Agricultural and Forest Meteorology, 40 (1987) 17-29 17 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

EFFECTS OF PARTIAL CLEARING ON MICROCLIMATE IN A HUMID TROPICAL FOREST

B.S. GHUMAN and R. LAL

International Institute of Tropical Agriculture, Ibadan (Nigeria)

(Received July 1, 1986; revision accepted October 15, 1986)

ABSTRACT

Ghuman, B.S. and Lal, R., 1986. Effects of partial clearing on microclimate in a humid tropical forest. Agric. Forest Meteorol., 40: 17-29.

We have measured and compared several microclimatic variables in a cleared area and under a high rain forest in the humid tropic belt of Nigeria. Measurements were made consecutively from July to December for the years 1984 and 1985. Throughfall under forest was about 12 and 32% less than rainfall received in the cleared area in 1984 and 1985, respectively. The relative humidity was higher in the forest than the cleared area during rainy months but the effect was less pronounced during dry months. Solar radiation was 25- to 30-times less, and open pan evaporation 4- to 6-times less under forest than in the cleared site. Wind speed, measured in 1986, was about 18 times less in the forest than the cleared area. On a clear day, the maximum soil temperature at 1 cm depth was 3~10°C lower under forest than in the cleared area. However, the difference in the maximum soil temperatures was less on cloudy days (3°C). Air temperature during the rainy season was lower by 1-5°C under forest than in the cleared area.

INTRODUCTION

Tropical rain forest is a diverse, closed, and stable ecosystem. Traditionally, the forest ecosystem has been used extensively for shifting cultivation and related bush fallow systems (Okigbo, 1984).

The paucity of data on the climatic variables under tropical rain forest, caused perhaps by the high cost of instrumentation and remoteness of the study sites (Pinker, 1980), limits our ability to understand and manage forest soils f o l l o w i n g c l e a r i n g . J a r v i s et al. (1976) r e v i e w e d l i t e r a t u r e o n c o n i f e r o u s forests , R a u n e r (1976) on d e c i d u o u s fores ts , a n d Do ley (1981) on t he h y d r o l o g y of t r o p i c a l a n d s u b t r o p i c a l fo res t s a n d w o o d l a n d s . L e i g h (1975) s t u d i e d t he ecolo- g ica l a spec t s of t r o p i c a l r a i n forests . Lee (1978) r e p o r t e d t h a t fores t o p e n i n g s p l a y a n i m p o r t a n t ro le i n t he r e g e n e r a t i o n of forest . S e e d l i n g s i n a fores t c l e a r i n g a re s u b j e c t e d to b o t h h i g h e r r a d i a t i o n a n d su r f a c e t e m p e r a t u r e d u r i n g t he day a n d to g r e a t e r e n e r g y losses a n d l ow e r t e m p e r a t u r e s a t n i g h t . Har - r i n g t o n (1984) c o m p u t e d t he effect of s o l a r r a d i a t i o n o n fo res t r e g e n e r a t i o n a n d c o n c l u d e d t h a t i n c l e a r e d s t r i p s t he i n t e n s i t y of so l a r r a d i a t i o n h a s a c r u c i a l effect o n g e r m i n a t i o n , s u r v i v a l a n d r a t e of g r o w t h of n e w seed l ings , t he

0168-1923/87/$03.50 © 1987 Elsevier Science Publishers B.V.

18

proliferation of insects and diseases, and the drying of forest fuels. Opara-Nadi and Lal (1984) reported that the maximum soil temperature at 5 cm depth was 0.7-2°C greater in mechanical ly cleared plots than in t radi t ional ly cleared and managed areas. Lal and Cummings (1979) reported similar effects of partial deforestation on soil temperature and at tr ibuted the differences among treat- ments to the amount of ground cover and the presence or absence of leaf litter.

This study was conducted to investigate the effects of partial clearing on rainfall parti t ioning, relat ive humidity, solar radiation, free water evapora- tion, wind run and air and soil temperatures in a high rain forest.

MATERIALS AND METHODS

Measurements of rainfall, throughfall , relative humidity, solar radiation, evaporation, air and soil temperatures were made in a recently cleared field from rain forest and in an adjacent forest plot in 1984 and 1985. The measure- ments were carried out at Okomu (6°25'N; 5°12'E; 76 m above mean sea level), near Benin City in Nigeria. This area falls in the humid tropics (Table I), with a rainy season from late April to October and a dry season from November to early April. About 30 ha of forest land were cleared by a D-8 bulldozer with a front-mounted shear blade.

