultural Meteorology, 17(1976) 401--431 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

BIOCLIMATE OF THREE VARIETIES OF OAT L. FORS*

Department of Plant Ecology, University of Lund, Lund (Sweden) (Received September 22, 1976: accepted October 20, 1976)

ABSTRACT Fors, L., 1976. Bioclimate of three varieties of oat. Agric. Meteorol., 17: 401--431. The bioclimate in plots of three oat varieties grown under three different water regimes (unmodified, drier, and wetter plots) was studied. The bioclimate measurements were combined with soil water determinations and growth and production measurements. White gutters, used to obtain drought conditions, disturbed the climate close to the ground: the zone they sheltered had high RH-values and low diurnal temperature variation at the soil surface. Though there was greater insolation on the dry plots than on the normal ones the maximum temperatures did not differ. In the wet plots, with higher and denser vegetation, the maximum temperature was less and the temperature range in the crops was smaller. Temperature recordings during irrigation showed abrupt changes. The temperature fall of 4°--7°C was, however, regained some hours after the irrigation was finished. Energy penetration into the crop was strongly correlated with phytomass, The net radiant flux into a dry plot was about 70% of that into a wet plot in the morning, but this difference decreased during the day and disappeared late in the afternoon.

INTRODUC~ON

The structure of the ecosystem and the vegetation strongly influence the bioclimate near the ground. Below spruce forest and deciduous forest in leaf there is a notably uniform temperature climate and a low radiant flux mostly of diffuse light (Nihlg~d, 1969; Tamai a n d Shidei, 1972). Compared to the forest the cultivated or near natural grassland has lower net radiant flux and lower evapotranspiration and has from a general point of view much more extreme climatic conditions (P~hlsson, 1966; Jakucs, 1968a; Denmead, 1969). In such "open ecosystems" the temperature maximum for the day is rather close to the ground, and the night minimum is rather close to the vegetation canopy. The plant density of the open ecosystem affects temperature and humidity in the vegetation stand (Gregory, 1926; Penman and Long, 1960; Mattsson, 1961; P~ihlsson, 1966; Ripley and Saugier, 1974; Peacock, 1975). Covering the soil with plastic reduces energy interception by the crop (Fritschen and Shaw, 1961). Irrigation-water temperature has *Present address: Weibullsholm Plant Breeding Institute, Landskrona (Sweden)

402

little effect on soil temperature and the effect is short-lived, probably having no influence on plant productivity (Wierenga et al., 1971; Kohl and Wright, 1974). Gutters, used to produce drought conditions, may effect the bioclimate near the ground (Grant, 1970; Rackham, 1972). The aim of this work was to elucidate the differences of temperature profiles, humidity and radiation climate at two stages of vegetation development in oat crops grown under each of three different water regimes. The present paper discusses the bioclimate of the oat crops, but also refers to the soil-moisture fluctuations and the oat growth and production as a result of the water-supply treatments. The soil-physics and soil-moisture regimes have already been discussed elsewhere (Fors, 1973). INVESTIGATION A R E A

The studies were made at Norrvidinge, at the southern tip of Sweden, about 27 km NNE of MalmS. The soil type is that known in Sweden as "South-West Moraine, restratified by wave action" (EkstrSm, 1950). The top soil is poor in humus, containing about 2.5% organic matter. T h e clay content is about 6%, and the dominating grain size is 0 . 6 - 0 . 2 mm (60%). The root zone of the sub-soil has 3--4% clay and 65--80% is in the range 0.6--0.2 mm diameter (cf. Fors, 1973). The site is on a plane about 28 m above sea-level and 11 km from the sea, which moderates the winter climate (Table I). M A T E R I A L AND T R E A T M E N T

One black oat variety (Stormogul II, Swedish) and two white ones (Pendek, Dutch and SSrbo, Swedish) were chosen for study. Stormogul II (Great Mogul II) is supposed to be drought-resistant, due to its late earing and high tiUering frequency. The plants were grown in monospecific plots 3 × 3 m with 1.5 m paths between. Each variety was grown at three different soil-moisture levels and there were three replicates (Fig.l). The middle row of plots was called "normal", for convenience. They got the precipitation for the actual site and actual year. One row of plots was called "wet". In addition to "normal" precipitation it received extra water from a watering-can filled from a sunken covered static water tank. The amount of water added corresponded to about 15 mm precipitation and was given every third day of bright sunshine. The irrigation treatment was not intended to be closely defined but merely to ensure that the treated plants suffered no water stress. The third row of plots was called " d r y " and received less water than the "normal" plots. This was achieved by placing gutters (white, 10 cm wide) between the plant rows, covering about 67% of the plot area (cf. Fors, 1973, p. 24). The technique is practised at the Plant Breeding Institute, Cambridge (Grant, 1970), but was here somewhat modified. The water from the gutters was collected in 20{M oil-barrels sunk in the ground. Depending on plant

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heights and varieties, between 25 and 50% of the precipitation was collected in the barrels with this arrangement. To lessen the change in the microclimate during the time when the plants were shooting (cf. Grant, 1970), 11 of the 21 gutters were laid out when the plants had reached a height of about 5--8 cm, and the remainder about two weeks later. This compromise between the need for early precipitation reduction and for plant establishment seems to have been successful. The oats were sown on April 14th with a ~ plot sowing machine with the seed-drills 15 cm apart. In each plot the soil-water content and the plant development were followed from sowing to harvest. For details see Fors (1973). METHODS

The precipitation was measured daily at 7 a.m. in the research area (Fig.l)

with a SMHI (Swedish Meteorological and Hydrological Institute) standard gauge (200 cm 2 collecting area) at about 1.5 m above the ground. The monthly precipitation at the investigation area is compared with measurements during the same time (and with long-term averages) from official meteorological stations in the neighbourhood (Table I).

