Soil properties and fast-growing tree species in Tanzania

Soil properties and fast-growing tree species in Tanzania

Forest Ecology and Management, 1 6 ( 1 9 8 6 ) 1 2 7 - - 1 4 7 127 Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N ...

1023KB Sizes 0 Downloads 22 Views

Forest Ecology and Management, 1 6 ( 1 9 8 6 ) 1 2 7 - - 1 4 7

127

Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

Soil Properties and F a s t - G r o w i n g Tree Species in Tanzania J U L I A C. A L L E N

Department o f Geography and Environmental Studies Program, University o f California at Santa Barbara, Santa Barbara, CA 93106 (U.S.A.) ( A c c e p t e d 11 M a r c h 1 9 8 6 )

ABSTRACT Allen, J.C., 1986. Soil p r o p e r t i e s a n d f a s t - g r o w i n g tree species in T a n z a n i a . For. Ecol. Manage., 16: 1 2 7 - - 1 4 7 . This s t u d y e x a m i n e d g r o w t h r a t e s a n d soil c o n d i t i o n s o f 1 1 - - 1 4 - y e a r - o l d f o r e s t p l a n t a tions a n d a d j a c e n t u n m a n a g e d n a t u r a l f o r e s t s a t t h r e e climatically a n d geologically dist i n c t sites in T a n z a n i a . T h e sites d i f f e r m a r k e d l y in g r o w t h rates, soil c o n d i t i o n s a n d pot e n t i a l for m a n a g e m e n t . A t t h e U l u g u r u F o r e s t Reserve o n q u a r t z o - f e l d s p a t h i c granulite a n d m i c a w i t h average rainfall o f 1 0 5 8 - - 1 3 7 7 m m y - ' , r e g e n e r a t i n g e v e r g r e e n f o r e s t a n d a p l a n t a t i o n o f Acacia mearnsii have g r o w n at 2.6--7.2 a n d 3.5 m 3 h a - ' y - ' , respectively. A t t h e East C h e n e n e F o r e s t Reserve o n granite w h e r e r a i n f a l l averages 5 7 3 - - 6 2 5 m m y - ' , m a t u r e d r y w o o d l a n d a n d p l a n t a t i o n s o f Cassia siamea a n d Eucalyptus citriodora have g r o w n at 0 . 2 - - 2 . 0 a n d 1.4--2.2 m 3 h a - ' y - ' , respectively. F e r t i l i t y o f t h e acid soils at t h e s e sites d e p e n d s o n litterfall a n d organic m a t t e r d e c o m p o s i t i o n , a n d it declines w h e n l e a c h i n g increases after forest clearing. A t t h e S a m b a s h a F o r e s t R e s e r v e o n volcanic ash soils w i t h average r a i n f a l l of 1 2 0 0 - - 1 8 0 0 m m y - ' , m a t u r e e v e r g r e e n f o r e s t a n d a p l a n t a t i o n o f Cupressus iusitaniva have g r o w n at 2--9 a n d u p t o 15 m 3 h a - ' y - ' , respectively. Soil fertility here is m u c h less d e p e n d e n t o n organic m a t t e r a d d i t i o n s . In forest m a n a g e m e n t w h e r e n e i t h e r fertilizer n o r irrigation is used, i n h e r e n t soil fertility has a m a r k e d e f f e c t o n g r o w t h . G r o w t h r a t e s in this s t u d y are r e l a t e d to r a i n f a l l a n d soil n u t r i e n t s t a t u s a t each site, w i t h t h e fastest g r o w t h o c c u r r i n g o n wet, r e c e n t v o l c a n i c soil ( S a m b a s h a ) . T h e slowest g r o w t h was m e a s u r e d a t t h e dry site (East C h e n e n e ) w h e r e soils are d e v e l o p e d f r o m n u t r i e n t - p o o r acid r o c k . A t t h e i r p r e s e n t slow rates o f g r o w t h , p l a n t a t i o n s will n o t m a k e a significant c o n t r i b u t i o n in t h e n e a r f u t u r e t o alleviating d e f o r e s t a t i o n a n d related scarcity o f f o r e s t p r o d u c t s in semiarid regions o f T a n z a n i a . F o u r a c t i o n s are u r g e n t l y - n e e d e d : (1) i d e n t i f i c a t i o n o f t h e b e s t sites for tree p l a n t a t i o n s ; ( 2 ) i d e n t i f i c a t i o n of t h e m o s t p r o m i s i n g e x o t i c a n d ind i g e n o u s species; (3) i d e n t i f i c a t i o n o f t h e c o n s t r a i n t s o n g r o w t h of p r o m i s i n g species; a n d (4) f o r m u l a t i o n o f e c o n o m i c a l l y a n d p h y s i c a l l y feasible m a n a g e m e n t t e c h n i q u e s t o overc o m e c o n s t r a i n t s t o g r o w t h . T h e s e c a n b e a c h i e v e d t h r o u g h increased use o f soil s u r v e y s ; e s t a b l i s h m e n t of c o m p a r a t i v e field trials w i t h v a r y i n g site/species c o m b i n a t i o n s ; r e c o g n i t i o n o f t h e limited p o t e n t i a l o f species t h a t r e q u i r e i n t e n s i v e m a n a g e m e n t b y local p e o p l e ; a n d e x p l o i t a t i o n o f t h e i n h e r e n t a d v a n t a g e s o f i n d i g e n o u s species.

0378-1127/86/$03.50

© 1 9 8 6 Elsevier S c i e n c e Publishers B.V.