Soil at the site is a deep reddish loamy sand near the surface that changes to sandy loam and sandy clay loam with increasing depth and is classified as Paleudult. The soil is free of gravel/Plinthite and freely drained. After clearing, 24 plots of 1 ha each (50 × 200 m) were prepared and allotted to eight land use systems with three replications to a system. The plots were made fairly smooth by a grader. In the centre of the 30-ha clearing, a meteorological observatory (MET) was set in a 20 × 20 m area to monitor different climatic variables. The ground surface in the MET was always under cover of grass that was periodic- ally slashed with a cutlass to a 5-10 cm height.

Surrounding the cleared area was a part ly cleared high rain forest. The forest vegetat ion had three storeys; the top storey consisted of trees of 40-50 m height, the middle storey of bushes, climbers and trees 10-15 m high and the bottom storey of tree seedlings and undergrowth of up to 1 m. The common tree species identified in the forest were Ceiba pentandra, Poga oleosa, Antiaris africana, Anonidium manii, Enantia chlorantha, Musanga cecropioides, Strom- bosia pustulata, Celtis zenkeri, Diospyros alboflavescens, etc.

According to one estimate, the Okomu forest is more than a century old (Jones, 1955). Before clearing, we demarcated 118 plots of 50 × 50m each and counted trees with their t runks thicker than 15 cm in circumference at 1 m height. The data were described well by a normal distribution function f(T):

f(T) = 0.001 exp [ - ( T - 1217)5/235092] (1)

where T is the number of trees ha 1. The mean tree density of 1217 trees ha- 1 classifies the forest as quite dense. The dense canopy and leaf l i t ter on the forest floor al ter the microclimate and soil physical properties considerably

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TABLE II

Rate of leaf fall under forest

Period Jun. ~ Jun. 27 July 17- Aug. 16- Sept. 10- Oct. 3 Average (1984) 20 July 9 Aug. 1 23 19 15

Rate of leaf fall 20.0 21.5 14.7 25.6 23.6 20.1 20.9 (kg ha - 1 day- 1)

(Lal and Cummings, 1979; Huluga l l e et al., 1984). The mean ra te of leaf fall under forest at Okomu was 21kg ha -~day 1 (Table II).

Throughfa l l unde r the forest was measured wi th a spider gauge because of its large surface-col lec t ion area. This was cons t ruc ted from two 10-cm d iamete r by 300-cm long galvanized steel pipes ha lved a long the i r length. The four ha l f pipes were a r r anged in the form of a cross, each wi th one end at the crossing point . The inner end of each pipe was provided wi th an ou t le t t ha t d ra ined into a wa te r co l lec t ion tank. Wate r from the t ank was measured wi th a ca l ibra ted bucke t a f te r each ra ins torm. To measure stem flow, a s lan t ing groove was made by giving a 3-cm deep cut a round the t ree t r u n k at about 1 m height from the base. A metal l ic sill was a t t ached at the lower end of the groove to d i rec t stem flow into a plast ic conta iner . Three trees, each of about 1 m in gi r th at 1 m height , were involved in the stem flow.

E v a p o r a t i o n from a class-A pan was measured nea r the spider gauge. In addit ion, an evapo ra t i on gauge was made to measure ra infa l l and evapora t ion in the c leared area. The evapora t ion gauge (Fig. 1) had th ree uni ts (i) a wa te r

W A T E R S U P P L Y U N I T

W a f e r tube

EV~PORIMETER UNIT ~tl F~I'~ UNIT

I I o io,.o .s,=o d// ',!'

maintaini~i~_{_~n~. , -__-;_~-___--Z-4_--~-__--~.'-- = ~ - - - - ~ -- = - I bo.,e

. n ,1-

Fig. 1. Schematic diagram of evaporat ion gauge.

, A i r tube

f- Rubber s topper

/ / j Wdfer in let

21

supply uni t to supply wa te r to the evapor imete r to ma in ta in a cons tan t wa te r level, (ii) the evapor imete r un i t to measure evapora t ion , and (iii) the ra in gauge uni t to measure rainfall . Dur ing rainless periods, the evapora t ion gauge acted as an evapor imete r by read ing evapora t ion direct ly from the wate r supply unit . Dur ing rain, the evapora t ion gauge acted as a ra in gauge by measur ing the ra in wa te r t ha t flowed into the ra in gauge uni t t h rough a siphon. Results obta ined with this evapora t ion gauge agreed well in compar i son tests aga ins t a class-A pan evapor imeter and a non-weighing ra in gauge. Rain was resolved to 1 mm and evapora t ion to 0.5 mm.