405

Official screen and soil temperatures (Table I) as well as the temperatures in the vegetation-covered areas were compared with long-term average thermograph measurements (Wilh. Lambrecht KG, GSttingen, type 257} in a bare-soil plot within the research area (Fig.1). The sensitive part and the connecting cable of the thermographs for above-ground measurements were shaded from direct insolation b y a covering o f aluminium foil. The temperature was measured at the levels - 5 0 , - 2 5 , - 1 0 , +0, +10, +25, +50 and +150 cm between May 1st and August 19th. The curves on the thermographs were analysed b y measuring the area b e t w e e n the curve and a base line (0°C or -10°C) per 24 h with a planimeter, giving the 24-h mean temperature. Except for the long-term bare-soil temperature measurements, the temperature profile in the nine second replicate plots (r2, Fig.l) were studied intensively during 24 h with one thermistor at each of the levels - 5 0 , - 2 5 , - 1 0 , - 5 , - 2 , +0, +2, +10, +25, +50, +150 cm and at the average plant heights. In the air, Pearl thermistors, type N, shielded with aluminium foils, were used. Soil temperatures were recorded with isolated thermistors t y p e B5. Both types are manufactured by HAFO, Stockholm. To avoid disturbing the roots, the soil thermistors at the four deepest horizons were placed out on April 30th (just after shooting) with a soil auger (23 mm ¢). The temperature of each thermistor was read once an hour with a t y p e R1 Normameter. Measurements with air thermistors must be regarded as rather uncertain during longer periods. Short-term studies can however be made with an accuracy of +0. 5 - 1.0°C (P~ihlsson, 1966). Soil thermistors are usually more capable of resisting the influence of the surroundings. Another temperature study was made in the nine second replicate plots with thermocouples connected to a Honeywell Brown Electronic recorder (cf. Nihlg~rd, 1969; P~hlsson, 1974). The thermocouples were placed in the centre of each plot at 15 cm height and at the average crop height. The mean of the 8 or 9 recorded figures from each measuring point (printing every 7th minute) is regarded as the mean temperature per hour (the average for 60 min centred on the stroke of the hour). Simultaneously the relative humidity of the air close to the ground was measured in each plot with Lambrecht-type hair hygrometers. The wind velocity was measured with R.Fuess-type cup anemometers at 10 cm height in the centre of the third replicate Stormogul-II normal plot and at 50 and 150 cm heights in the open field. The measurements were made for 5 min before to 5 min after the stroke of every hour. For albedo studies fight was measured with an " E E L Lightmaster Photometer" (cf. Nihlg~rd,! 1969) at a b o u t 1 m height. The radiation (300--2,500 nm) was measured in the centre of each plot with a Kipp & Zonen type solarimeter connected to a "Multicolour Print Recorder" (Joens, Dfisseldorf). The radiation climate of the different crops at 15 cm height was measured at the same stage of plant development; the single solarimeter was moved every 24 h (at sunset) from one plot to the next one in the following order; Pendek dry, normal, wet, SSrbo dry, normal

406

and wet from July 21st to 27th and Stormogul II dry, normal and wet from August 5th to 8th. The recorded measurements were transformed to mWh cm -2 day -1 (1 ly = 1.163 mWh cm -2 ). The radiation climate in the plots were compared with simultaneous measurements in the open field at 2 m height and with the measurements at the SMHI-stations at Sval6v and Bulltofta (cf. Tables I and VIII). As the time usually did not permit more than one day of measurement in each plot, changes in the climate could not be avoided. Several days in the period showed some clouds, a few days resulting in a few millimetres of rain (cf. Table VIII and Fig.12). The relative radiation in the plots seems, however, not to have been influenced b y the cloudiness. The radiation balance was measured with a Schultze direct-reading net radiometer (300--10 s nm) connected to the Joens-recorder. Only comparisons between dry and wet plots were made and no calculations of absolute values were done. The evaporation was measured following Andersson (1969) at the soil surface and at 150 cm height in the centre of the third replicate Stormogul-II normal plot (Fig.l). The water level was noted on the stroke of every hour. The soil moisture was determined in all three replicates with the gravi' metric (0--20 cm) and the neutron scattering (20--200 cm) methods a b o u t once a fortnight. Soil textural and structural analyses were also made (Fors, 1973). Every twelfth day parts of a fourth replicate (Fig.l, between r3 and T) were harvested (6 x 35 cm) for plant growth studies. On this plant-material height, green-leaf area index (LAI ~- m 2 green-leaf area on one leaf side per m 2 soil surface), total green area index (TAI -- green area on one leaf side plus total stem and panicle areas) and dry weight, were determined (Fors, 1973). On June 5--6, temperature studies with thermistors, B,H, wind velocity and evaporation were made. On July 6--7 the temperature was measured with thermocouples. Simultaneously RH, wind velocity and evaporation were determined. It was realized that n o t all the meteorological measurements were made to the same degree of accuracy. In particular, relative humidity and evaporation were thought to be the most susceptible to instrumental and/or sampling error.

