Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

Journal of Arid Environments (2001) 47: 499–511 doi:10.1006/jare.2000.0734, available online at http://www.idealibrary.com on Growth performance of e...

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Journal of Arid Environments (2001) 47: 499–511 doi:10.1006/jare.2000.0734, available online at http://www.idealibrary.com on

Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

Gufu Oba*-, Inger Nordal?, Nils C. StensethA, Jo/ rn Stave?, Charlotte S. Bjoras ?, Josphat K. MuthondekiB & William K. A. BiiB *Noragric, Centre for International Environment and Development Studies, Agricultural University of Norway, PO Box 5001, N-1432 As s, Norway ? Department of Biology, Division of Botany and Plant Physiology, University of Oslo, PO Box 1045, N-0316 Oslo, Norway A Department of Biology, Division of Zoology, University of Oslo, PO Box 1050 Blindern, N-0316 Oslo, Norway B Kenya Forestry Research Institute (KEFRI), PO Box 468, Lodwar, Kenya (Received 6 April 2000, accepted 22 September 2000; published electronically 19 February 2001) In the arid zone of central Turkana, north-western Kenya, where soil salinity affects 15–20% of the rangelands, growth performances of trees planted in saline soil rehabilitation trials have not been evaluated. Tree-planting trials have emphasised exotic species over indigenous ones. However, advantages and disadvantages of promoting exotic tree species have not been examined. The current study was aimed at evaluating growth performance of seven exotic and nine indigenous tree species used in saline soil rehabilitation trials. The tree species were established from 6-month-old saplings using microcatchments (FT1) from 1988 through 1990 and pitting treatment (FT2) from 1989 through 1992. The soils in FT1 and FT2 treatments were moderately to highly saline. The exotic tree species produced greater cover and volume during the first year (FT1) but by the second year, production was not sustained due to greater mortality (FT1 & FT2). The indigenous species in general had higher survival rates. Relative growth rates (RGR) of exotic and indigenous species did not differ (FT1 & FT2). Tree mortality was negatively correlated with RGR for exotic species in FT1 but not for indigenous ones. However, changes in plant performance were not in response to salinity alone. Rather, water scarcity superimposed on soil salinity might have influenced plant growth performance. Greater water and salinity stress and subsequently greater mortality in exotic species provided a more convincing reason for promotion of indigenous tree species. In the future, knowledge of salinity distribution and selection of indigenous species to match this will be a better way of rehabilitating sites affected by soil salinity in the arid zone of central Turkana, north-western Kenya.  2001 Academic Press Keywords: Kenya; microcatchments; pitting; range rehabilitation; relative growth rates; soil salinity; tree growth performance; tree mortality; tree planting; water harvesting

R Corresponding author. E-mail: [email protected] 0140-1963/01/040499#13 $35.00/0