For each of the remain ing parameters , i n s t rumen ta t ion was the same in the cleared field as under the forest. Relat ive humidi ty was measured with hygro- t he r mogr aph housed in a vent i la ted wea the r cage 1 m above the ground. Four h y g r o t h e r m o g r a p h s were tested under the same env i ronment for 2 days, and the two which gave similar readings were selected to measure humidi ty in the forested and cleared plots. Incoming solar rad ia t ion was measured with pyr- he l iograph at 1 m height (USDA, 1962). The posi t ions o f h y g r o t h e r m o g r a p h and pyrhe l iograph placed in the forested plot were changed twice a week to mini- mize errors in the average values of humid i ty and rad ia t ion in the forest due to sunflecking. Air at 1 m height and soil t empera tures at 1 and 50 cm depths were measured with cyl indr ical thermis tors a t t ached to a 28-channel Gran t recorder. Wind speed was recorded dur ing some months of 1986 with a digital anemomete r fixed 1.5 m above the ground. The anemomete r in the forested plot was instal led at a d is tance of 25 m from the edge.

RESULTS AND DISCUSSION

Rainfall

Throughfa l l under forest was a lways less t h a n rainfal l received in the cleared area (Table III) due to canopy in te rcep t ion and stem flow. The volume

TABLE III

Monthly throughfall (TF) and rainfall (RF) from July to December 1984 and 1985. Water intercep- ted (I) by the vegetation is also given

Month TF (mm) RF (mm) I = RF - TF (mm)

1984 1985 1984 1985 1984 1985

July 198.7 281.2 255.1 364.9 56.4 83.7 August 211.5 317.8 232.3 434.3 20.8 116.5 September 286.3 132.2 320.7 176.6 34.4 44.4 October 195.1 185.1 203.6 305.8 8.5 120.7 November 0.0 3.6 0.0 8.0 0.0 4.4 December 0.0 0.3 0.0 1.1 0.0 0.8 Total 891.6 920.2 1011.7 1290.7 120.1 370.5

22

of stem flow could not be converted into the depth dimension (i.e. mm) because of our inability to measure the crown area of the involved trees, and, therefore, results are not presented. Though the rainfall was 22% more in 1985 than 1984, throughfall was 1.5% less in 1985. This difference was caused by the spatial variation of gaps in the forest canopy when the spider gauge was shifted to a new location in 1985. The difference could also be attributed to varying wind speed and direction because the gauge was at a distance of 75 and 25 m from the forest edge during 1984 and 1985, respectively. Throughfall (TF) was related to rainfall (RF) by the relation (based on 1984 and 1985 data)

TF = 0.78 RF r = 0.97 (2)

in which TF and RF are in mm. The water intercepted (I) by the vegetation (canopy interception plus stem flow) was related to rainfall by the relation

I = 0.21RF r = 0.79 (3)

Eqns. 2 and 3 show that the amounts of throughfall and intercepted water are linearly related to storm size. However, from two of his studies in the Usambara Mountains, Tanzania, Jackson (1971, 1975) established semilogarithmic rela- tionships.

Rainfall, throughfall and stem flow were collected during two rainstorms in 1985 for chemical analysis. Throughfall and rainfall had identical pH values (Table IV). The pH of stem flow was higher than that of throughfall and rainfall by 0.2 unit, probably due to contamination of the stem flow by sap oozing from the edge of the groove made in the tree trunk. Direct rainfall contained considerably less Ca, Mg, K and NO3-N than stem flow and throughfall. How- ever, PO4-P level was higher in the direct rainfall.

