Dwarf shrub ecology in Kenya's arid zone: Indigofera spinosa as a key forage resource

Journalof Arid Environments (1990)18, 301-312

Dwarf shrub ecology in Kenya's arid zone: Indigo/era spinosa as a key forage resource M. B. Coughenour,* D. L. Coppock,*t M. Rowland*:f: and

J. E.

Ellis*

Accepted 1 August 1989 Dwarf shrubs are a very important forage resource in Northwest Kenya and probably contribute to the stability of pastoral systemsin that area. In an effort to learn more about the ecology of these species we measured their density, size, morphometric traits and relations, and responses to defoliation. These studies indicate that while dwarf shrub foliage is less abundant than grass, these plants can produce a significant quantity of palatable forage (8 g!m2/year) for the pastoral system, and the forage is availableat critical times. The most abundant species, I. spinosa was patchily distributed, but production rates within patches werehigh (220 g!m2/year). A defoliationexperiment indicated that I. spinosawas tolerant of herbivory. Defoliation also had little net effect on plant size because tissue mortality during a dry period resulted in greater tissue lossesfrom larger undefoliated plants. Tolerance of herbivory and drought, ability to respond to little rainfalland preferencefor sandy soilsindicated substantial valueas a forage speciesin arid tropical regions.

Introduction There is a need in sub-Saharan Africa for plants that can provide palatable forage on a reliable basis, tolerate the herbivory, persist under periodic drought and compete successfully with other plant species, such as annual grasses and forbs. Arid and semi-arid environments in Africa are highly susceptible to increasing human and livestock use. Climatic variability coupled with increased livestock numbers are placing great demands on forage species and their environments. While forage needs have resulted in attention to biology and use of indigenous trees in sub-Saharan regions (Lamprey et al., 1980; Le Houerou, 1980), very little attention has been given to indigenous dwarf shrubs (one exception is Herlocker & Dolan, 1980). Dwarf shrubs have proven to be an extremely important resource for pastoralists in northern Kenya. Despite a long history of use by pastoral livestock, plants have persisted. Nomadic pastoralists in southern Turkana derived approximately 43% of their calorific intake indirectly from dwarf shrubs (Coughenour et al., 1985; Galvin, 1985). Two-thirds of camel forage and one fourth of goat forage intake was comprised of dwarf shrub species in a 1981-82 study of nomadic pastoral livestock in southern Turkana District (Coppock et al., 1986a, 1988a). Most of these dwarf shrubs were Indigofera spinosa Forsk., but I. cliffordiana Gillet was also utilized in lesser amounts. Indigofera was palatable for cattle in wet seasons, and to camels and goats all year (Coppock et al., 1986b, 1988a, 1988b). I. spinosa is also an important browse species in regions east of Lake Turkana (Herlocker & •Natural Resource EcologyLaboratory, Colorado State University, FortCollins, Colorado 80523, U.S.A. tCurrent address: International Livestock Center forAfrica, P.O. Box 5689, Addis Ababa, Ethiopia. *Current address: P.O. Box 555, Big Piney, WY 83113, U.S.A. 014Q--1%3/90/030301