RE S UL TS

To show the crop bioclimate during active growth and during r i p e n s , two periods were selected for detailed 24-h studies. On the first occasion, June 5--6 1971, the plants were in the fast4rowing log phase with almost no dead leaves. On the second occasion, July 6--7 1971, nearly t w o thirds of the leaves were dead, b u t the total green area was at maximum, 25% on average being in the panicle.

407

Macro-clima te The precipitation at Norrvidinge before sowing (88 mm) was small compared with the long-term average for the area (155 mm), and there was also little rain in May (Table I). The plants developed normally, however, because the soil-water content was at or near field capacity in the early critical growth periods. In June the rainfall was a little more than the average, while July was drier and August much drier than usual (Table I). The water supply between sowing and harvest (14/4--9/8) was 95--100 m m to the dry plots (depending on variety), 166 m m to the " n o r m a l " ones and 275 mm to the wet plots, compared with the long-term average precipitation of about 225 mm. The air temperature at 150 cm height over bare soil was rather low during May, but in June and July it was near average. The first week in June was 4°C above and the third week was 4°C below the average temperature.

Bio-climate Temperature At a few horizons no temperature measurements were reade in the earlyJune study. These levels are consequently n o t indicated in Figs.2 and 3. The most striking differences in the temperature regime were seen just after noon in the wet plot (Fig.2C) compared to the other ones. The two drier treatments as well as the bare-soil plot (Figs.2 and 3), produce maxima which were more sharply delimited and were closer to the ground. It was a remarkable fact that the m a x i m u m temperature at soil surface was about 10°C lower in all the dry plots than in the normal ones (Table II), b u t the differences at +10 cm were trivial. The gutters were obviously creating a protected zone near the ground with little influence from above (cf. Fig.6). The gutters also diminished the long-wave radiation at night, and this kept the m a x i m u m temperature difference relatively low and of the same magnitude as that of the wet plots (Table II). The soil surface of normal plots was the earliest to warm in the morning: up to twice as fast as in the dry plots (Fig.4). In the second 24-h study, the crop heights were between 48 and 73 cm (cf. Table III). The Pendek and, to some extent the SSrbo kernels, had started to turn yellow, while Stormogul II still had but 50% of the panicle visible (Fors, 1973). The LAI and TAI were 1.3--2.8, and 4.3--8.9 m 2 m -2 , respectively, depending on variety and t r e a t m e n t (Fors, 1973). The thermocouple measurements showed that the temperature was nearly always highest in the dry plots both at 15 cm height and at the top of the crop (Table III). The lowest m a x i m u m temperature was, as would be expected, in the densest crop (Stormogul II wet, LAI = 2.8 and TAI = 8.9), which also had the lowest daily mean temperature (Table III). The lowest night temperature was f o u n d in the open field with free long-wave radiation. The highest day temperature, however, was over the white gutters. The increase in the daily mean temperature at 10--15 cm height between

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early June and early July was about 3°C, but the dry plots had the greatest increase (3.4°C) and the wet plots the lowest one (2.9°C) (cf. Tables II and III). The mean air temperatures for June and July showed similar differences (Table I). The maximum temperature difference within 24 h is, however, 4--7°C less in July (Tables II and III), presumably as a result of the 2--3 times greater phytomass in July (Fors, 1973). The temperature of the irrigation water was not measured, but was estimated from the bare-soil temperature measurements to be about 12°C. Due to the press of other work, the watering could not be started before about 10 o'clock in the morning, when the air temperature had already reached high values. The temperature fall near the ground (+15 cm) was greatest where temperature was highest before irrigation. At 23.5°C the fall was 4.5°C and at 28°C the temperature was lowered 6.5°C (Fig. 5). The temperature course during the watering (10 watering-cans of 14 1) was irregular depending on how much water fell directly on the thermocouples. It should be noted that though the temperature courses were quite different during irrigation the final temperatures were very similar (Fig. 5). The temperature in the wet plots was about 4°C below the cluster of temperature curves in the dry and normal plots. This difference was not caused by the

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irrigation, b u t was of the same magnitude as the m a x i m u m temperatures when no irrigation was given (cf. Table III). The initial temperature decrease of a b o u t 4. 5--6.5°C was thus recovered quite rapidly.

Wind, RH, evaporation During early June in the normal plots high temperature was established relatively early in the day (Fig.4) and was associated with a high evaporation from early in the morning and a rather low relative humidity after n o o n (Fig.6). The relatively high RH-values in the dry plots are exclusively an effect of the gutters though air movements are also small near the ground in plots w i t h o u t gutters. At times when windspeed is easily measureable in the open it is undetectable inside a plot (Fig.7). Changing wind directions inside the plot at low wind speeds caused the a n e m o m e t e r to go sometimes forwards and sometimes backwards. The net result was a low (or even zero} reading, and such results have been omitted from Fig. 7. Despite the low wind velocity, evaporation from the soil is more than one third of the total evapotranspiration from the plot (Table IV). The rather high evaporation from a " n o r m a l " plot is, however, n o t surprising when the relative humidity at the