Relative humidity

The relative humidity data for two representative days of the rainy seasons of 1984 and 1985 for the two sites are shown in Fig. 2. Relative humidity in the forest stayed more or less constant around 90% throughout the day on 23rd May 1984. However, it dropped to 70% at midday on 17th July 1985. There was no appreciable variation in the corresponding air temperature under forest. In the cleared area, however, relative humidity decreased with increasing air temperature during the day and was the lowest (50 and 65% on 23rd May and 17th July, respectively) at the highest air temperatures (38°C at 1620 h and 30°C

TABLE IV

Chemical properties of stem flow, throughfall, and rainfall of rain events on September 9 and October 4, 1985

Source pH Electrical conductivity (× 10 5dSm 1)

Concentrations (mg kg- 1 )

Ca Mg K PO 4-P NO 3.N

Stem flow 7.50 3.40 6.57 1.78 9.60 0.10 1.40 Throughfall 7.30 1.70 4.33 1.14 6.10 0.30 0.86 Rainfall 7.30 1.50 1.82 0.21 0.40 0.40 0.24

23

I

W n~

O0 02 04 06 08 I0 12 14 16 18 20 22 24

- 25 rd MAY 1984 56 2

--la", l [ I ~ ^~ Jl Iq I

60-

50- CLEARED AREA

40 0052 O~ 06 08 '6 ~2 'I 4 '6 18 ~0 22 214

HOUR OF DAY (23 rd MAY 1984)

O0 02 04 06 08 I0 12 14 16 18 20 22 24 L I I ] I L I L I I I I

= - - . . . . . . . . L 2 o ~ ~

f°° 1 ~ . ~

<~ 80 ~4 UNDER FOREST ~" ., W ~

~ : : 2= 7 0 I I i L I I L I I I

z O.u~-] . ̂/IA ,/1

1

~% o9

06 08 I0 12 14 16 18 08 IO 12 14 16 18

HOUR OF THE DAY

Fig. 2. Diurnal fluctuations in relative humidity and corresponding air temperature under the forest and in an adjacent cleared area on two representative days selected from the rainy seasons of 1984 and 1985. Solar radiation for the two days is also shown.

at 1400 h). There was a sudden increase in the relative humidity at 0800 h and then at 1400 h due to clouding of the sky on 23rd May 1984 as evident from the sharp drop in insolation (Fig. 2). When the sky became relatively clearer (at 0900 and then 1500 h), relative humidity started declining. A similar effect of cloud on the relative humidity was observed on 17th July 1985 at 1500 h when humidity increased sharply to 95%. This effect of overcast skies on relative humidity was absent under forest. Therefore, data show that clearing caused the relative humidity to fall by 5-40% at the peak air temperature, depending on sky conditions. However, during the night there was no difference between the relative humidity values of the cleared and forested areas.

24

Figure 3 illustrates changes in the relative humidity during some days of the dry season. Comparing Fig. 2 with Fig. 3 shows that the forest canopy had a markedly different effect on relative humidity during the rainy season com- pared to that in the dry season. On December 4-6 1984, the day-time relative humidity in the forest was 40-50% higher than in the cleared area. This difference narrowed down to 10% on December 11, and this was more or less maintained during the months of January, February and March 1985 (data not shown). The concomitant fall in the day-time relative humidity under the forest close to that of the cleared area was perhaps caused by the increasing fall in the vapour pressure due to increasing air temperature during the dry season. The dry season of 1984/85 became rapidly intense after its beginning in late November 1984, as indicated by the data for December. The arrival of dry and dust-laden winds from the Sahara desert during this period (called "harmat- tan") also contributed in the rapid intensification of the dry season.

Solar radiation

On a sunny day, considerably more solar radiation was received in the cleared area than under forest (Fig. 4). Although the insolation was less on a cloudy day the ratio of radiation under forest to the cleared area was similar. The closed canopy of the forest effectively intercepted the radiation, prevent- ing it from reaching the ground surface; consequently, there was little under- growth of plants in the forest.

A similar trend of distribution of solar radiation at the two sites (cleared and forested) was observed during the dry season (Table V). On average, 10.5 and 11.5 MJ m 2 day=l of insolation was received on the cleared site in comparison to 0.4 and 0 .3MJm ~day i in the forest in the dry seasons of 1984 and 1985, respectively.

There was no appreciable difference in the solar radiation received under

I00-

90-

80-

5 0 -

> 40-

2 0 -

rO

I -e-UNDER FOREST -& -

~ ' ~ -v-CLEARED AREA -#r

i i i f r r i ; ;2 fs 2~, ; ,~, ,s 2~, ; ,~ I~ 2, ; ;2 Is 2'4 ; ,2 ,s 2', s ,'2 ,s 2~

TIME OF THE DAY

DEC. 4 DEG. 5 DEC. 6 DEC8 DEC9 DEC:. i l

Fig. 3. Relative humidity under forest and in the cleared area for some days during the dry seasons of 1984 and 1985.