 2001 Academic Press

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Introduction Rehabilitation of saline soils in arid rangelands requires careful selection of tree species that tolerate salts, going hand in hand with the improvement of soil water by appropriate water-harvesting methods to increase productivity (Gindel, 1973; Evanari et al., 1982; Kessler & Breman, 1991; Minhas et al., 1997). In the arid zones of central Turkana, north-western Kenya, where 15–20% of the rangelands are affected by soil salinity (Van Bremen & Kinyanjui, 1992), the effect of water harvesting on growth performance of trees in saline soils has not been evaluated. Tree-planting trials using exotic (i.e. introduced) tree species in arid zones in Kenya have had little success relative to those in humid areas (Synott, 1979). In the Sahel, low successes of tree planting trials have been attributed to the poor performance of exotic species compared to indigenous ones (Catterson et al., 1987). The exotic tree species may have faster initial growth and produce more biomass than the indigenous species (Gosseye, 1980; Taylor & SoumareH , 1984; Kessler & Breman, 1991), but given that information on survival is usually lacking, choice of exotic over indigenous species cannot be justified (Sinha & Ghial, 1997). Rather, tree-planting trials need to assess growth performance of tree species as criteria for selection. In this paper, we report on two tree planting field trials FT1 — using microcatchments, and FT2 — using pitting to promote water harvesting. The field trials to assess the growth performance of seven exotic and nine indigenous tree species were conducted in central Turkana, north-western Kenya. We assessed the growth performance of the exotic and indigenous species in terms of height, relative growth rate (RGR), cover, volume and total mortality. Study area description The study was conducted within the East African Great Rift Valley in Turkana, north-west Kenya. The climate is characterized by erratic rainfall, high temperatures and periodic drought. Patterns of rainfall are bimodal with major and minor peaks in April–June and October–November, respectively (Fig. 1). The mean day temperature is about 303C and the mean annual rainfall is 200 mm (over a 60-year average, Ellis, 1994) and evapotranspiration exceeds 2000 mm year!1 (Pratt & Gwynne, 1977). The geology and soils of the central Turkana rangelands have been described (Makin, 1969; Van Bremen & Kinyanjui, 1992). The soils of Turkana have high concentrations of the carbonates and chlorides of sodium and calcium (Makin, 1969). High salt concentration causes surface sealing, reducing water infiltration (Van Bremen & Kinyanjui, 1992). Such sites have either low natural vegetation cover or are unproductive (Skjaeveland, 1979). In the Turkana rangelands, the distribution of indigenous woody plants is influenced by soil salinity and water scarcity. The woody plants provide land cover and are a critical dry season browse resource for the livestock of Turkana pastoralists (Morgan, 1980; Amuyunzu, 1988). Material and methods Microcatchments and pitting techniques Plant growth in the arid zone is usually constrained by lack of water. However, runoff from erratic rainstorms can be harvested (Gindel, 1973; Evenari et al., 1982; Babaev, 1995) to support tree growth (Sharman, 1986). Microcatchments with 5–10m-long water catchment aprons of 5–15% slope (Orev, 1988; Suleman et al., 1995) and pittings are usually used. Pittings vary in size from minor depressions to 0)4-m-deep by 0)4-m-wide pits. In microcatchments, usually a single seedling is planted above the water level (Fig. 2), while in pitting; a single seedling is planted in the water collecting pits.

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Figure 1. Monthly rainfall (1988–1990) in (a) FT1 (Site 1), 1988 ( ), 1989 ( ), 1990 ( ) and (b) FT1 (Site 2) and FT2 1988 ( ), 1989 ( ), 1990 ( ), 1991 ( ), 1992 ( ) in Turkana, Kenya.

Since plants need rainwater for establishment and to sustain growth, inadequate rainfall during planting season may result in establishment failure, reduced growth or increased mortality after establishment (this study).

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Figure 2. Water harvesting microcatchments in FT1 (Site 2) showing tree regeneration. The tree in the picture is Prosopis chilensis. Photo by G. Oba 1990.

Soil salinity survey The soils in FT1 varied from deep sand (Site 1) to sandy-clay loam (Site 2), while in FT2 they comprised moderately deep sand and clay, with the surface covered by salt crusts. Six 50-cm-deep soil pits in the FT1 and two in the FT2 treatment were dug. Soil samples were collected from three depths (0–20 cm, 20–40 cm and '40 cm depth) and analysed for physical and chemical composition. (Samples were analysed by KARI laboratories.)