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soil surface is taken into consideration (Fig.6). The more vigorous growing wet plots (Table VII) all had saturated air during the night (Fig.6), b u t this state was never reached in the " n o r m a l " plots, and only rarely in t h e dry ones. L o w RH-values were recorded in Stormogul II wet plot during daytime (Fig.6C). These did n o t fall within the c o m m o n pattern and were perhaps results from a faulty instrument. For all varieties it was obvious that in early July the wet plots had the highest daily average relative humidity ( F i ~ 8 ) . The relative h u m i d i t y at the soil surface continued to be lowest in the " n o r m a l " p l o t s , though t h a t in Pendek dry plot was as low as the "norm_al" plots. The relatively dry a k in the open field during the night was particularly noteworthy. Just as in early

413 T A B L E II T e m p e r a t u r e s at soil surface a n d at 10 c m h e i g h t o n J u n e 5--6, 1971

Variety and treatment

Minimum

Maximum

Difference

24-h m e a n

Pendek, d r y normal wet

11.0 10.1 11.1

25.4 36.8 27.4

14.4 26.7 16.3

16.7 19.1 17.2

S6rbo,

dry normal wet

10.5 11.9 10.0

27.1 36.7 23.5

16.6 24.8 13.5

17.2 20.9 15.8

S t o r m o g u l II, dry normal wet

9.7 11.2 11.9

24.5 34.4 28.2

14.8 23.2 16.3

16.2 19.9 18.1

Pendek, dry normal wet

7.1 8.4 10.2

34.5 31.9 30.1

27.4 23.5 19.9

18.0 18.0 17.8

S6rbo, dry normal wet

11.6 9.0 6.7

33.8 36.6 25.9

22.2 27.6 19.2

18.7 19.1 15.2

S t o r m o g u l II dry normal wet

7.0 8.3 10.4

31.5 31.6 27.0

24.5 23.3 16.6

16.5 17.8 17.8

+0 c m :

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June, the high RH-values in the dry plots were caused b y the shelter of the gutters. The evaporation in July was less than that in June (Table V), b u t the difference at 10 cm height was small.

Light, albedo The illuminance was followed during most of June 6th. The cloud cover (mostly cumulus-clouds) varied between 0 and 6/8 and the m a x i m u m luminous flux density reached 76,000 lux (Table VI). Early in the morning the albedo values were high. The lowest values were recorded just before noon (Table VI). The albedo increased successively with decreasing LAI (Table VII). A vigorously growing plot under no water stress and having a LAI of 5.4 had

414 T A B L E III

Temperatures a t 15 c m and the average plant heights on J u l y 6--7, 1971

Variety and treatment

Height

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normal wet

Stormogul II, dry normal wet

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24-h mean

52 15 63 15 67 15

12.7 12.7 12.0 12.4 12.3 12.8

29.6 32.7 27.9 29.6 28.5 27.9

16.9 20.0 15.9 17.2 16.2 15.1

20.8 21.7 20.5 20.2 20.7 19.8

52 15 66 15 67 15

11.8 12.1 13.0 13.6 13.0 12.9

29.3 29.8 28.2 30.8 28.5 27.3

17.5 17.7 15.2 17.2 15.5 14.4

20.9 20.0 21.1 21.3 21.0 20.4

48 15 57 15 73 15

13.1 12.5 13.0 13.1 12.0 12.4

31.2 34.7 28.5 32.0 27.0 27.0

18.1 22.2 15.5 18.9 15.0 14.6

22.2 21.7 21.4 22.6 20.0 19.4

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28.5 33.1

16.5 21.4

20.5 21.8

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an albedo of about 4 (Tables VI and VII). A LAI of 3.6 or more resulted in an albedo of 3.9. This was true o f the "normal" and wet plots, where the ground had the usual grey soft colour. In the dry plots, however, the gutters caused a limited water supply, revealed in both dry weight and leaf area (Table VII; cf. Fors, 1973). The leaf area was, however, Slightly less in the dry plots, compared to the "normal" ones, while the albedo values were strikingly high (Table VII) because o f the high reflectance from the white gutters. Table VII shows however a very good correlation between LAI and albedo values within each treatment. Radiation The shortrwave radiation in and above the crops was studied at a rather late stage in the plant development, when the TAI was about 2 m 2 m -2 . The variation in solar radiation at +200 cm is shown in Table VIII. If the relative mean radiation (the radiation in t h e vegetation as percentage of t h e ~ o m i n g solar radiation) in the plots is c o m ~ with the plot biomass on e.g. July

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18, when plant growth had almost ceased (Fors, 1973), the regression coefficient is f o u n d to be - 0 . 9 4 (Fig.9). The dense canopies of the w e t plots resulted in small fluctuations in the short.wave radiation during the day, while the amplitude, especially in the dry plots, was considerably greater (Fig.10). Around July 9th two Pendek plots were studied more intensively. The short, wave radiation at 15 cm height in the Pendek wet plot averaged a b o u t 39% of that above the canopy (Fig.11), b u t only a b o u t 29% a fortnight later (Fig. 10). The correlation b e t w e e n phytomass and relative radiation was high and the relationship to the measurements two weeks later was close. The simultaneous radiation balance measurements are shown in Fig. 11. The net radiation in the Pendek dry plot in the early morning was around 70% of that in the wet plot. The ratio steadily decreased during the d a y and late in the afternoon the difference between plots almost disappeared. Soil water