/

r~g~r ~tir~ bushland

/

100 m

j

/

/

/

. - ~

- - ~

~--4 __t__

/1

slo

FOREST

cas/a slamea

J

/

/

25 m

J

cltrlodora

Eucalyptus

i

t % t t

Zambia

1,........ d ~ ~anzania

,.r

\

;
Sambasha Forest Reserve

I

/

,~e~ / / "

//"

/

/

,~eve

Uluguru Forest Reserve

Usa River Forest Reserve

'0o

129 INTRODUCTION In recognition of d e f o r e s t a t i o n and the i m port ance o f forest p r o d u c t s t o rural people, the Forest Division of the Ministry of Natural Resources and Tourism in Tanzania initiated a vilage afforestation program nearly t w e n t y years ago (Mnzava, 1980). T h e program provides free seedlings and advice to encourage villagers to plant trees and establish c o m m u n i t y w o o d l o t s for fuelwood, lumber, poles and o t h e r p r o d u c t s (Skutsch, 1983). In addition, the Forest Division has established over the years a n u m b e r o f trial plantations o f exotic species (native to Australia, Latin America, and Asia) in di fferent parts o f Tanzania. Some of these trials are n o w 15 years old. It is well k n o w n t hat gr ow t h rates o f forest plantation in semiarid areas are far lower than in areas o f higher rainfall. However, it is less well k n o w n that the actual growth of so-called fast growing pl ant at i on species at certain field sites m a y fall far short o f predicted growth for t h a t species as a whole (Webb et al., 1980; Burley and Wood, 1976). Because rural and urban develo p m e n t project planning (by agencies such as the World Bank, U.S. AID, the U.N. FAO, and others) relies u p o n published tree growth rates, the failure of plantations to achieve d o c u m e n t e d rates is of considerable concern. Plantations of exotic species are desirable if t h e y can p r o d u c e lumber, fuel, fodder, poles or ot her p r o d u c t s m ore quickly and efficiently than existing natural forests. Thus, t he success of exotic tree plantations can be measured against the p e r f o r m a n c e o f natural forests. In areas where all natural woodland has been removed, indigenous species still provide a good index o f site potential with which to c o m p a r e the p e r f o r m a n c e o f exot i c species. This study examined the survival and growth of some early trial plantations at each o f th r ee climatically and geologically distinct trial sites. T o assess their p er f o r man ce, the biomass and a p p r o x i m a t e growth o f natural forests growing adjacent t o forest plantations were det erm i ned also. This evidence, along with data on soil properties, was used to compare exotic species t o indigenous species as fast-growing sources of w o o d and o t h e r products. METHODS Field m e t h o d s

S t u d y sites were chosen at t hr ee locations of differing climate and geology in Tanzania (see Fig. 1) where f ue l w ood or lumber plantations have been Fig. 1. Location of study sites, and sampled plots in Tanzania. (a) Sambasha Forest Reserve, Mount Meru, Arusha region; (b) Usa River Forest Reserve, Mount Meru, Arusha region; (c) Uluguru Forest Reserve, Uluguru Mountains, Morogoro region; and (d) East Chenene Forest Reserve, Chenene Hills, Dodoma region. In East Chenene Forest Reserve, the 25 by 25 m plots are plantations of Cassia siamea and Eucalyptus citriodora and regenerating bushland; 20 by 20 m plots are of mature dry woodland in reserved forest. Plots 1, 2 and 3 in the Usa River Forest Reserve are mature evergreen forest; plots 1, 2 and 3 in the Uluguru Forest Reserve are regenerating evergreen forest. No soil samples were taken in Usu River Forest Reserve.

130

established in reserved natural forest. These sites were in the Uluguru Forest Reserve in Morogoro region, the East Chenene Forest Reserve in D o d o m a region, and the Sambasha and Usa River Forest Reserves in Arusha region. As each site, 25 by 25 m plots (eight altogether) with the same slope, aspect, and apparently same original soil were laid out in natural forest and adjacent forest plantations and cropped fields. In addition, 27 plots, 20 by 20 m, were laid o u t in mature dry m i o m b o woodland in East Chenene, on shallow skeletal sandy soils (see Fig. ld). The type and age of vegetation, mean annual temperature, mean annual precipitation, slope, elevation, aspect, and geology of each site were noted (see Table 1). A soil pit was dug at the center of each plot, and soil profiles were described and sampled by horizon. In the 27 m i o m b o woodland plots, composite soil samples were taken at 2-m intervals along 10-m transects within each plot. In other plots, 15 surface soil samples were taken from the top 30 cm. The diameter at breast height (dbh) and height of all stems 1> 2 cm (in the 20 by 20 m plots) or/> 5 cm (in the 25 by 25 m plots) were recorded. C o m m o n names and local uses of all species were recorded. Genera and species names were determined for those species whose available plant material permitted identification. Vegetation samples were donated to the Herbarium of the Forestry F a c u l t y at Sokoine University of Agriculture in Morogoro. Soil samples were shipped to the United States for laboratory analysis. TABLE 1 C l i m a t i c and e n v i r o n m e n t a l c h a r a c t e r i s t i c s o f t h e sites in T a n z a n i a Site

Vegetation type

Age (years)

Uluguru

Evergreen forest Fallow field Acacia mearnsii

14

1058--1377

24.3

1

1058--1377

24.3

14

1058--1377

24.3

20--32

1860

Mature

573---625

22.7

17--30

1280--1710

Granite

Mature

573--625

22.7

17--30

1280--1710

Granite

Eucalyptus citriodora

11

573--625

22.7

2

1325

Granite

Cassia

II

573--625

22.7

2

1325

Granite

Evergreen forest

Mature

1200--1811

--

28

1661

V o l c a n i c ash, trachyte

Cupressus lusitunica

14

1200--1811

--

28

1950

V o l c a n i c ash, trachyte

East Chenene

Deciduous miombo woodland Woodland-shrubland

Mean annual precipitation (ram)

Mean Slope annual (o) temperatuxe (°C)

Elevation (m.a.s.L)

Geology

20---32

1860

20--32

1860

Granulite, m i c a schist Granulite, m i c a schist Granulite, m i c a schist

siamea Sambashaa

- - N o t available. a D a t a f o r e v e r g r e e n f o r e s t also a p p l y f o r t h e Usa R i v e r F o r e s t R e s e r v e .

131

Laboratory methods The < 2 mm fraction of the soil samples was fractionated by sieving and the clay percent determined b y centrifuging. Soil pH was determined as the mean of t w o measurements using a glass electrode and a 1:1 soil:water mixture. Organic carbon (C) was determined by potassium d i c h r o m a t e - f e r r o u s sulfate titration (Walkley, ! 9 3 5 ) . Calcium (Ca), magnesium (Mg), and potassium (K) were extracted from 10 g samples using a 1 N a m m o n i u m acetate (NH4Ac) solution, filtered, and analyzed b y atomic absorption spectroscopy (AAS) (USDA Soil Conservation Service, 1972). The mineralogy of the clay fractions (< 0.002 mm) of all depths in the soil profiles was determined by X-ray diffractometry. Histograms of the vegetation plot data were constructed, and the basal area of each plot was calculated. Biomass of each plot was determined assuming conical geometry of stems. The growth rate (mean annual increment) of each vegetation t y p e was calculated as the total biomass divided by the age; in the case o f the natural forests whose age was unknown, mean annual increments were calculated for assumed ages o f 50, 100, and 150 years. RESULTS