Field trial 1 Microcatchment treatments (FT1) were conducted at Site 1 south-west (2358N; 35345E) and Site 2 west of Lodwar (3307N; 35335E). The two sites are about 25 km apart. The dominant naturally growing woody species at Sites 1 & 2 of FT1 are shown in Table 1. On average, the native tree species density at the two sites was 20)6 trees ha!1 and 10)4 trees ha!1, respectively (Oba, unpublished data). The microcatchments were constructed with the help of the local community. The size of microcatchments used in the current study was 5;5 m. Each microcatchment had a pit 0)5 m deep and 0)5 m wide at the sloping end (5–15%) of the water-harvesting basin. The microcatchments had a total catchment area of 21,250 m2 planted with 1700 saplings. In individual microcatchments (25 m2), a single 6-month-old tree sapling (15–20 cm in height) was planted during the October rains in 1988. Tree planting trials at Sites 1 and 2 comprised 4 exotic species that were widely used in tree planting trials in Turkana compared with 6 common indigenous tree species (Table 1) (note two indigenous species that had unbalanced distribution between sites were excluded from the present study). A total of 850 saplings were randomized within Site 1 and 730 saplings within Site 2. At Site 1, the exotic species (n"4) and the indigenous (n"6) each comprised a total of 425

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Table 1. The indigenous and exotic woody species used in Field Trial 1 ( FT1) located south-west (2358N; 35345E), west (3307N; 35335E) and Field Trial 2 (FT2) located north of Lodwar (3307N; 35336E in Turkana, Kenya

Species Indigenous species Acacia nilotica Acacia tortilis Acacia seyal Acacia senegal Acacia mellifera Salvadora persica Cordia sinensis Dobera glabra Ziziphus mauritiana Exotic species Prosopis chilensis Parkinsonia aculeata Leucaena leucocephala Azadirachta indica Acacia holosericea Atriplex lentiformis Atriplex aurionformis

Authority

Family

FT1

FT2 

(L.) Willd. Ex del (Forsk.) Hayne Del, (L.) Wild. Benth. L. Lam. (Forsk) R. Br. Lam.

Mimosideae Mimosideae Mimosideae Mimosideae Mimosideae Salvadoraceae Boraginaceae Salvadoraceae Rhamnaceae

   

Stunz L. (Lam.) De Wit A. Juss A. Cunn. Ex G. Don S. Wats.

Mimosideae Caesalpiniaceae Mimosideae Meliaceae Mimosideae Chenopodiaceae Chenopodiaceae

   

 

          

The nomenclature of all woody species follows Dale & Greenway (1961). The sign () shows species involved in field trials in FT1 and FT2.

saplings, while at Site 2, exotic (n"4) and indigenous (n"6) comprised 400 and 330 individuals, respectively. Following transplanting, 14% at Site 1 & 27% of total saplings at Site 2 (exotic and indigenous combined) failed to establish due to drought stress. Among the exotics Azadirachta indica and Leucaena leucocephala had complete establishment failures at Sites 1 & 2, respectively. Among the surviving exotic species, Prosopis chilensis (69%) had a relatively greater establishment success at both sites. The indigenous species that experienced establishment failures at Site 1 were Cordia sinensis, Acacia senegal and A. nilotica, while none failed entirely at Site 2. Great establishment failure by the exotic and indigenous species resulted in unbalanced ratios of exotic and indigenous species at both sites. In the present study, measurements were taken only on surviving saplings. The trials were protected from livestock grazing by forest guards and growth of saplings monitored from 1988 to 1990. Field trial 2 Pitting treatment (FT2) was conducted north of Lodwar (3307N; 35336E) located about 4 km from Site 2 in FT1. In Field Trial 2 (FT2), land was bare of vegetation cover. The site was 908 m2 divided into 5)5;5)5 m blocks (n"30) including buffer zones. For the purpose of the present study, three blocks were selected randomly. In each block, three rows of 12 pits (with rows spaced at 1)6 m excluding buffer area and pits spaced at about 42-cm intervals) were established. Into each pit (0)4 m wide;0)4 m deep), a 6-month-old tree sapling (15–20 cm in height) or 36 saplings per block comprising three individuals of each species of five exotic and seven indigenous species were planted randomly (Table 1).