The soil moisture fluctuations in all plots were followed during the vegetation period. The soil moisture tension fell considerably in the r o o t zone of the dry plots during the summer. The soil-mois~tre decrease in the wet plots was mainly limited to a lowering of the ground-water level, from a depth of a b o u t 100 cm sowing time t o a b o u t 180 cm in August, while the top soil remained rather moist t l ~ o u ~ h o u t the season of growth. The soil-water tension in the Pendek plots is shown in Fig.12. The wet plots evapotranspired a b o u t twice as much water as the dry plots. The water supplied was a b o u t 100 mm to the dry plots and 275 mm to the wet plots, while the decrease in soil-moisture store was a b o u t 55 mm in

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Date: Sol. radiation:

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Growth, production and biomass Leaf area was calculated from leaf width (b) and leaf length (l). After several tests the best general equation for varieties and leaf sizes was found to be Y = c(0.9 + 0.7 x l x b), where Y is the leaf area and c is a factor (~1), depending on the green-leaf area and estimated in percentage of the total leaf area. The LAI was at m a x i m u m around June 14th in the Pendek plots (LAI = 3--4), around June 17th in the SSrbo plots (3.5--4) and around June 20th in the Stormogul-II plots ( 3 . 5 - 6 ) . As a result o f delayed flower development and greater height growth, the Stormogul-II plots had up to 3 m 2 m -2 greater

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LAI than the other varieties and retained green plant parts a fortnight longer. The relative growth rate (g m -2 day -L } was lower in the $tormogul-II plots until around July 1st, when the vegetative growth in Pendek and SSrbo had almost ceased. The greatest Stormogul-II production occurred one week later, whereafter the changes in biomass were very irregular. There were several causes: grain production; some late shooting after rainy periods; and reactions to the different water treatments. The root biomass was considerably lower in the wet plots for all varieties in 1971 both around heading time and at harvest, Other years have however shown different results. The dry plots had a relatively greater biomass at

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deeper horizons than the other treatments. No obvious varietal differences in r o o t biomass were found in this year (cf. Fors, 1973). DISCUSSION AND CONCLUSIONS

T e m p e r a tu re

In early June the temperature regime in the wet plot differed from those of the other plots (Figs.2 and 3). The higher and denser c a n o p y of the wet plots (TAI = 5.8) resulted in a lower maximum temperature and almost no temperature gradient. Isothermal conditions occurred briefly in field grown crops during clear days (Peacock, 1975), b u t only if TAI was at least 5--6 (Table VII). The two drier growing crops had lower productivity and a smaller green area (Table VII). As a consequence the air inside the plot warmed up more effectively, and these plots had more sharply defined temperature maxima a n d these were closer to the soil surface. Though there was greater insolation into the dry plots (TAI = 3.6 on average for three varieties) than into the normal ones (TAI = 4.3), the maximum temperatures did not differ. The sheltering effect of the gutters is i.a. shown in a near 10°C lower m a x i m u m temperature difference in 24 h at the soil surface in the dry plots than in the normal ones. Earlier measurements have shown that the day maximum temperature was f o u n d close to the ground, while the minimum night temperature was at higher levels, relatively closer to the t o p of the c a n o p y (cf. Cordukes and Robertson, 1963; P~hlsson, 1966; and Ripley and Saugier, 1974). In this investigation the maximum day temperature (about 37°C) was close to the ground, and the minimum night temperature (below 10°C) was around 35 cm height. The energy accumulated in the soil surface during a clear day

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keeps the layers near the ground warmer than the top of the c a n o p y during the night. In early July drought conditions in the dry plots were still more pronounced, causing the highest temperatures both in the vegetation and at the crop surface. The weakly developed dry plots were thus letting more radiation in, but the plants were unable to use a lot of it in photosynthesis. Some energy was reflected from the gutters. The temperature increase from early June to early July was consequently greater in the more open dry plots than in the closer growing wetter ones. All plots covered larger soil surfaces in July, however, giving rise to a smaller temperature amplitude. The higher m a x i m u m temperatures in June (~37°C) were probably not harmful to the plants, as the soil water was still rather easily extractable (Fors, 1973). The cooling of the leaves from the transpiration stream was probably sufficient to prevent injuries. As shown in Fig. 5 the temperature of the irrigation water had some effect on the temperature in the crops. When the irrigation water was at 12°C then the crop temperature was lowered ~ I . 0 ° C for each 2.5°C drop in temperature between crop air and irrigation water. The initial temperature decrease of about 4. 5 to 6.5°C soon disappeared and is probably of no importance for plant production. Wierenga et al. (1971) showed that the soil temperature at 5 and 10 cm depth, initially at 15°C, increased about 3°C when the soil was irrigated with 100 m m water at 27°C. But this effect lasted less than 24 h and was of little importance. Kohl and Wright (1974) showed that the temperature of sprinkler water had little or no effect on plant growth. The temperature in the wet plots before irrigation was on average for the three varieties 2°C lower than in the other plots, and 4°C lower after irrigation (cf. Fig. 5). The difference was probably partly caused by the shelter of the leaf canopy, stopping m u c h of the incoming radiation. This was also shown in the other temperature measurements and in the radiation measurements. The lower temperature was however mainly caused by cooling due to evaporation of water from the plant surfaces.