Soils Soil properties by site Soil profile descriptions of the sites are shown in Table 2. Soils at the East Chenene Forest reserve site are Lithic Dystropepts, those at Sambasha are Mollic Vitrandepts, and those at Uluguru are Typic T r o p o h u m u l t s (USDA Soil Conservation Service and U.S. Agency for International Development, 1983). Soil texture, structure, and clay mineralogy vary between, b u t n o t within sites, although plots at East Chenene have silica-cemented hardpans (fragipans) at 25 and 50 cm. Clay minerals at Uluguru include kaolinite, gibbsite, and traces of illite, indicative of high rainfall, advanced weathering, and intense leaching. Clay minerals at East Chenene include kaolinite, quartz and mica~ with some plagioclase and orthoclase feldspars, illite and interlayered minerals; unweathered minerals and kaolinite clay are characteristic of soils developed in arid regions. Volcanic ash (amorphous aluminosilicates or allophane) is the only clay mineral at Sambasha, confirming the recent deposition of the ash, which has n o t had time to alter to clay despite relatively high rainfall and leaching. Soil organic C, pH, and available nutrients (Ca, Mg, and K) are shown in Fig. 2 and summarised in Table 3. Softs at Uluguru are rich in organic C but acidic, and have high nutrient levels only in the organic surface layer of the forest soil. Soils at East Chenene are very low in organic C, acidic, and nutrient poor. Soils at Sambasha are high in organic C and nutrients, and have a high pH.

132

East Chenene (a)

pH 3.6 4.0 orgC(%)

Organic

5.0 5.6 6.0 , i

).%..

Uluguru

Sambasha Carbon

4.0

5.0 5

(%)

and

6.0 10

7.0 1,5

O.C.! / ~ ~

pH

i

,

pH

3.6 4.0 0

l

i

i

5.0 5

i

6.0 10

i

15 i

,,/, pN

25 /

5O depth

50

I

(cm)

/

75"

,

25

/

i!

75 100

I I,

125'

l'

1, ',I

r/

OOli.7, ,,,.~

1

'

t!"?'

# II

75 I I

100

t

,,

"

[II

100

125"

I % \

150

N

",%

(b) Available meQ ~OOg°

Calcium, ~

2

3 ~

,:,~l,=

.

4~

251, ~. ,., 50

II ~'ai II t Ii: II

depth

(cm)

75-

Magnesium, 10

~r/~ / I

M~

~ ~

and 20

Mg

Potassium(meq

100g -1 )

30

10

_~'~,,,

6

lS

" '"

~

1'

S

Mg

~Mg

t

100"

£i

125"

tI li

150"

"\

~ natural forest --.-- plantation \\

......

,,°,d

Fig. 2. Organic C (%), pH, and available calcium (Ca), magnesium (Mg), and potassium (K) (meq 100 g-i oven-dry soil) under natural forest, forest plantations, and a fallow t'ie]d at East Chenene,Sambasha, and U]uguru.

133

Soil properties and vegetation Organic C, pH, and available cations in the soils differ markedly according to vegetation type. Surface organic C ( 0 - 1 0 cm depth) is significantly higher under natural forest vegetation than under forest plantations at almost all sites (see Fig. 2 and Table 4). Organic C is significantly higher under the TABLE 2 Color, texture, and structure of soils u n d e r n a t u r a l forest, plantations, and fallow field a t d i f f e r e n t sites in Tanzania Site Uluguru

East Chenene

Sambasha

Depth (cm) Munsell color

Texture

Structure

Regenerating evergreen forest

0--2 2--8 9--13 18--38 39--56 57--

n/a 1 0 Y R 3/4 I O Y R 4/4 7.SYR 4/4 1 0 Y R 4/6 7.SYR 5/8

Dense Sandy Sandy Sandy Sandy Sandy

root mat l oa m loam l oa m clay loam clay loam

F i brous Fine granular Fine angular b l o c k y M e d i u m angular b l o c k y Large angular b l o c k y Large p l a t y

Fallow field

0---8 9--24 25--57 58--

7.SYR 3/4 7.SYR 4/4 7.5YR 516 1 0 Y R 6]4

Sandy Sandy Sandy Sandy

loam l oa m l oa m clay loam

Fine gl-anular Medium granular Medium angular b l o c k y Large p l a t y

Acacia mearnsii

0--2 3--25 26--57 58--91 92--

n/a 1 0 Y R 414 7.SYR 4/4 5YR 516 5YR 718 to 7.5YR 8/4

L i t t e r~ Light sandy l oa m Light silty clay l o a m Silty clay Silt

F i brous Large subangular b l o c k y Large subangular b l o c k y Large angular b l o c k y Large p l a t y

Regenerating bushland

0---13 14--25 26--51 52--82 83--86

10YR 10YR 10YR 10YR 10YR

5]2 512 413 514 5/4

Sandy Sandy Sandy Sandy Sandy

clay loam clay loam clay loam clay clay

Medium Medium Medium Medium Me di um

Eucalyptus citriodora

0--13 14--28 29---66 67--

10YR 10YR 10YR 10YR

413 514 514 6/2

Coarse Coarse Coarse Coarse

sandy sandy sandy sandy

l oa m l oa m l oa m l oa m

Medium angular b l o c k y Fine angular b l o c k y Medium angular b l o c k y Medium angular b l o c k y

Cassia siamea

0--2 3--18 19--28 29---61 62--102 103--132 133--165 166--

10YR 10YR 10YR 10YR 10YR 10YR 10YR 10YR

4/8 313 312 312 312 312 4/2 512

Coarse sandy l oa m Coarse sandy c l a y Coarse sandy clay; Coarse sandy clay Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam

Medium granular Large angular b l o c k y Medium angular b l o c k y Me di um angular b l o c k y Large angular b l o c k y Large angular b l o c k y Large angular b l o c k y Large angular b l o c k y

Mature evergreen forest

0--2 2--8 9--30 31--36 37--61 62--91 92--117

n/a 1 0 Y R 414 7.5YR 414 7.SYR 414 7.SYR 4/4 7.SYR 4/4 7.SYR 4/4

Litter b Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam

Granular Granular Granular Massive Massive Massive

Cupressus lusitanica

0--8

7.SYR 4]4 1 0 Y R 4/4 1 0 Y R 4]4 1 0 Y R 4]4

Sandy Sandy Sandy Sandy

Granular Granular Massive Massive

a Stems~ seed pods, leaves. b Dried leaves, twigs. n/a Not determined.