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The selected saplings were marked and monitored for growth performance. The choice of species was influenced by two factors. First, exotic tree species introduced mainly from Australia were selected on a trial basis with the intention of using them in the rehabilitation of saline soils in Turkana, Kenya. Second, indigenous tree species that are natural to saline soils in the arid zone of Turkana (Amuyunzu, 1988) were selected and their growth performances compared with those of the exotic species. The establishment success for the exotic (80%) and the indigenous tree species (87%) in FT2 were comparable. The trials were conducted during the October rains in 1989 and protected from livestock grazing by the forest guards. Monitoring continued until October 1992. The data compared only the means for 1989 and 1992 periods.

Assessment of tree growth performance The trees raised using microcatchments (FT1) and pitting treatment (FT2) were assessed for growth performance using different criteria. Height, RGR and mortality were assessed in both treatments, while cover and volume were assessed only in FT1. Equation 1 determined tree cover for each species: WC"1/4 (CD1#CD2)2 ) n,

(Eqn 1)

where CD1 is the shortest and CD2 the longest tree crown diameters (m). n is phi Wood volume for each species in FT1 was determined by equation 2: WV (m3)"1/2 (D)2 ) nh,

(Eqn 2)

where h is height (m), D is stem diameter (mm) and WV is wood volume. Relative growth rates (RGR) of exotic and indigenous tree species in FT1 and FT2 were determined by Equation 3. RGR is an index of the functional efficiency in plants that has close relationship to species performance (Hunt, 1982). RGR"In (H2!H1)/t2!t1

(Eqn 3)

Where H2 is height at final time (t2) and H1 is height at the beginning (t1) (i.e. cm.year!1). Mortality of exotic and indigenous species in FT1 and FT2 were calculated as: Mortality (%)"SES!M/SES.

(Eqn 4)

Where SES is the total of successfully established saplings for the species and M the total mortality at the end.

Data analysis Because the data from the two field trials were dissimilar in terms of species and time, direct comparisons were not made between them. Moreover, differences between individual species among treatments were not compared because of unbalanced species number caused by establishment failures. Comparisons were made within treatments between exotic and indigenous species. We used partially nested ANOVA with repeated measures of year, sites and species nested in exotic and indigenous species, while height, RGR, cover; volume and mortality were used as response variables. Sapling mortality was correlated with tree height and RGR (FT1 and FT2). All analyses were done using SYSTAT Inc. (1992).

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Table 2. Soil physical composition and chemical extracts sampled in Sites 1 and 2 of Field Trial 1 (FT1) located south-west (2358N; 35345E), west (3307N; 35335E) and Field Trial 2 (FT2) located north of Lodwar (3307N; 35336E), in Turkana, Kenya

Soil properties

Silt (%) Clay (%) Sandy (%) pH)HO Electro-conductivity (EC) Cation exchange capacity (CEC) Exchangeable cations (meq 100 g!1) Ca Mg K Na C% N%