+ 12 $03'00

© 1990 Academic Press Limited

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Dolan, 1981; Lamprey & Yusuf, 1981; Lusigi, 1981). Ethnobotanic studies indicate the importance of such species to the Turkana (Morgan, 1981). The objective of this paper is to synthesize results pertaining to attributes of Indigofera carried out in the south Turkana ecosystem project (McCabe, 1983; Little et ai.1984; Coughenour et al., 1985; Coppock et al., 1988a, 1988b). Results will hopefully suggest that future work with these species could be pertinent for other sub-Saharan regions similarly stressed by climate and exploited by livestock. To meet these objectives we will report on several aspects of the ecology of Indigofera spp. in our study area: their spatial distributions, patchiness and affinities with soil types, rooting depths, responses to defoliation, responses to small rainfall events, drought responses and productivities. Plant sizebiomass relations will be reported and used together with cover estimates to calculate forage productivity over the region. We will relate these findings to other studies of livestock use (Coppock et al., 1988a) and experimentation with Indigofera in controlled environments (Bamberg, 1986; Mugambi, 1989). We conclude with a brief discussion of current and potential dwarf shrub importance for sub-Saharan livestock ecosystems. Study area Study sites were approximately 70 km south-west of Lake Turkana and located entirely within the low-lying Rift Valley (500-2000 m elevation). Approximately 1'2 pastoralists/km/ were found in southern Turkana, with about nine sheep and goats, one cowlbullock, one camel and 0·5 donkey per person (Ecosystems Ltd. 1983/84). Climate in the study area ranges from very arid to arid. The arid zones of Kenya (300550 mm rainfall/year) are primarily dry thorn bushland, while the very arid zones (I50350 mm/year) are mostly dwarf shrub grassland (Pratt & Gwynne, 1977). Average daily temperature is 30°C with little seasonal variation. Rainfall usually occurs in a single rainy season of 2-3 months followed by a dry season of 9-10 months (Little & Johnson, 1984). Mean annual rainfall at four southern Turkana weather stations from 1971-1987 was 309 mm. In the years of the study (1981-1984) the mean annual precipitation at the two available weather stations (Kaputir and Lokori) was 390 mm in 1981and 409 mm in 1982. In 1983at corresponding weather stations (Katilu, Lokichar and Lokori) there was a mean of 145 rnm. During the study period January through September 1984 the mean rainfall (at Kaputir, Lokichar and Lokori) was only 83 mm, Soils in the South Turkana area are mostly deep sands, but basement-derived pediment stone-mantle and lava-derived stone mantle soils are also widespread (Hemming & Trapnell 1957). Gravelly loam soils have also developed on much of the ancient lava. Indigofera spp. are found primarily on sandy soils and lava-derived loam, but also along runnels of dissected pediment and pediplain. Methods Dwarf shrub density was estimated in 1981-82 in a survey of five sites with the third nearest individual in two sectors method (McNeill et al., 1977; Coppock, 1985). There were 30 sample points on each of four transects at each 6- to 8-ha site. Cover was estimated by multiplying mean crown area by density. Current annual growth (CAG) per m 2 was measured by clipping terminal, weakly lignified portions of plant stems encountered in 20 0'25-m2 sample quadrats randomly placed along a series of parallel transects at each site. In 1983 densities of Indigofera spinosa were measured in 20 4_m2 quadrats per site. Quadrats were located randomly within large contiguous patches of I. spinose. In 1984 quadrats were located randomly at two points on perpendiculars to line transects of 13S to 300 m. The 10-14 perpendicular lines were evenly spaced along each transect. One transect was established per site. Nine sites were sampled in 1983 and 11 in 1984. The