Wind, RH, evaporation From the temperature measurements it was obvious that the space between and below the gutters had a different microclimate. The same conclusion m a y be drawn from the relative-humidity measurements, showing high RH-values in plots lacking water. If the hair hygrograph had been placed above the gutters, the RH-value would probably been below 10% at lowest. The soilwater c o n t e n t (0--20 cm) was m u c h lower in the dry plots compared to the other ones (Fors, 1973} and there is no connection with the high RH-values above soil surface. Theoretically the evaporation from the soil in the dry plots should have been m u c h lower than in the plots without gutters. The watersaving effect was probably not real, as the water addition from this source

426 was limited and transpiration continued. The relatively still air reduced heat flux and evaporation was thus diminished. It should however be emphasized that the evaporation is always low when t o p soil moisture supply is scarce. This is often the case in the dry plots. The high RH-values in the wet plots may be explained b y the higher total plant area in these plots (Figs.6 and 8, and Table VII). Although the soil is moist, so one might have expected evaporation to be great, yet the dense foliage impeded movement of air near the ground so that in fact evaporation from the soil was small. As there was no lack of soil moisture, there was a relatively high transpiration from the green plant parts, and this increased the relative humidity near the ground too. A thin crop has a lower average relative humidity and a shorter period of saturation at night (Penman and Long, 1960). The wind speed inside a normal plot was on average a b o u t 2% of that in the open field at 150 cm height (July 7th); the wind velocity in a wet plot was probably even less. The sensible heat flux at zero wind speed is less than 0.1 ly min -1 for a temperature gradient of 10°C between leaf surface and air (Rosenberg, 1974). The greatest temperature difference during 24 h was a b o u t 16°C inside a wet plot (Tables II and III). This means that the temperature was rather stable and the insolation very limited. The temperature gradient between leaf surface and air was probably lower than 10°C, so the sensible heat flux from the lowest levels in the crop was probably around 0.05 ly min -1 . Within the crops the air was nearly saturated during the night, b u t in the open field the air was far from saturated. The explanation is probably to be f o u n d in the evapotranspiration potential. The temperature fall during the night stops at a b o u t 12--13°C resulting in a limited, b u t still important, evapotranspil ation from the soil and crop. Investigations of soil moisture have shown that in June the cultivated normal plots had a small volume percentage less water in the t o p soil than did the bare ones. In all soils at this time, however, there was a lot of readily available water (cf. Fors, 1973). Evapotranspiration exceeded the bare-soil evaporation. From the main r o o t zone (0--50 cm) the water losses between sowing and harvest were 62, 44 and 20 mm greater in the cultivated dry, " n o r m a l " and wet plots than in the corresponding bare-soil plots (Fors, 1973). Johansson (1969) f o u n d a close correlation between the evaporation from Andersson's evaporimeter (Andersson, 1969) and the actual evapotranspiration from a grass cover on a sandy soil. If Johansson's (1969) calculations are used on the figures from this investigation the average daily evaporation from the evaporimeter is predicted to have been a b o u t 5.4 mm. The measured values, 6.8 in June and 4.8 in July (Tables IV and V) did not differ conspicuously from these calculated values. The a m o u n t of water available in the soil is however of great importance in this type of comparison. The two 24-h mean evaporation figures were strongly correlated with the mean wind velocity at 150 cm height.

427

Light, albedo The albedo over a sorghum canopy with LAI = 3 was found to be about 20% at high sun angles (Kanemasu and Arkin, 1974) and a wet bare soil reflects about 15--20% of the incoming solar radiation (Idso et al., 1975). Fritschen (1967) has f o u n d that the albedo of different crops varies between 16 and 27%, while it is reported in Graham and King (1961) that the reflection from a maize field, obtained with an airborne beam reflector, was 8--12%. Perttu (1970) has measured an albedo of 6% from an aircraft over a green field in early June in central Sweden. Other authors have f o u n d albedo values varying between 2 and 20% over treeless tundra and coniferous forests. Early in the morning the solar radiation reflectance is comparatively high due to the small sun angle (Graham and King, 1961). Most of the direct sunlight only reaches the uppermost leaves and a rather large part of the green area receives only diffuse light. The relatively high albedo measured in this work at 06h00 is puzzling, and was not a quirk of the instrument. A high albedo around 8--9 o'clock in the morning, and to some extent also in the evening, was caused by a high reflectance of the near-infrared wavelengths at these times of the day (Kanemasu and Arkin, 1974). The minimum albedo around 10--11 a.m. (Table VI) may have been caused by a high absorption of photosynthetically active radiation at low moisture stress (Allen, 1974). The high moisture stress about 14h00 (Allen, 1974) was however delayed nearly 2 h (Table VI) due to good moisture supply in the wet plot. Investigations made with a selenium barrier layer photoelectric cell in meadow vegetation on slopes in southern Sweden have given albedo values of around 5% (L. P{~hlsson, personal communication, 1976; cf. P~hlsson, 1966 and 1969). The sensitivity of a selenium cell is limited to about 400--700 nm (Mattsson, 1961), while most of the albedo values already referred to were measured with solarimeters ( ~ 3 0 0 - - ~ 5 , 0 0 0 nm). This means that the near-infrared light, which c o m m o n l y forms three quarters of the reflected light (Kanemasu and Arkin, 1974), is measured with solarimeters but not with the photoelectric cell. Therefore, an albedo of 5% measured with a photoelectric cell would be comparable to an albedo of 20% measured with a solarimeter. It has been shown that the reflectance from gutters is rather great. This reflectance may be of importance for photosynthesis in the dry plots during cloudy days (Kanemasu and Arkin, 1974). A low soil-moisture c o n t e n t is then compensated by a higher energy supply.