FT1

FT2

Site 1

Site 2

21)0$22)26 20)0$10)73 59)0$31)13 8)28$0)54 0)78$0)91

12)0$11)1 33)66$28 37)66$25)56 8)45$0)62 1)69$2)89

13)0$1)3 34)8$3)4 51)9$5)4 7)5$0)7 10)02$1)0

8)96$6)20

20)56$10)13

34)3$3)2

11)0$7)54 1)48$2)23 0)70$1)01 1)22$0)92 0)39$0)21 0)03$0)03

19)18$10)0 0)81$0)55 0)51$0)28 4)94$3)43 0)23$0)18 0)018

17)7$12)5 1)3$0)4 0)6$0)01 7)0$0)7 0)17$0)02 0)0027

Results Soil extracts Interactions between soil extracts and pits and soil depths by sites in FT1 (F"0)34, df."2, p"0)558) and FT2 (F"0)005, df."1, p"0)995) were not significantly different. Mean values of percentage silt, clay and sand contents are shown in Table 2. Soil pH in FT2 (7)54$4)23) reflected greater salinity, while it reflected greater sodicity in FT1 (8)28$4)87). The soils in FT1 and FT2 had low organic carbon as well as available nitrogen. Moreover, the electro-conductivity (EC) of the extracts at Sites 1 & 2 (FT1) was less than in FT2. In FT1 cation exchange capacity (CEC) of the extract was generally lower than those in FT2. The exchangeable bases were greater for calcium and sodium cations in FT1 & FT2 (Table 2). Field Trial 1 Tree growth performance Mean tree heights between exotic (3)25$0)17 m) and indigenous (1)87$0)15 m) species (F"30)09, df."1, p(0)0001) were significantly different. Among the exotic species, Prosopis chilensis had greater growth performance than other exotic species (Table 3). Among the indigenous species, greater growth was recorded in A. nilotica and A. senegal and least growth in Ziziphus mauritiana (Table 3). However, weighted mean RGR for exotic (1)7$0)3 m year!1) and indigenous (0)77)4$ 0)27 m year!1) at site 1 (F"4)57, df."1, p"0)122) and exotic (1)66$ 0)22 m year!1) and indigenous (1)36$0)15 m year!1) at Site 2, respectively showed no significant differences (F"1)21, df."1, p"0)307) (Fig. 3).

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Table 3. Mean ($S.E.) of height (m), cover (m2) and volume (m) of the exotic and indigenous tree species planted in microcatchments (FT1) in Sites 1 and 2 located south-west (2358N; 35345E) and west of Lodwar (3307N; 35335E) in Turkana, Kenya, respectively.

Planted tree species

Sites Height (m) 1 2

Exotic species Prosopis chilensis 4)1$0)1 Parkinsonia aculeata 2)1$0)9 Leucaena leucocephala 1)2$0)5 Azadirachta indica n/a Indigenous species Acacia nilotica Acacia tortilis Cordia sinensis Ziziphus mauritania Acacia senegal Acacia seyal

3)3$0)2 2)4$0)4 1)0$0)9 0)9$0)2 3)2$1)5 1)3$0)3

Cover (m2) 1 2

1)4$0)2 21)3$1)5 13)3$2)5 2)0$0)3 6)8$6)5 6)1$4)5 n/a 20$0)1

1)5$1)4 n/a

Volume (m3) 1 2 0)07 0)006

0)004 0)003

n/a 3)7$1)3 n/a 4)0$0)2 n/a 3)2$1)3

2)8$0)2 12)6$2)4 11)4$3)0 2)3$0)7 12)5$6)4 5)5$4)3 1)0$0)4 1)0$0)8 2)4$2)6 1)1$0)2 2)2$2)8 1)8$2)2 1)2$0)4 14)5$13)2 2)2$6)7 1)8$0)3 3)4$4)2 4)4$3)7

0)02 0)005 0)001 0)006 0)04 0)002

0)015 0)006 0)002 0)003 0)005 0)004

n/a"not available.

Woody cover and volume Mean cover in FT1 between exotic (17)52$1)35 m2) and indigenous (6)09$1)19 m2) species (F"40)06, df."1, p(0)0001) showed significant differences. Among the exotic species, greater cover was contributed by P. chilensis, while among the indigenous species, A. senegal, A. nilotica and A. tortilis (Site 1) contributed greater cover (Table 3). Mean volumes between exotic (0)051$0)006 m3) and indigenous (0)008$ 0)006 m3) species (F"24)9, df."1, p(0)0001) were significantly different. Despite high initial failure Leucaena leucocephala (Site 2) and Azadirachta indica (Site 1) produced greater wood volume than P. chilensis (Sites 1 & 2, respectively). Among the indigenous species, A. senegal and A. nilotica (Site 1) contributed greater volume (Table 3). Mortality Differences in total tree mortality between exotic and indigenous species was highly significant (F"13)8, df."1, p"0)004).). On average, greater total mortality occurred among the exotic (70)04$8)84%) than indigenous species (27)0$7)5%). Total mortality by species by site interactions showed no significant differences (F"2)014, p"0)133), while mortality by time interaction was highly significant (F"89)55, df."1, p(0)0001). At Site 2, both exotic and indigenous tree species experienced less mortality during the first year compared to Site 1 (Fig. 4). Mortality among exotic and indigenous species at Site 2 occurred in the second year. Parkinsonia aculeata (Site 1) experienced greater mortality during the first year than L. leucocephala and P. chilensis