9--23 24--56 57--81

clay clay clay clay

loam loam loam loam

angular b l o c k y angular b l o c k y angular b l o c k y angular b l o c k y angular b l o c k y

134

Acacia mearnsii plantation than under the two other vegetation types at Uluguru, but the soil is very acid. At East Chenene, organic C and pH are both significantly higher under the Cassia siamea plantations than under the regenerating bush or the Eucalyptus citriodora plantation. At Sambasha, organic C is significantly higher under the Cupressus lusitanica plantation than under the mature evergreen forest, but pH is unchanged. Soil nutrient content at Uluguru depends upon litterfall and nutrient TABLE 3 O r g a n i c C, p H , a n d e x c h a n g e a b l e b a s e s Site

Uluguru

East Chenene

Sambasha

Depth (cm)

Organic C (%)

pH

A v a i l a b l e n u t r i e n t s ( m e q p e r 1 0 0 g soil) Ca

Mg

K

Ca + Mg + K

Regenerating evergreen forest

2--8 9--13 18--38 39--56 57--

12.4 5.3 3.3 2.1 1.6

5.5 5.2 5.0 4.6 4.7

15.0 8.7 3.6 2.2 0.6

10.0 5.8 1.8 0.7 0.3

0.4 0.2 0.2 0.2 0.1

25.4 14.7 9.6 3.1 1.0

Fallow field

0--8 9--24 25--57 58--

6.4 4.3 3.0 1.1

4.4 4.3 4.1 4.6

3.9 2.1 1.0 0.4

2.4 1.2 0.5 0.2

0.3 0.2 0.2 0.1

6.6 3.5 1.7 0.7

Acacia mearnsii

0--25 26--57 58--91 92--

5.8 3.1 0.7 0.1

3.8 4.1 4.7 4.7

0.5 0.3 0.2 0.2

0.7 0.2 0.0 0.2

0.2 0.1 0.1 0.0

1.3 0.6 0.3 0.1

Regenerating bushland

0---13 14--25 26--51 25-82 83--86

0.8 0.4 0.3 0.2 0.2

5.6 5.1 4.5 4.7 4.5

2.4 1.3 1.3 1.2 1.3

1.2 1.0 1.3 1.4 1.5

0.3 0.2 0.2 0.2 0.2

3.9 2.5 2.8 2.8 3.0

Eucalyptus citriodora

0--13 14--28 29--66 67--

4.9 0.2 0.3 0.2

1.1 4.3 4.5 4.6

0.8 1.3 1.1 1.3

0.2 1.1 1.8 1.9

2.1 0.4 0.2 0.2

3.1 2.8 3.1 3.4

Cassia siameu

0--2 3--18 19--28 29--61 62--102 103--132 133--165 166--

0.4 0.6 0.3 0.2 0.2 0.2 0.1 0.1

5.6 5.7 5.4 4.5 4.1 4.1 3.9 4.8

-2.5 1.6 0.6 0.6 0.5 0.7 1.4

0.8 0.6 0.5 0.5 0.7 0.6 1.5

0.2 0.2 0.2 0.2 0.2 0.2 0.2

-3.5 2.4 1.3 1.3 1.4 1.5 3.1

Mature evergreen forest

2--8 9--30 31--36 37---61 62--91 92--117

12.9 7.1 5.0 4.8 4.5 4.5

6.9 6.8 6.9 6.8 6.6 6.7

32.3 19.0 13.7 13.8 12.0 11.3

13.8 10.4 8.0 9.4 6.8 7.9

Cupressua lusitanica

0--8 9--23 24--56 57--81 82--91

10.0 6.0 3.7 4.8 3.4

6.6 6.8 7.0 6.8 6.7

29.7 24.2 18.5 16.8 18.5

5.5 6.0 7.2 5.0 7.2

-- Not determined.

4.6 4.1 4.5 3.8 3.52 4.1

50.7 33.5 26.2 27.0 22.3 23.3

3.9 3.0 2.8 3.9 3.6

39.1 33.2 28.5 25.7 29.3

135 TABLE 4 Significance o f d i f f e r e n c e s in organic C a n d p H in s u r f a c e s a m p l e s o f soils f r o m U i u g u r u , East C h e n e n e , a n d S a m b a s h a ( e a c h n u m b e r is t h e m e a n o f 15 s a m p l e s f r o m 0 - - 3 0 c m d e p t h in a 25 b y 25 m p l o t ) Site, p r o p e r t y

Vegetation

Uluguru o r g a n i c C (%) pH

Evergreen f o r e s t 5.5 c'd 5.0 d

F a l l o w field 4.4 c'e 3.8 d

Acacia mearnsii

East Chenene o r g a n i c C (%) pH

Regenerating bush 0.47 b 5.4

Eucalyptus citriodora

Cassia siamea

0.44 a 5.3 d

0.62 a,b 6.2 d

Sambasha organic C (%) pH

Evergreen f o r e s t 6.4 c 6.8

Cupressus lusitanica

7.4 d'e 4.4 d

7.7 c 6.8

a ' b S i g n i f i c a n t l y d i f f e r e n t at 0.05 level f r o m o t h e r s in s a m e r o w w i t h s a m e letter. c S i g n i f i e a n t l y d i f f e r e n t at 0.01 level f r o m o t h e r s in s a m e r o w w i t h s a m e letter. d ' e S i g n i f i c a n t l y d i f f e r e n t at 0 . 0 0 1 level f r o m o t h e r s in s a m e r o w w i t h s a m e letter.

cycling in the system, as shown by the relationships between organic C, pH, and nutrients. Available cations are closely related to organic C under all three vegetation types at Uluguru (r 2 = 0.95 to 0.98). Soil pH is positively related to organic C and cations under natural forest (r 2 = 0.92 and 0.98, respectively), n o t related under the fallow field (r 2 = - 0 . 2 7 and - 0.06), and negatively related under the Acacia mearnsii plantation (r 2 = - 0 . 9 8 and - 0 . 9 3 , respectively). Soil nutrients at East Chenene also depend upon nutrient cycling; total available cations are positively related to organic C at all three plots (r 2 = 0.95, 0.86 and 0.77 under regenerating bushland, Eucalyptus citriodora, and Cassia siamea, respectively). Nutrients are held in organic matter in these soils, but they have been stripped away by increased leaching following removal of the natural forest. Soil nutrient c o n t e n t at Sambasha is d e p e n d e n t on litterfall and nutrient cycling but pH is not. Organic C is positively related to total cations under mature evergreen forest (r 2 = 0.99) and under Cupressus lusitanica (r 2 = 0.93). Some nutrients are held by organic matter in these soils, but nutrients that are leached away are constantly replenished by weathering of the nutrientrich volcanic ash. Vegetation Histograms of vegetation data from forest plantations and natural vegetation are shown in Fig. 3. The negative exponential shape of the histograms from the dry woodland and woodland-shrubland (East Chenene) and mature