DWARF SHRUB ECOLOGY IN KENYA'S ARID ZONE

303

diameter of the longest and perpendicular crown axes and plant height were measured for three randomly located plants in all sampled quadrats for those years. Morphometric relations between crown volume (rn") and plant biomass were established by harvest of 80 of the plants measured in the density survey of 1983. An additional 40 plants were measured, harvested, dried, and weighed in 1983. Twenty plants were the subjects of a more extensive morphometric study in 1984. Leaves and stems were separated, twig lengths and heights determined, and roots fully excavated, dried, and weighed. Regression analyses for relations between biomass and size or cover were performed with least mean squares techniques, in order to develop predictive rather than causal or allometric relationships. Responses of plants to simulated rainfall levels were observed in 1984 to determine the minimal amount of rainfall necessary to reverse drought-induced dormancy. Water was sprinkled within a metal ring (30'5 em diameter) placed over individual plants. It was applied weekly to eight plants in each of three watering levels (plus an unwatered control) for 6 weeks, and biweekly to six plants in each of four watering levels for 4 weeks. Soil was wetted 7'6 em beyond the circumference of the ring. Simulated rainfall levels (em H 20 ) were calculated from the volumes of water added and the total area of wetted soil. Obserations were made of numbers of green plants eight times over the 6-week period and five times over the 4-week period. Growth rates of individual marked branches were measured after a 36-mm rainfall event which broke a long drought in 1984. The ends of 30 twigs on nine plants were marked with ink the day after the rain, and the new growth distal to the mark was measured 10 and 16 days later. On the sixteenth day, 10 plants were marked and these were measured 30 days later. Responses to defoliation were measured by establishing one grazing exclosure on patches of I. spinosa at two sites in May 1983 and at two sites in July 1983. Two exclosures were established at each of two additional sites in 1984. Subsets of the enclosed plants were subjected to two levels of defoliation. All terminal branches of 10 randomly chosen, marked plants in each exclosure were clipped to 25% or to 50% of their original length. Adjacent plants were similarly clipped to equalize competition. The clipped material was dried and weighed. Ten non-defoliated plants were also included as controls. Canopy heights, lengths, and widths were measured before and after clipping and on subsequent sampling dates. Results of defoliation experiments were analyzed with two-way analysis of variance, and Duncan's multiple range tests were employed for comparisons of means. Annual above ground primary production was calculated from the sum of positive growth increments, and relative production rate was calculated as the annual total production rate divided by initial standing crop.

Results Sampling in 1982showed Indigofera spinosa accounted for 61% of total dwarf shrub cover. Total dwarf shrub cover was also comprised of Barleria acanthoides (22%), Sericocomposis hildebrandtii (7'5%), Blepharis linariifolia (3'6%), and Indigofera cliffordiana (1'7%). At least six other species constituted the remaining 4'2% of dwarf shrub cover. The density of dwarf shrubs at five sites sampled by the plotless technique in 1982 was 0'2-1'5 plants/m", (mean = 0'63, SD = 0'52), while cover was 0'7-4'8% (mean = 2'35, SD = 1'42). Current annual growth was 0-26· 3 g/m 2 in 1981 and 0-34'1 g/m 2 in 1982 (Table 1). An inverse relation was observed between the density and sizes of I. spinose plants on sampled patches in 1983 and on sampled sites in 1984 (Fig. I). Total cover per m 2 was calculated from the product of cover per plant and plants per m for each sampled patch or site. Mean cover values were 81% for patches and 19% for sites. Isolines of these cove-

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Table 1. Current growth (g/m2/year; x ± 1 SE) ofdwarf shrubs for various sites onsandyplainsin South Turkana, Kenya, during 1981 and 1982. Based 00200'25-,,( plotsat all sites (exceptstream sides, where n = 15) entirely composed of Indigofera spinosa (95%) and I. cliffordiana. Plants along ephemeral streams wererestricted to shadedareas underlarge trees

1981 season Site

Toma

Lokosimaekori (A) (B)

Katamanak (A)

(B)

KadapadweU (A) (B)

Streamsides (A)

(B)

(C)

Naroo Kadangoi (A)

(B)

Lobokat, open bush (A)

(B) (C)

Lobokat, heavy bush (A)

(B) (C)

1982 season

Late wet (jun)

Mid dry (Nov)

Late dry (Apr)

Late wet (Jun)

4·3 ± 0·9 2·8 ± 1-6 26'3 ± 5'9 3'4 ± 1·9 2'1±1-4 2'6 ± 1-4 o·o± 0·0 1·5 ± 1'1 0'0 ± 0·0

0·9 ± 0·3 5·5 ± 0·9 6·5 ± 1'3 19'0 ± 3'8 0'0 ± 0·0 4'0 ± 2·2 0·0 ± 0·0 0'0 ± 0'0 0'0 ± 0·0

18-1 ± 4'2 24·3 ± 3·6 15·7 ± 4·8 8'8 ± 3'3 0·0 ± 0'0 1·9 ± 1·9 0·9 ± 0·6 4'5 ± 2'3 0·0 ± 0'0