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Figure 3. Relative growth rates (RGR) of the exotic and indigenous tree species in Sites 1 ( ) and 2 ( ) of FT1. The species were; Pc"Prosopis chilensis, Pa"Parkinsonia aculeata, Ll"Lucaena leucocephala, Ai"Azadirachta indica, Cs"Cordia sinensis, As"Acacia seyal, Ase"Acacia senegal, An"Acacia nilotica, At"Acacia tortilis, and Zm"Ziziphus mauritania.

(Fig. 4). The surviving P. chilensis had mean dieback (i.e. mortality of the current years twigs) of 74% and 56% at the first and second sites, respectively. By comparison, the indigenous species experienced(10% dieback on average. Correlation between total mortality and tree heights in the exotic (r 2"0)64, n"4, p"0)557) and indigenous species (r 2"0)42, n"6, p"0)482) were positive but not statistically significant. Correlation between total mortality and RGR for exotic was negative (r 2"!0)99, n"4, p"0)032), while that of indigenous was not (r2"0)142, n"6, p"0)782). Field Trial 2 Tree growth performance Mean sapling heights in FT2 between the exotic and indigenous species (F"6)72, df."1, p"0)013) and between years (F"11)90, df."1, p"0)001) showed highly significant differences. Generally, mean sapling heights of the exotic [32)9$7)5 cm (1989) and 90)8$11)03 cm (1992)] exceeded that of the indigenous species [37)6$7)3 cm (1989) and 40)5$8)8 cm (1992)] only in 1992. Mean RGR of exotic (13)7$2)2 cm plant!1 year!1) and indigenous species (10)5$1)89 cm plant!1 year!1) did not differ significantly (F"1)17, df."1, p"0)304) (Table 4). Mortality Year-effects had no significant influence on plant mortality in FT2 (F"0)17, df."1, p"0)676), although mortality showed highly significant differences (F"16)76, df."1, p(0)0001) between exotic [74)7$6)5% (1989) and 60)3$10)5% (1992)] and indigenous species [29)8$6)9% (1989) and 37)3$8)4% (1992)]. Among the exotic species, mortality was greater in A. holosericea, L. leucocephala, P. chilensis and At. lentiformis, in that order (Table 4). Among the indigenous species, Z. mauritiana had the greatest mortality (70)3$14)7%) compared to A. mellifera and A. senegal which had

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Figure 4. Total mortality of the exotic and indigenous tree species during year 1 ( ) and year 2 ( ) at Sites 1 (a) and 2 (b) of the FT1 south-west (2358N; 35345E) and west of Lodwar (3307N; 35335E) Turkana Kenya. The species were; Pc"Prosopis chilensis, Pa"Parkinsonia aculeata, Ll"Lucaena leucocephala, Ai"Azadirachta indica, Cs"Cordia sinensis, As"Acacia seyal, Ase"Acacia senegal, An"Acacia nilotica, At"Acacia tortilis, and Zm"Ziziphus mauritania.

a 100% survival (Table 4). Neither in the exotic nor in the indigenous species was mortality of the saplings in FT2 correlated with sapling height (r2"(0)2, p'0)1 in both cases). Moreover, the correlation between RGR and mortality in the exotic (r 2"0)18, n"5, p'0)1) and indigenous species (r 2"0)22, n"7, p'0)1) was not significant. Discussion In FT1 and FT2, the CEC of the saturation extract reflected levels of salinity, while sodium cations and pH reflected sodicity (Waisel, 1972). Thus, scarce vegetation cover