2 5

CLEAR DAY CLOUDY DAY

0.06~ 17th MAY 1984 ' ~ / ~ CLEARE_~D ARE~

't ,9.4 o.o2- l..Ir~ I ~1 0 , 5 t (2.7) ",i. / (2.9)

~ o o o , r . . . . . ,'3 o o l . k " - - - - - '~ . 0406 OS I0 t2 14 I& I~ 26 0 4 6 6 0 ~ 6 I~ I~ I~ I~ 2b

o o4] o n L U.D o.Es

i v,

o.o 1 (,.o,llt i,.o, °.°l 0.000~ 4 ]~ _ ~ | (0.4) (0 4)

HOUR OF THE DAY

Fig. 4. Incoming solar radiation received under the forest and in the cleared area on a clear and a cloudy day. Bracketed values are the MJ m- 2 of radiation received on a given day.

fo res t d u r i n g r a i n y a n d d ry s e a s o n s (Fig. 5 v s . T a b l e V). H o w e v e r , so l a r r ad ia - t i o n on c l e a r days i n t he c l e a r e d a r e a was r e l a t i v e l y m o r e d u r i n g the r a i n y s e a s o n t h a n in t he d ry season . T h i s was c a u s e d by the hazy a t m o s p h e r i c c o n d i t i o n s d u r i n g t he d ry s e a s o n due to s u s p e n d e d d u s t p a r t i c l e s i n t he a i r

b l o w n f rom the S a h a r a d e s e r t t h a t d i m i n i s h e d r a d i a t i o n r e a c h i n g soi l sur face . O n the o t h e r h a n d , t he c l o u d l e s s days d u r i n g t he r a i n y s e a s o n were excep t ion - a l ly c l e a r a n d t he a i r was devo id of a n y s u s p e n d e d m a t t e r , a l l o w i n g m a x i m u m so l a r r a d i a t i o n to r e a c h t he g r o u n d sur face .

TABLE V

Solar radiation received under forest and in the cleared area l or selected days ofthe dry seasons of 1984 and 1985

Day Solar radiation (mJm ~day -1)

Under forest Cleared area

1984 1985 1984 1985

December 4 0.4 0.2 11.3 11.7 December 6 0.4 0.3 9.6 10.7 December 8 0.3 0.2 9.6 11.3 December 10 0.4 0.3 10.2 12.1 December 12 0.4 0.3 9.6 11.9 December 14 0.5 0.3 11.3 12.1 December 16 0.4 0.3 11.8 11.3 Average 0,40 + 0.06 0.27 _+ 0.05 10.48 + 0.95 11.58 _+ 0.51

26

NOV DEC, 85

6 9 12 15 18 21 24 27 30 5 6 9 12 15 18 21 24 27 50 >~ 5 I I I I I I l I I I I ] I 1 I I I /

E 4-

uP 5- i _o 2-

I- UNDER FO R - :

uJ

12-

JO-

8 -

6 -

4 -

2-

0

,5 2; z~ 3; SEPT

; i; is 2; 25 3o 4 ; i~ ; z~ ~; ~ 9 14 OCT. NOV. DEC.,84

D A Y O F T H E M O N T H

Fig. 5. Open pan evapora t ion under forest and in the cleared area dur ing some months of 1984 and 1985. E 0 is the average E 0.

Wind run

Wind speed was about 18 times more in the cleared than forested plot (Table VI). Wind movement into the forested plot was restricted by the windbreak effect of trees.

Pan evaporation

Open pan evaporation in the cleared area was always higher than under forest (Fig. 5). In the open, pan evaporation varied with incoming solar radia- tion, wind speed and water deficit of the atmosphere. However, during the rainy

TABLE VI

Wind speed in the cleared and forested plots dur ing some months of 1986. The values are month ly means

Month Wind speed (m s- 1 )

Cleared Forested

J u n e 0.60 + 0.17 0.02 _+ 0.02 Ju ly 0.49 + 0.18 0.03 _+ 0.02 Augus t 0.61 + 0.15 0.03 + 0.01