136

evergreen forest (Sambasha) indicate that these forests are relatively undisturbed. Of the four forest plantations measured, Cassia siamea, Eucalyptus citriodora, and Cupressus lusitanica are even-aged. The Acacia mearnsii plot has several mid-sized individuals, 2 9 - - 3 5 cm dbh, and approximately 200

2oo~ 3o

oE

25

~2o

a) Plantations

i

East C h e n e n e - C a s s i a siamea

~

i

---

East C h e n e n e - E u c a l y p t u s citriodora

-.-

S a m b a s h a - C u p r e s s u s lusitanica

......

Uluguru-Acacia mearnsii

i

~0 is. Z

i

iz

.:

tO

i

s

in]

"

i -i - -

11"~

I

;-JL~. L3""

=

5

10

t5

;"

i; ,

!.~_.~ "L ":..! ....

-i ,-t

20

25

30

D i a m e t e r at B r e a s t Height

35 (cm)

80, - - -

b) Natural Forest -O.

3O

East C h e n e n e - m a t u r e dry woodland

•- -

East C h e n e n e - m a t u r e woodland/shrubland

---

Sambas, h a - m a t u r e e v e r g r e e n forest

...... Uluguru-regenerating evergreen forest

x .E o~ 20.

~ Z

~0"

I

.-i

i _.J

-J. !

t__ t0

20

--'

..... 30

_._ 40

50

D i a m e t e r at B r e a s t Height

100 (cm)

Fig. 3. Histograms of diameter classes at East Chenene, Sambasha, and Uluguru. (a) Plantations: n u m b e r of stems in 25 by 25 m plots, (b) Natural forest: m e a n s of 20 by 20 m plots; n = 25 for mature dry woodland, n = 2 for mature dry woodland/shrubland, n = 4 for mature evergreen forest, and n = 3 for regenerating evergreen forest.

137

seedlings smaller than 2 cm in diameter. T he n u m b e r originally planted, spacing, age, percent survival, mean diameter, and mean height o f t he plantations are shown in Table 5. T h e Cassia siamea pl ot had the highest survival rate of the three even-aged plots; the low survival rate of the Cupressus lusitanica plot is due to illegal cutting o f 50% of the trees, whereas t he low survival rate of the Eucalyptus citriodora appeared to be related t o edaphic conditions. TABLE 5

N u m b e r p l a n t e d , s p a c i n g , p e r c e n t survival, a g e , m e a n d i a m e t e r , m e a n h e i g h t , a n d m e a n a n n u a l i n c r e m e n t o f 2 5 b y 2 5 m p l o t s in f o r e s t p l a n t a t i o n s Species

Number planted

Spacing (m)

Survival Age (%) (y)

Mean

Mean

Mean

diameter

height

annual

(cm)

(m)

increment

im 3 ha-~ y-l) A c a c i a mearnsii Cassia siamea

Eucalyptus citriodora Cupressus lusitaclica

24 156 100 70

-2 by 2 2.5 by 2.5 3 by 3

-76 22 50

14 11 11 14

16.2 6.5 10.0 21,3

+ 7.2 +- 2 . 0 +- 2 . 8 +- 2 . 9

5---10 10 15 18

3.5 2.2 1.4 8.7

-- N o t applicable.

The basal area, biomass, and n u m b e r o f stems and individuals in 20 by 20 m plots o f natural forest and plantations are shown in Table 6. Regenerating evergreen forest (Uluguru) has the lowest basal area (12.3 + 4.5 m s ha-l ), while the mature evergreen forest at Sambasha has the greatest basal area (33.3 +- 4.1 m s ha-l). The m a t u r e dry w o o d l a n d at East Chenene has a basal area of 13.1 + 4.7 m s ha - ' . T h e estimated biomass of the forest t ypes mirrors the differences in basal area, ranging f r o m 53.8 +- 26.9 m 3 ha -~ for m a t u r e dry woodland to 346.2 +- 97.8 m 3 ha -~ for mature evergreen forest. Higher basal area and biomass were due to denser spacing and m ore large individuals in the mature evergreen forest plots which have on average 36 +- 4 individuals ( o f which 10 -+ 6 are larger t ha n 20 cm dbh), com pared to an average o f 25 + 8 individuals (of which only 5 +- 2 are larger than 20 cm dbh) in m a t u r e dry woodland plots. Most of t he basal area of dry w oodl and plots is a c c o u n t e d for by the trees of over 20 cm dbh as the data in Fig. 4 show (r 2 = 0.78 for dry woodland). Basal areas of plantations of Eucalyptus citriodora, Cassia siamea, and Acacia mearnsii are n o t significantly different f r o m those of mature dry w o o d l a n d (at East Chenene) or equal-aged natural forest (at Uluguru). Gr o wth rates of natural forest and w o o d l a n d were calculated assuming th at these f o r m at i ons ranged f r o m 50 to 150 years in age (see Fig. 5). The mean annual i n c r e m e n t of forest plantations is n o t significantly di fferent f r o m th at estimated f or natural forests at each site, e x c e p t for t he Cupressus lusitanica plantation. More precise growth estimates were n o t possible because the ages of the natural forests are unknow n. A t t e m p t s t o date the dry wo o d lan d by d e n d r o c h r o n o l o g y were unsuccessful; the species (Pterocarpus

138 TABLE 6 Basal area, n u m b e r o f s t e m s and individuals, a n d b i o m a s s o f natural f o r e s t and p l a n t a t i o n plots in Tanzania Site

Plot a

N u m b e r in 20 by 20 m p l o t o f Stems

Uluguru

East Chenene

Sambasha

Individuals

Biomass (m 3 ha -1 )