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Table 4. Mean ($S.E.) of seedling height (cm) and seedling mortality (%) of the exotic and indigenous tree species established using pitting (FT2) north of Lodwar (3307N; 35336E), Turkana, Kenya Tree species

Height

RGR

Mortality

Exotic species Acacia holosericea Atriplex lentiformis Leucaena leucocephala Atriplex aurionformis Prosopis chilensis

31)5$31)2 44)6$12)6 88)3$31)2 32)2$13)3 77)5$11)9

7)9$0)4 11)6$0)6 22)07$2)2 8)05$0)4 19)37$1)5

77)5$27)1 57)5$10)9 77)5$19)3 57)6$10)9 62)0$10)5

Indigenous species Acacia mellifera Acacia nilotica Acacia senegal Cordia sinensis Dobera glabra Salvadora persica Ziziphus mauritiana

48)3$22)3 61)9$22)3 46)7$31)2 33)4$10)8 16)2$15)5 39)5$12)7 49)9$18)3

12)06$1)2 15)49$0)7 11)68$0)6 8)35$0)4 4)03$0)2 9)87$0)5 12)47$1)3

0 21)9$19)3 0 39)7$9)5 33)4$13)5 24)2$11)0 70)0$15)8

(n"2 years).

in FT2 probably reflected greater salinity. The soil data alone was, however, insufficient to separate factors such as water stress from salinity stress, while water stress superimposed on soil salinity stress might have greater implications for tree survival than either of them alone (Sun & Dickinson, 1995; Sinha & Ghial, 1997). Generally, RGR of exotic and indigenous species in both treatments were each not significantly different, even though the exotic species had greater cover and volume than the indigenous tree species during the first year. However, the superior cover and volume of the exotic species in FT1 were not sustained after the second year due to greater mortality compared to that of the indigenous species. Among the indigenous species, Z. mauritiana had greater mean total mortality in both treatments. Greater mortality by Z. mauritiana may be related to its adaptation to wetter environments such as drainage lines, where it relies on ground water (Oba, 1991). Other indigenous species had less mortality or dieback and were highly tolerant to salinity and drought stress (Synott, 1979). RGR in the exotic species was negatively correlated with mortality in FT1 but not in both treatments in the indigenous species. Both dieback and total mortality reflected environmental stress. Dieback in P. chilensis begins after the species grows to over 2)5 m. In tree planting field trials throughout the Turkana District, Obado and colleagues (unpublished data) reported 64% dieback in P. chilensis. They found greater dieback among the older trees, but less in saplings. Similarly, Parkinsonia aculeata experiences dieback (Grewal & Abrol, 1986). In FT2, patterns of dieback were observed in L. leucocephala, A. holosericea, At. lentiformis and A. aurionformis. Atriplex species, despite their greater capacity to tolerate high levels of salinity (Black, 1968), experienced greater total mortality in our study; this was perhaps due to site-specific factors (cf. Sharman, 1986). In the Sahel, Le Houe` rou (1992) reported that A. lentiformis had high rates of failure on sandy soils; which probably applied to conditions in FT2. Overall, exotic tree species in the current study had poor performance after the initial year than the indigenous species. Exotic tree species did not sustain improved growth performance compared to the indigenous species that sustained growth. Failure to rehabilitate saline soils in Turkana during the past was, therefore, due to the emphasis

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put on planting exotic instead of indigenous species. The field trials give strong support to promotion of the indigenous rather than exotic species. In the future, knowledge of salinity distribution and selection of indigenous species to match them will be a better way of rehabilitating sites affected by soil salinity in the arid zone of central Turkana, north-western Kenya. We thank the staff of the UNESCO-TREMU, the Forestry Department in Turkana and KEFRI for help with fieldwork and the KARI Laboratories for analysis of soil samples. Data analyses and writing was made possible by funds from the Norwegian Research Fund (NFR) and Noragric (to GO) and from NORAD (to CBS and JS). We thank Tom Warren and two referees for their comments on the earlier version of the manuscript.

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