30'7 ± 5'5 34·1 ± 7·9 21-1 ± 7'9 32-8 ± 11'3 16·3±11·2 9·3 ± 5'6 1·3 ± 1·3 2·7 ± 2-7 4·5 ± 4'5

3·0 ± 2·7 O·O± 0'0 o·o± 0·0 2'3 ± 1-6 8·2 ± 2·3 O·O± 0'0 0'0 ± 0'0 3'3±1-6

0·0 ± 0'0 0'0 ± 0·0 0·0 ± 0'0 0'8 ± 0·6

0'0 ± 0'0 0'0 ± 0'0 0'0 ± 0·0 0·0 ± 0'0

0·0 ± 0·0 0'0 ± 0'0 0·0 ± 0·0 0·0 ± 0·0

values are plotted in Fig. 1. Densities on these sites and patches were considerably higher than the average densities over sites sampled in 1981-82. Relationships between above ground biomass per plant and canopy area or volume were highly significant (Table 2, Fig. 2). Regression of log-transformed canopy volumes were better than for untransformed volumes for I. spinosa but not I. cliffordiana. Regressions involving canopy cover were usually as accurate as those involving volume. I. cliffordiana plants were more massive per unit of cover or volume than I. spinosa plants. A sample of I. spinosa from a site in the Nadikam (an area of lava hills interspersed with sandy alluvium) contained unusually large specimens. However, the slope of the regression for only those plants was not significantly different from the regression for all other I. spinosa plants (p < 0'05). The slope of the regressions in Fig. 2 can be usefully interpreted as g/m 2 per % cover. Thus, 1% cover of I. spinose results from a standing crop of 1'4 g/m 2 • Morphometric studies of 20 I. spinose revealed a mean rooting depth of 48 ± 17 em, a mean root weight of 5·8 ± 4·6 g, and a mean shoot: root ratio of 1·5 ± 0·56. A highly significant relationship between twig weight and length was observed (Table 2). The mean above ground weight of these plants was 8·11 ± 6'0 g. Their mean leaf: stem ratio was 0·17 ± 0·06 g. A weak inverse relation between canopy volume and rooting depth was observed (Table 2). Results of defoliation experiments varied among sites and dates (Fig. 3). Non-defoliated plants grew significantly faster than defoliated plants between May and July 1983 at two exclosures, and in July-September at one exclosure (Table 3). Over all other intervals there were no significant effects of defoliation on growth rate. At Katapadwell-4 a few clipped plants grew very fast, but the large variance among plants resulted in non, significant difference among means. Of plants defoliated in 1984, there were no significant defoliation effects on growth rates from April to May. At the N. Site (exclosures 5 and 6),

305

DWARF SHRUB ECOLOGY IN KENYA'S ARID ZONE 60

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M. B. COUGHENOUR ET AL.

306

Table 2. Morphometric relations forIndigofera spp. Units are nI for cover, "r for volume, gfor weight. Forall regressions p < 0'001 except root depth-canopy volume (p = 0'02) Y I. spinosa n = 107

n = 20 I. cliffordiana n = IS

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Intercept 4·7 2'3 -0'9 2'1

weight log(weight) weight log(weight) twigweight root depth-em

volume log(volume) cover log(cover) twiglength-mm canopy volume

353 0'74 138 1'0 0'001 -880

weight log(weight) weight log(weight)

volume log(volume) cover log(cover)