1 2 3

43 35 32

8 5 0

26 21 20

17.0 12.0 8.0

100.8 69.2 28.1

1b

25

6

25

12.4

49.6

10 13 17 21 21 23 24 25 26 26 26 28 30 30 31 34 35 36 38 39 40 45 45 60 68 104 147

5 3 6 0 3 5 4 6 3 3 9 1 5 4 4 3 8 4 4 6 2 6 5 10 7 3 7

10 12 17 17 19 21 20 25 19 22 26 23 27 29 17 31 28 21 33 24 38 33 33 33 37 48 89

15.3 6.3 12.6 5.0 11.1 10.5 12.0 14.9 6.6 10.1 16.2 5.1 13.8 9.7 9.8 11.8 21.0 14.1 12.4 11.2 11.5 15.7 14.6 21.0 19.7 19.0 21.5

100.2 14.8 13.9 13.5 82.3 28.7 26.6 60.3 25.0 26.6 68.2 53.3 74.3 56.8 60.5 47.1 87.2 102.5 45.7 51.8 37.5 52.7 42.7 79.4 33.7 74.3 100.6

22 119

0 0

22 119

3.8 9.2

15.2 24.5

11 18 3

34 41 37

27.5 36.4 33.2

257.5 266.5 417.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Ic 2d

Usa River

Stems > 20cm dbh

Basal area (m 2 ha -1)

1 2 3

37 46 37

1

32

7

32

36.0

442.9

ie

35

23

35

25.4

121.9

aUnless o t h e r w i s e i n d i c a t e d , p l o t s are in natural forest. b A cacia m e a r n s i i ; CE u c a l y p t u s

ci t r i o d o r a ; d Cassia S iarnea ; e C u p r e s s u s l u s i t a n i c a .

139 1.6

1.4 ¸

E. oE 1 . 0 ¸

"6

.8

.6' •

t,-, .4'



I

Sambasha and Usa Rtver. matte everq~eenfores1 Sambasha. Cu~cessu~kJs~a~a

• Easl Chenene.malure dry woodland. woodLar~/shr ublar~ Easl Chenene. Cassia slamea and EuCalyptus cltr~odora

= .2 ¸

• Uluquru,req,eneralr.q evergreen IOrest D Ulu~uro.Acacia mearnsii

1'4 Number

of Individuals

>

t'6

20cm

1'8

~)~)

30

dbh

Fig. 4. Basal area o f 2 0 b y 2 0 m plots ( m 2 ha - j ) as a f u n c t i o n o f n u m b e r o f individuals > 20 cm dbh. • Sambssha and Usa R~er, mature ev~green l~est

9 •

z~ SambtShB. Cupressu$ k}sit~v~a

8\

• East Chenene, mat~e dry woodland, woo~andlshr ubland o East Chenene. Cassia $iamea and Eucalyptus citriodora • Uluguru, regenerating evergreen forest £3Uluguru. Acscia mearnsii

7

T~ x:

\

6

E

~ 5 E E

~3 o 2 c

10

30

50

100 Age

150

(Yr)

Fig. 5. M e a n a n n u a l i n c r e m e n t ( m ~ h a -1 y - ~ ) a s a f u n c t i o n o f e s t i m a t e d a g e ( y ) o f n a t u r a l forest and forest p l a n t a t i o n s at Sambasha, East Chenene, and Uluguru.

7

Local name (Latin n a m e )

m z o n a (Oncoba routledgii) msusula munguti (Maesa app.) msenene (Euphorbia usambarlca) mkwego muhenge (Aphloia thelformls) msaada (Rublaceae) msembelele (AlZophylus abysslnicus) m d u g u t u ( V e r n o n i a abriculifera) mkwele ( D o m b e y a torrlda) nyambande gogosesi utumbunuku mldrlngala

mgondamkwasa munghunungu muwumbu msumvugo msinamhemu msonahanga mwiombwe mtundikimeho mwerewere mpedge hawadge mtarawanda mpelemehe mfulu

Site

Uluguru

East Chenene S S S S S S S S S S S S S

T T T T T T T T S S V V V V

Form

X X

X X

X X X X X X X X X

X X

X

X

X X X X X

X X X X X

X

X

X

X X

X X

2 0

X

X

X

X

e~

Uses of e v e r g r e e n forest a n d b u s h l a n d species• I n c l u d e s all trees ( T ) , s h r u b s (S), h e r b s (H), a n d lianas (V) f o u n d in 0 . 2 h a a t U l u g u r u , 0•25 h a at S a m b a s h a a n d .Usa River, and 0.06 h a at C h e n e n e

TABLE

0

Usa River

Sambasha

olarashirashi olmabara olmoret'kaal engebasher 'kon olnagsboli oleteti olmuko'ngora oldjeani lengai