Short-wave radiation The regression coefficient of - 0 . 9 4 between radiation and production in late July, suggests that there must have been a high positive correlation between the absorbed or reflected light and the phytomass. The radiation

428

measurements in late July and early August were made when rather little ol ~ the plant area was still green. Much of the light energy not reaching the solarimeter at 15 cm height must have been reflected from the yellow plant parts and not used in photosynthesis. The total leaf area (independent of leaf colour) is somewhat smaller at the later stage of plant development, as a result of leaf inrolling or leaf fall. The results did agree closely however with those made some weeks earlier (cf. Figs. 10 and 11). Radiation balance

The radiation balance measurements in early July showed interesting results ( F i g . l l ) . The instruments used were simple and did not, unfortunately, allow distinction of the negative terms in the radiation balance equation, so it is n o t possible to assess fully the significance of the results in F i g . l l . Some possible causes may, however, be discussed. Reflection from the gutters was probably of importance. For the vegetation the TAI was 4. 3 in the dry plot and 6.8 in the wet one. The turgor was somewhat lower in the dry plot plants with a plant.water c o n t e n t of 62% in the dry and 72% in the wet plot (Fors, 1973). The minimum night temperature in the dry plot was just a few tenths of a degree below that in the w e t plot (Table III). The temperature rise was however much faster in the dry plot, especially between 07h00 and 12h00. The heating of the plants, gutters and air was thus relatively fast and less o f the long-wave radiation was reflected. A dry soil surface is warmed up during increasing solar radiation and no latent heat is lost (Kanemasu and Arkin, 1974). In the wet plot long-wave radiation was needed to evaporate water, which resulted in sensible heat flux to the air. This heat transport increased when the solar radiation increased. The net-radiation difference was thus diminishing before noon. The net radiation started to decrease a b o u t half an hour earlier in the wet plot than in the dry one, while the temperature m a x i m u m was a b o u t one hour later in the wet plot. The differences were small, however, b u t probably of some importance for the comparison between the t w o treatments. Maximum temperature in the dry plot coincided with the m a x i m u m incoming shortwave radiation, namely 12h00--13h00. This means that the dry plot really was lacking water and the a m o u n t of latent heat was negli~'ble. The normally occurring lag of temperature behind radiation (Rosenberg, 1974) was however seen in the wet plot. After a b o u t 13h00 the temperature in the dry plot quickly decreased. The phytomass was less and rather dry and had a comparatively small heat capacity, so the sensible heat flux to the air was relatively high. This cooling effect resulted in condensation, so the RH between the gutters started to rise at a b o u t 14h00--15hO0 (Fig.8). The evaporation rate was rather great until a b o u t 16h00--17h00 (Table V). Due to ample water supply b o t h in the plants and in the soil in the wet plot much of the radiation was used in evapotranspiration from the large phytomass.

429 The time displacement and the latent heat in the wet plot resulted in a relatively greater out-going radiation during the afternoon, compared with the dry plot. Every afternoon a b o u t 18h30 the net radiation in the dry plot exceeded that in the wet plot and the latent heat was thus still of great importance in the balance. Large amounts of heat were lost from the wet plot and during such a long time that the minimum temperature in the night became almost as low as in the dry plot (Table III). The ratio of net radiation in dry to that in wet plots during the day time was thus constantly decreasing and became zero late in the afternoon. Thereafter the outgoing radiation from the wet plot was the higher until some time during the night. Connection between the parameters studied

The different treatments gave rise to different soil-water supplies, resulting in diverging production rates. The vigorous growth in the wet plots resulted in a comparatively large leaf area. Due to this high LAI, the diurnal variation in the bioclimate was rather small inside the wet plot. As the water supply was ample, the high evapotranspiration resulted in a high relative humidity in these plots. The great vegetation cover strongly diminished the soil evaporation and most of the water loss was through transpiration. m m e dry plots the soil-water loss was also diminished, b u t here as a result of the gutters. Unfortunately, the water-supply reducing gutters caused an artificial climate near the ground, with a rather high relative humidity and a s o m e w h a t low temperature amplitude. Though the conditions between the gutters must be regarded as unnatural, the real difference from the natural conditions was probably rather small. The processes normally happening at the soil surface were here moved up to the level of the gutters, while the space between the gutters had a climate between that of the soil and that of the air above the gutters. The reflectance from the dry plots was however much higher as a result of the gutters. The low production in the dry plots was primarily a result of the low soilwater supply. However, as the plants were short and the leaves were small too, the rain water was less likely to follow the plant surface and infiltrate into the soil. Plants in the dry plots were thus trapped in a vicious circle.

ACKNOWLEDGEMENTS I thank FD Lars P~hlsson, Prof. Nils Malmer and civil engineer Bo Wiman for valuable advice and criticism, Miss Eva Hagen for assistance with the field work, Mrs. Mimmi Varga for diagram drawing, and, for financial support, The Swedish Council for Forestry and Agricultural Research, The R o y a l Physiographical Society in Lund and the University of Lund.