-Mean 0.55 + 0.17 0.03 +_ 0.02

27

season pan e v a p o r a t i o n u n d e r the fores t was less af fec ted by v a r i a t i o n in the above fac to r s as shown by c o n t i n u o u s l y low e v a p o r a t i o n ra tes . In D e c e m b e r 1984 (beg inn ing of the dry season), p a n e v a p o r a t i o n unde r the fores t inc reased to a b o u t 2 m m day 1 and stil l the v a r i a t i o n was not as g r ea t as exper i enced in the c lea red area . In D e c e m b e r 1985 the inc rease in the e v a p o r a t i o n unde r the fores t was no t as g r ea t as expe r i enced in 1984. A v e r a g e pan e v a p o r a t i o n was 4- to 6-times h ighe r in the c lea red a r ea c o m p a r e d to the fores t in 1984 and 1985, respec t ive ly . Ef fec t ive sh ie ld ing of i ncoming r a d i a t i o n by fores t canopy (Fig. 5 and Tab le V) and re s t r i c t ed a i r m o v e m e n t inside the fores t (Table VI) were the usua l f ac to r s r educ ing p a n e v a p o r a t i o n unde r the forest .

Temperatures

Diu rna l soil t e m p e r a t u r e f luc tua t ions a t 1- and 50-cm dep ths for the two e x p e r i m e n t a l s i tes for two consecu t ive days dur ing the r a iny season are illu- s t r a t ed in Fig. 6. F o r the c lea r day, the soil t e m p e r a t u r e a t 1 cm was s l ight ly lower in the c lea red a r e a t h a n unde r fores t f rom midn igh t to 0700 h, a f t e r which inve r s ion t ook place. The m a x i m u m t e m p e r a t u r e occu r r ed a round 1500h at bo th s i tes bu t the r ise in the t e m p e r a t u r e was s t eeper in the c leared compared to the fores ted area . Af te r 1500 h the r a t e of fall of t e m p e r a t u r e was fas te r in the c leared a r ea t h a n the forest . The m a x i m u m t e m p e r a t u r e was 10°C h ighe r in the c leared a r e a t h a n unde r forest . The lower soil t e m p e r a t u r e a t 1 cm dep th

CLEAR DAY (17 fh MAY 1984) CLOUDY DAY (18 fh MAY 1984)

371 ~ I CM DEPTH

34 -o- CLEARED AREA

/ X"I 224 o,o'e"u'~e"~ 22 i

19 ~ T - , __ , , , - ~ 19i ..... n r-- , , ~

ooo3 o& o9 ~2 I~ 18 2~ 2,~ ooo'3 o~ o9 ~ ~5 ~8 2, 24

50 - CM DEPTH

2 31~

m 28

22~ 22-

O0 03 06 09 12 f5 t8 211 24 (30 03 06 09 12 15 18 21 2'4

HOUR OF" THE DAY

Fig. 6. Diurnal fluctuations in soil temperature under forest and in the cleared area on a sunny and a cloudy day.

28

before sunr i se in the c lea red a r ea was a t t r i b u t e d to the absence of a p ro t ec t ive canopy , r e su l t i ng in the u n h i n d e r e d emiss ion of long wave r a d i a t i o n to the a tmosphe re . On the o the r hand, the t r a n s m i s s i o n of long wave r a d i a t i o n to the a t m o s p h e r e was i n t e r cep t ed by t ree c anopy and leaf l i t t e r in the fores t si te thus keep ing the su r face soil wa rmer .

The soil t e m p e r a t u r e a t 1 cm dep th a t 1500 h on a c loudy day was on ly 3°C h ighe r in the c leared a r ea t h a n t h a t unde r forest , and the d i f ference be tween two si tes was e l imina ted a f t e r 2000 h.

The re was no app rec i ab l e d iu rna l v a r i a t i o n in soil t e m p e r a t u r e a t the 50-cm dep th a t e i the r si te on a c lear or a c loudy day (Fig. 6). The 50-cm soil tem- p e r a t u r e was a lways lower unde r fores t by abou t 3°C t h a n the c leared si te i r r e spec t ive of the a t m o s p h e r i c condi t ions .

I t is i n t e r e s t i ng to c o m p a r e soil t e m p e r a t u r e a t a g iven dep th on a sunny and on a c loudy day (Fig. 6). At 1 cm depth in the c leared a r e a the m a x i m u m t e m p e r a t u r e was 37°C on the s u n n y day c o m p a r e d to 28°C on the c loudy day. Cor r e spond ing t e m p e r a t u r e s were 27°C and 24.5°C under the forest . However , the sky cond i t ion (c lear or c loudy) had no effect on soil t e m p e r a t u r e a t the 50 cm dep th in the c leared v s . fores ted sites.