oldaztllya olseyai oldazwot olmorasha oldap'kalya orsanguwes oldanilongiri

olroiseremai olorondo

oldeas'go

olangoruwa olmadudu olsin'di

olosholo olyabiyap olmatata olnaranguwe or~umi

olmaduoi engiyamasembi orgelai

olmafimu ohnorichol

mlimbera mchibe

muriakutu m waga

mzambao

mhanghana mkakatiku kunyuansabi msega rebuke

T T T T T T T T T T T T V V T

T T T T T T T T T T T T H H V V

S S S S S S S S S S

X

X X X X

X

X

X

X

X

X

X

X

X X

X X

X

X

X

X

X

X

X

X

X

X X X X

X X

X

X

X

X

X

X

X X

X

X X X

X

X

X

X

X

X

~=~

142

angolensis) which has been reported to have annual rings elsewhere in Tanzania (Boaler, 1966) had no annual rings in D o d o m a . A wide variety of uses o f the c o m m o n indigenous forest species was cited by local people at each site (Table 7). For example, of fourteen species found within 0.2 ha in the regenerating evergreen forest at Uluguru, nine are used for fuelwood, six for construction, five for rope, twine or thread, and one for poles, medicine, or tea. At Sambasha and Usa River, of thirty-one species in 0.25 ha of the mature evergreen forest, sixteen are used for fuelwood, ten as goat fodder, eleven for medicine, three for construction, four for rope, twine, or thread, t w o for cattle fodder, and one each for arrowshafts, edible fruits, soup, beer, tinder, tea, sandpaper, charcoal, shade, beehives, or ear cleaners. At East Chenene, of twenty-three species in 0.06 ha of regenerating bushland, eight are used for construction, t w o for rope, twine, or thread, five for medicine, six for fuelwood, three for edible fruit, t w o for fencing, and one each for goat fodder, or thatch (Allen, 1983). Soil--vegetation interactions The success of plantations for fuelwood or lumber is certainly dependent upon the inherent climatic and edaphic features of each site. Cupressus lusitanica is the most successful plantation, b u t its high yield (up to 15 m 3 ha -~ y-~) is due to high rainfall and high soil fertility, which occur together in only a few highly prized areas in Tanzania. The plantation of Acacia mearnsii at a site with acid softs and high rainfall has had much slower growth (3.5 m 3 ha -~ y-~), and the extreme acidity of its litter has removed almost all of the nutrients initially available under the natural evergreen forest (see Fig. 2), rendering the site useless for all b u t highly acid-tolerant crops or tree species. The slowest growth of b o t h natural and planted forests occurred at East Chenene, whose climate and softs are typical of the large central region of Tanzania and much of the East African Rift Valley. At none o f these three sites (except possibly Sambasha) did plantation growth rates significantly exceed the estimated growth of unmanaged natural forest (Fig. 5). DISCUSSION The results presented here raise two distinct issues. First, h o w important are soil characteristics for forest productivity? Second, can exotic species meet local needs better than indigenous species?

Soils The fact that available soil macronutrients (Ca, Mg, K, Na) are high in soils with high organic C and pH is important for forestry operations that depend on natural soil fertility. Because organic C, pH, and nutrients are especially

143 high in wet, recent volcanic soils, these sites can be expected to have the most productive forests, both natural and planted. Wet soils developed on acid rocks are likely to have lower macronutrients status, and slower growth of both natural and planted forests may be expected. Sites under high rainfall whose fertility is derived solely from n u t r i e n t recycling via litterfall and organic matter decomposition are less productive and more vulnerable to undesirable losses of soil fertility as a result of plantation establishment than sites under high rainfall whose fertility is continually replenished by weathering of basic rock material. Contrary to the findings of Lundgren (1978) in the Usambara Mountains, our findings on similarly weathered softs at Uluguru suggest that fertility losses resulting f r o m plantation establishment may affect plantation productivity even in the short term. At arid sites on acid rocks both moisture and nutrients are constrained, so litter production, organic matter, pH, soil nutrients, and soil moisture are all low, and the slowest forest and plantation growth is likely to occur here. These results are based on a restricted data set, and a n u m b e r of other factors could explain the differences in performance. For example, some species might grow much faster than those t h a t were chosen to be planted at these sites; or growth might be controlled by management practices or soil characteristics not considered here. Nevertheless, the data presented here are the best data available for comparison of the performance of unmanaged natural forest and present plantations in Tanzania.

Species The data presented here indicate that neither natural forest nor forest plantations are growing very quickly in semiarid central Tanzania. Neither exotic nor indigenous species are clearly superior because of the wide margin of error around the growth estimates. Growth o f plantations of so-called fast growing species at sites with low rainfall (e.g. East Chenene) seems to be constrained principally by the lack of moisture, especially at depth, and its effect on nutrient production, leaching and uptake, but pests also play a role. At East Chenene Cassia siamea had a higher survival rate, but it was planted on a soil w i t h o u t a hardpan and is apparently n o t bothered by termites, whereas the Eucalyptus citriodora seedlings, planted less than 150 m away, were attacked by termites and failed to establish r o o t systems in the silicacemented hardpan below, which may also have waterlogged roots during the wet season. Despite these differences, both species grew at less than 20% of the rate of the successul Cupressus lusitanica plantation at Sambasha, and a b o u t 90% slower than their reported rates under this rainfall (Webb et al., 1980; Burley and Wood, 1976). It may be argued that these two examples are n o t representative, or that their poor performance is due to u n f o r t u n a t e management decisions regarding site, species, pesticides, and weeding. However, while these are only two of m a n y trial plantations in semiarid Tanzania, they are among the earliest

144

and involve species typical of plantations trials throughout the central semiarid region. Other plantations in the region are at most 3--5 years old, and field examination in July 1985 indicates that they are not growing significantly faster than the ll-year-old trials measured in this study. The narrow range of species used in plantation trials in Tanzania (Cassia siamea, Eucalyptus citriodora, E. maidenii, Acacia mearnsii, Grevillea robusta, Schinus molle, and Tectona grandis) reflects a preoccupation with lumber production. None of these species provides edible fodder, fruit, medicine, or rope, although G. robusta and E. citriodora can be used for honey, and A. mearnsii produces tannin (National Academy of Science, 1980, 1983; Webb et al., 1980). The results presented here cast grave doubts on the ability of plantations of exotic species as currently managed to successfully substitute for disappearing natural forests and to provide products useful for local people in the rural areas of Tanzania. If the growth rates measured at East Chenene are representative, 11--17% of the land and 16--30% of the population in Dodoma would be employed full time simply to provide local fuelwood demand by 1986 without further cutting of the natural forest (Allen, 1985). There is little doubt that growth rates of exotic species would increase if present management techniques were improved; but so might the growth rates of indigenous species if they too were managed. In Dodoma and other similar areas, rapid improvement in growth and forest product diversity is urgently needed. Achievement of these goals requires: (1) identification of the best sites for tree plantations, (2) identification of the best exotic and indigenous species and/or provenances for multiple use, (3) identification of the constraints on growth of promising species, and (4) formulation of economically and physically feasible management techniques to overcome constraints to growth. Actions (1) and (2) involve the immediate establishment of comparative field trials of a range of exotic and indigenous species on sites of varying moisture and nutrient status. Identification of appropriate sites for plantation trials requires more detailed soil surveys than are currently available in many areas, but encouraging results have been obtained recently through the use of satellite imagery and aerial photography to survey softs at a suitably small scale (Kalyango and Wen, 1983). Improved site identification could greatly improve tree performance, judging from the considerable variation in growth among individuals of indigenous tree species as a function of site characteristics (Trapnell, 1959; Boaler and Sciwale, 1966; Jeffers and Boaler, 1966; Strang, 1974; Rutherford and Kelly, 1978; Allen, 1983). With better site selection and management, especially in mixed plantings, exotic tree species may more nearly attain fast growth and multiple uses. However, the estimated growth rates of unmanaged natural forests indicate that many indigenous species (e.g. Acacia spirocarpa in central Tanzania) may have equal potential for rapid growth as well as the ability to produce