2102 0'79 484 1·1

0'01 57'0 7'9 3'0

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plants defoliated with a 50% intensity suffered a smaller tissue mortality rate than nondefoliated plants (Fig. 4). Control plants were often, but not always, significantly larger than clipped plants. By the last sampling dates in 1984 (Fig. 3), defoliated plants were significantly smaller than non-defoliated plants at only two of the six exclosures (Katapadwell-4 and N. Site-5) (p < 0'05). Thus, while tissue mortality rates were not significantly different among treated plants and controls, lack of rainfall in 1984 tended to cause greater tissue mortality and stem breakage among larger non-defoliated plants. Annual primary production calculated from the sum of positive growth increments ranged from 70% to 250% for the year-long period May 1983-Apri11984 (Table 3). For the period April to September 1984, relative growth rates were 3-59%. It was possible to have a positive mean relative growth rate despite a negative weight change because growth rates were considered nil when individual plants experienced a weight decrement. These zero sample values were retained in the analysis. It was also possible to have a positive relative production rate for the population with little positive increment in mean plant weight because decrements represented a transfer of production to litter, not a loss of production from the plant. Relative production rate was stimulated by 50% defoliation at two exclosures, while at the others there was no defoliation effect. Among all plants defoliated in 1983 the mean relative growth rate was significantly greater in 50% defoliated plants (182%) than undefoliated (115%) or 25% defoliated plants (116%). The mean relative production rate over all 120 plants was 137%. Among plants defoliated in 1984, there were no significant effects of defoliation at any single exclosure. However, among all plants, relative production rate was higher (p < 0,05) in 50% defoliated plants (33%) than in controls (9'5%) or 25% defoliation (15'4%). Greater quantities of simulated rainfall caused an increased fraction of watered I. spinOsa plants to flush leaves at some time over the experimental periods (Fig. 5). This experiment suggested that about 8 mm per week of rain is necessary to induce leaf growth in a significant number of plants. Significantly more plants were green with 8 mm/week than wi~h 3-4 mm/week wate~ing (p < 0'01, r-test, n = 13). Watering infrequently but heaVily (biweekly) produced a slightly greater response per amount of added water than watering weekly, although the difference was not significant (p = 0'4-0'6, r-test, n = 5,8). I. spinosa twig growth rate over the first 10 days following a 36-mm rainstorm was

0'85 ± 0'65 mrn/day(n = 30), while over days 11-16 it was 1'64 ± 1'20 mrn/day(n = 30) and over days 17-47 it was 0·09 ± 0'17 mrn/day (n = 52). Discussion and conclusions

DWarf shrub above ground primary production varied considerably among sites and years. Mean annual production on sites sampled in 1981 and 1982 was 4'1 and 10·5 g1m2 (Table 3). The mean cover on sites sampled in 1984 (Fig. 1) implied a standing crop of 2 26 g1m • An annual production rate of 137% in May 1983-Apri11984 (Table 3) would produce 36 g1m2 • These production estimates can be divided by corresponding cover

M. B. COUGHENOUR ET AL.

308

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Figure 3. Changes in plant weight (calculated from log[crown volume], Table 2) of Indigo/era spinosa following defoliation (0 .... " defoliated 25%; defoliated 50% of distal stems) compared with undefoliatedcontrols (.--) at two sites. Significant differencesamongmeanson a given date exist and are distinguished by dissimilar letter labels. Plants defoliatedin Mayor July of 1983 and resampled in April or September 1984at (a) Lokosimaekorisite (exclosures1 and 2), and (b) Katapadwellsite (exclosures 3 and 4). Allplants in a givenexclosurewereofthe samesizeprior to defoliation (p < 0'01).

+- -,

2/year

values [2'35% in 81-82, 19% in 83-84 (Fig. 1)] to yield 1'8,4'5, 1·91 g!m per % cover for the 3-year-long periods (mean = 2·7 g1m2/year per % cover). A 1987 survey of herbaceous and dwarf shrub vegetation at 34 O'25 X O'25 km sites in southern Turkana (M. Mugambi, pers. commun.) indicated approximately 3·2% cover of Indigo/era species. This density multiplied by the mean production/cover ratio of 2'7 would result in 8·6 g!m 2/year average annual production. An earlier estimate of dwarf shrub production of 8·1 g!m 2/year (Coughenour et al., 1985) was based on estimates of cover made during an aerial survey. These estimates are just above an estimated