L ike soil t e m p e r a t u r e , a i r t e m p e r a t u r e before dawn on a c lear day was h ighe r unde r fores t t h a n in the c leared a r ea (Fig. 7). Af ter sunr ise , a i r tem- p e r a t u r e rose f a s t e r in the c leared area , a t t a i n i n g a m a x i m u m va lue of 36°C at 1500 h. U n d e r the forest , a m a x i m u m air t e m p e r a t u r e a t 31°C occur red a t 1330 h. On the c loudy day, a i r t e m p e r a t u r e in the c leared a r e a r ema ined h ighe r t h a n unde r the fores t t h r o u g h o u t the day.

D iu rna l soil and a i r t e m p e r a t u r e v a r i a t i o n s du r ing the dry season m o n t h of December were s imi la r to t h a t shown for the c lear day in Figs. 6 and 7. However , the m a x i m u m t e m p e r a t u r e was 1-2°C lower dur ing the dry season c o m p a r e d to the r a iny season due to hazy sky condi t ions in the dry season as exp la ined ear l ier .

U

Q~ ~J CL

W F-

Q/

57-

54-

51

28

25

22-

19 t O0 03 06

CLEAR DAY (f7thMAy 1984) CLOUDY DAY (18thMAy 1984)

~ CLEARED AREA 311 ~ UNDER FOREST

25

22

I , i i

o; ,) ,; ,& 2; 2~ ooo3 06 09 ,2 ,5 ,8 2, 24

HOUR OF THE DAY

Fig. 7. Diurnal fluctuations in air temperature measured 1 m above the ground surface under forest and in the cleared area on a sunny and a cloudy day.

29

CONCLUSIONS

R e s u l t s p r e s e n t e d i n t h i s p a p e r s u p p o r t the c o n c l u s i o n t h a t c l e a r i n g of fores t

exe r t s a c o n s i d e r a b l e effect on t he m i c r o c l i m a t e . D e f o r e s t a t i o n i n c r e a s e d in- s o l a t i o n o n t he soi l su r face , i n c r e a s e d m a x i m u m a n d l o w e r e d m i n i m u m soi l t e m p e r a t u r e s a t t he 1-cm d e p t h by a b o u t 3 10°C a n d 1-2°C, r e spe c t i ve l y , in-

t r o d u c e d c o n s i d e r a b l e d i u r n a l v a r i a t i o n i n r e l a t i v e h u m i d i t y , i n c r e a s e d p a n e v a p o r a t i o n 4- to 6-fold, i n c r e a s e d w i n d r u n by 18 t imes a n d i n c r e a s e d max im- u m a i r t e m p e r a t u r e by 2-5°C.

ACKNOWLEDGEMENT

T h i s r e p o r t is a p a r t of a n o n - g o i n g e x p e r i m e n t s p o n s o r e d by t he U n i t e d

N a t i o n s U n i v e r s i t y u n d e r the a u s p i c e s of i ts o v e r a l l p r o j e c t on " C l i m a t i c ,

B io t i c a n d H u m a n I n t e r a c t i o n s i n the H u m i d Trop ic s " . The p r o j e c t is j o i n t l y m a n a g e d by I I T A a n d U N U .

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Hulugalle, N.R., Lal, R. and Ter Kuile, C.H.H., 1984. Soil physical changes and crop root growth following different methods of land clearing in western Nigeria. Soil Sci., 138:172 179.

Jackson, I.J., 1971. Problems of throughfall and interception assessment under a tropical forest. J. Hydrol., 12: 234-254.

Jackson, I.J., 1975. Relationships between rainfall parameters and interception by tropical forest. J. Hydrol., 24: 215-238.

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Lee, R., 1978. Forest Micrometeorology. Columbia University Press, New York, 276 pp. Leigh, E.G., Jr., 1975. Structure and climate in tropical rain forest. Annu. Rev. Ecol. Syst., 6:

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Pinker, R., 1980. The microclimate of a dry tropical forest. Agric. Meteorol., 22: 249-265. Rauner, Y.L., 1976. Deciduous Forest. In: J.L. Monteith (Editor), Vegetation and the Atmosphere.

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