130 REFERENCES Allen Jr., L. It.. 1974. Model of light penetration into a wide-row crop. Agron. J., (~6:4 ! 47. Andersson, S., 1969. Markfysikaliska unders6kningar i odlad jord, XVIll. O m e n ny (,ch enkel evaporimeter. GrundfOrb~ittring (Uppsala), 2 2 : 5 9 - - 6 6 (English summar.~ )~ Cordukes, W. E. and Robertson, G. W,. 1963. T e m p e r a t u r e distribution within an ()at. crop. Can. J. Plant Sci., 43:23r, -239. Denmead, O. T., 1969. Comparative m i e r o m e t e o r o l o g y of a wheat field and a forest of Pinus radiata. Agrie. Meteorol., 6:357--371. Ekstrom, G., 1950. Sk'~nes ~kerjordsomr~de. Sockerhandlingar, (Malm6), 6:3, pp. 53-61 (English summary). Fors, L., 1973. Field research on the response of oats to different water supply. Meddelanden l'r~n Avdelningen for ekologisk botanik. Lunds Univ., Lund, 7{1:7 ), 197 pp. Fritschen, L. J., 1967. Net. and solar radiation relations over irrigated field crops. Agric. Meteorol., 4:55--62. Fritschen, L. J. and Shaw, R. H., 1961. Transpiration and evapotranspiration of corn as related to meteorological factors. Agron. J., 53:71--74. Graham, W. G. and King, K. M., 1961. Short-wave reflection coefficient for a field of maize. Q. J. R. Meteorol. Sot., 8 7 : 4 2 5 - - 4 2 8 . Grant, D. R., 1970. Some measurements of evaporation in a field of barley• J. Agrie. Sei., Camb., 7 5 : 4 3 3 - - 4 4 3 . Gregory, F. G., 1926. The eftk.~ct of climatic conditions on the growth of barley. Ann. Bot., XL: 1--26. Idso, S. B., Jackson, R. D., Reginato, R. J., Kimball, B. A. and Nakayama: F. S., 1975. The dependence of bare soil albedo on soil water c o n t e n t . J. Appl. Meteorol., (Boston), 1 4 : 1 0 9 - - 1 1 3 . Jakucs, P., 1968a. Comparative and statistical investigations on some microclimatic elements of the biospaces of forests, shrub stands, w o o d l a n d margins and open swards. Aeta Bot. Acad. Sei. tiung., (Budapest), 1 4 : 2 8 1 - - 3 1 4 . Jakues, P., 1968b. A new representation m e t h o d for the daily course of mieroelimates. Acta Bot. Aead. Sci. Hung., (Budapest), 14:59--61. Johansson, W., 1969• Meteorologiska elements inflytande p~ avdunstningen frhn Anderssons evaporimeter. Grundforb~ittring, (Uppsala), 2 2 : 8 3 - - 1 0 5 (English summary). Kanemasu, E. T. and Arkin, G. F., 1974. Radiant energy and light e n v i r o n m e n t of crops. Agric. Meteorol., 14:211--225. Kohl, R. A. and Wright, J. L., 1974. Air temperature and vapor pressure changes caused by sprinkler irrigation. Agron. J., 66:85--88. Mattsson, J. O., 1961. Mieroclimatic observations in and above cultivated crops with special regard to t e m p e r a t u r e and relative humidity• Lurid Studies in Geography, Set. A, Physical Geography, No. 16, Lund, 117 pp. Nihlg~rd, B., 1969. The microclimate in a beech and a spruce forest -- a comparative study from Kongalund, Scania, Sweden• Bot. Notiser, (Lund), 1 2 2 : 3 3 3 - - 3 5 2 . Peacock, J. M., 1975. T e m p e r a t u r e and leaf growth in Lolium perenne, I. Thermal microclimate : Its measure m e n t and relation to crop growth. J. Appl. Ecol., 12:99--- 114. Penman, H. L. and Long, I. F., 1960. Weather in wheat: an essay in m i c r o - m e t e o r o l o g y . Q. J. R. Meteorol. Sot., 86:16- 50. Perttu, K., 1970. Radiation measurements above and in forest. Studia Forestalia Sueeiea, (Stockholm), 72:49 pp. PRhlsson, L., 1966. Vegetation and mieroelimate along a belt transect from the esker Kmvsas. Bot. N o t i s e r ( L u n d ) , 119:401 418. Pi~hlsson, L . 1969. Vegetation, microclimate and soil moisture of b e e c h w o o d and open pasture land on the esker Knivs~ts, Central Seania. Oikos Suppl., 12:87--103.

431 P~ihlsson, L., 1974. Influence of vegetation on microclimate and soil moisture on a Scanian hill. Oikos, 25:1--11. Rackham, O., 1972. Responses of the barley crop to soil water stress. In: Rees, Cockshull, Hand and Hurd (Editors), Crop Processes in Controlled Environments. Academic Press, London, New York, pp. 127--138. Ripley, E. A. and Saugier, B., 1974. Microclimate and production of a native grassland. A micrometeorological study. Oecol. Plant., 9:333--363. Rosenberg, N. J., 1974. Microclimate: The biological environment. Wiley, New York, N.Y. 315 pp. Sveriges Meteorologiska oc h Hydrologiska Institut, (SMHI), 1971. ]krsbok 53 (1972-1975), (Stockholm). Tamai, S. and Shidei, T., 1972. Light intensity in the forest, I. Bull. Kyoto University For., 43:53--62. Wall6n, C. C., 1966. Global solar radiation and potential evapotranspiration in Sweden. Tellus, 18:786--800. Wierenga, P. J., Hagan, R. M. and Gregory, E. J., 1971. Effects of irrigation water temperature on soil temperature. Agron. J., 63:33--36.