145 fodder and other products of local use, and t h e y are already adapted to the moisture stress and other edaphic conditions peculiar to the areas where they occur. Research to develop the potential of these indigenous species is urgently needed. Action (3) requires the controlled laboratory experimentation on indigenous and exotic species, using soil typical of field sites, to determine their growth responses to increased moisture, nutrients, light, and biota (particularly N-fixing bacteria and mycorrhizae). Action (4) is the most challenging because it requires the mobilization of the entire forestry bureaucracy and the interest and participation of local people to disseminate and implement management practices. In Dodoma, as in many areas severely affected by forest removal, the local people are inexperienced in farming of any kind, and tree farming seems to them to be a radical action. While tree planting of itself is n o t unfamiliar, villagers apparently are not aware of the scale of planting that w o u l d be required (Skutsch, 1983). Moreover, irrigation, plowing, fertilizer, herbicides and pesticides, which may be required for successful rapid tree growth in these areas, are rarely, if at all, used by local farmers even for f o o d crops. In D o d o m a , the expansion of local forestry is unlikely to exceed the pace of adoption of agricultural innovations, and social and cultural barriers to change may de facto favor plantations of indigenous species, which are familiar to local people and may require less intensive management. CONCLUSION Presently the potential of forest plantations in Tanzania is limited by climatic and edaphic conditions. In general, faster growth can be achieved at high-rainfall sites, but productivity also varies according to the species planted and the geology of the site. The site--species combinations examined here, which are typical o f trials in semiarid Tanzania, showed extremely poor performance, and the arid plantations achieved less than 10% of their d o c u m e n t e d growth for that rainfall. While the sample size is small, the results presented here indicate that plantations of exotic species are not contributing to increased forest productivity by improving u p o n the growth rates of the natural forests they replace. Even in deforested areas, there is no assurance that managed stands of exotic species would outperform managed stands of selected indigenous species in terms of growth rate and multiple products. Under these circumstances, increased research attention should be directed toward the development of indigenous as well as exotic multipurpose species for semiarid areas and the identification of productive site--species combinations suitable for management by local people. ACKNOWLEDGEMENTS The research for this study was provided by Resources for the Future under Contract No. 53-319R-2244 between Resources for the Future and the

146 U.S. D e p a r t m e n t o f A g r i c u l t u r e , O f f i c e o f I n t e r n a t i o n a l C o o p e r a t i o n Development {John Hyslop, Project Manager). Additional funding was vided by the Academic Senate of the University of California at Santa b a r a . T h a n k s a r e d u e t o J o h n G. C a d y , P. S c o t t J o n e s , F r a n k W. D a v i s , Vivian Ackerson for valuable comments, field and laboratory assistance, data processing.

and proBarand and

REFERENCES Allen, J.C., 1983. Deforestation, Soil Degradation, and Wood Energy in Developing Countries. Ph.D. Dissertation, The Johns Hopkins University, Baltimore, MD, 315 pp. Allen, J.C., 1985. Wood energy and preservation of semi-arid woodland in developing countries: The case of Dodoma region, Tanzania. J. Developm. Econ., 19: 59---84. Boaler, S.B., 1966. The ecology of Pterocarpus angolensis DC in Tanzania. Ministry of Overseas Development. Overseas Research Publication no. 12. London. 1958, 35 pp. Boaler, S.B. and Sciwale, K.C., 1966. Ecology of a m i o m b o site, Lupa North Forest Reserve, Tanzania. 111. Effects on the vegetation of local cultivation practices. J. Ecol., 54: 577--587. Burley, J. and Wood, P.J., 1976. A Manual on Species and Provenance Research with Particular Reference to the Tropics. Tropical Forestry Papers. No. 10. Commonwealth Forestry Institute, University of Oxford, Oxford, 226 pp. Jeffers, J.N.R. and Boaler, S.B., 1966. Ecology of a m i o m b o site, Lupa North Forest Reserve, Tanzania. 1. Weather and plant growth, 1962--64. J. Ecol., 54: 447--463. Kalyango, S. and Wen, T., 1983. Remote Sensing of Natural Resources in Eastern and Southern Africa. Soil Survey and Mapping. Regional Remote Sensing Facility, Nairobi. Lundgren, B., 1978. Soil Conditions and Nutrient Cycling Under Natural and Plantation Forests in the Tanzanian Highlands. Report on Forest Soils 31. Department of Forest Soils, Swedish University of Agricultural Sciences, Uppsala, 352 pp. Mnzava, E.M., 1980. Village Afforestation: Lessons of Experience in Tanzania. FAO, Rome, 62 pp. National Academy of Sciences, 1980. Firewood Crops. Shrub and Tree Species for Energy Production. Volume 1. National A c a d e m y of Sciences, Washington, D.C., 237 pp. National Academy of Sciences, 1983. Firewood Crops. Shrub and Tree Species for Energy Production. Volume 2. National A c a d e m y of Sciences, Washington, D.C., 93 pp. Rutherford, M.C. and Kelly, R.D., 1978. Woody plant basal area and stem increment in Burkea africana--Ochna pulchra woodland. S. Aft. J. Sci., 14: 307--308. Skutsch, M., 1983. Why People Don't Plant Trees: The Socioeconomic Impact of Existing Woodfuel Programs: Village Case Studies, Tanzania. Discussion paper D-73P, Energy in Developing Country Series. Resources for the Future, Washington, D.C., 100 pp. Strang, R.M., 1974. Some man-made changes in successional trends on the Rhodesian highveld. J. Appl. Ecol., 11(1): 249--263. Trapnell, C.G., 1959. Ecological results of woodland burning in northern Rhodesia. J. Ecol., 47: 129--168. USDA Soil Conservation Service, 1972. Soil survey laboratory methods and procedures for collecting soil samples. Soil Survey Investigations Report no. 1. Government Printing Office, Washington, D.C., 63 pp.

147 USDA Soil Conservation Service and U.S. Agency for International Development, 1983. Keys to soil taxonomy. Soil Management Support Services Technical Monograph No. 6. Cornell University Department of Agronomy, Ithaca, NY, 244 pp. Walkley, A., 1935. An examination of methods for determining organic carbon and nitrogen in soils. J. Agric. Sci., 25: 598--609. Webb, D.B., Wood, P.J. and Smith, J., 1980. A Guide to Species Selection for Tropical and Subtropical Plantations. Tropical Forestry Papers no. 15. Department of Forestry, Commonwealth Forestry Institute, University of Oxford and the Overseas Development Administration, London, 304 pp.