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consumption of5'2 g!m2/year by all livestock in South Turkana in 1981-82 (Coughenour et al., 1985), and suggest that herbivory levels on these species approximate 60% of total production. While 8-9 g/m 2 is a low density of biomass, the quality is high, it is persistent and available at critical periods (see Coppock et aI., 1986a, 1986b). Although overall density was low, Indigofera was usually concentrated into patches. Patches sampled in 1983were on average 81% covered, which could result in a within-patch concentration of production of 219 g!m2/year. Patchy spatial distributions of plants may have two consequences. First, livestock feeding rates while in a patch would likely be greater than if plants were uniformly distributed. Less time would be expended moving from plant to plant. Second, patchy distributions may reduce competition with other plant species. The defoliation experiment indicated that absolute growth rates during favorable wet seasons were not inhibited by defoliation. Faster relative growth rates of many defoliated plants indicated that compensatory growth was occurring. Although some plants responded favorably to defoliation, the trend was probably not strong enough to derive a general conclusion for the species or the region. However, Bamberg (1986) studied responses of I. spinose in a controlled environment experiment. She found that total yield was unaffected by clipping one-third or two-thirds of stem length. Although residual

M. B. COUGHENOUR ET AL.

310

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biomass was reduced by clipping, plant responses of equal regrowth suggested that photosynthetic activities of residual biomass were stimulated by clipping. Mugambi's (1989) garden experiment with A and ~ defoliation showed no significant effect of defoliation on total above ground production or nitrogen yield. In his field experiment clipping reduced stem production, but not leaf production or nitrogen yield. An inverse relation between plant size and density (Fig. 1)indicated that small plant size could be compensated by an increased total number of plants. Conversely, however, high plant density may reduce productivity by individual plants through competition. This suggests that if undefoliated plants achieve larger sizes, this may be at the expense of reductions in plant number through intraspecific competition. It is unknown whether plant sizes here were regulated by browsing or were simply a reflection of densities achieved during germination and establishment. However, when competition from neighbouring plants was reduced by defoliation, production of individual I. spinosa plants was much higher (Mugambi, 1981). In addition to their capacity for tolerating herbivory in a dry environment, these dwarf shrubs produced leaf growth with even small quantities of rain. An irrigation experiment suggested that green-up could occur after as little as 8-mm rainfall, and that greeness could be maintained with as little as 8 mm1weekof rainfall (Fig. 5). A 36-mm drought-breaking rainfall was sufficient to induce significant rates of stem elongation. Similar rainfall levels would induce little or no growth among the herbaceous annuals that provide the bulk of primary production in this ecosystem. Dwarf shrubs also remained green longer into the dry season than herbaceous plants (unpubl. phenological data). Below-normal rainfall during the 1984 part of the defoliation experiment showed that tissue loss during drought reduces dwarf shrub biomass relatively more in larger unbrowsed plants, indicating that plants would be reduced to a similar size whether drought or browsing were responsible for tissue losses. This levelling effect of dry-seaSon tissue mortality shows that drought will reduce standing crop irrespective of herbivory. The moderate rooting depth (0'5 m) of this species suggests that it can utilize water from deeper soil than annual plants, but possibly not the deeper water available to larger shrubs and trees. Since these species tend to grow on sandy soils, water from small rainfall events is likely to infiltrate to a moderate depth. The non-fibrous root system would be advantageous in coarser soils where water moves easily to individual roots.

DWARF SHRUB ECOLOGY IN KENYA'S ARID ZONE

311

Dwarf shrubs such as I. spinosa are a key element of pastoral subsistence in this arid ecosystem. Key attributes which make it a valuable forage species are its palatability (Coppock et al., 1986b, 1988a), its resistance to herbivory (this study; Bamberg, 1986; Mugambi, 1989), and its ability to respond to small rainfall events (this study). The perennial, deep-rooted growth form would also prove important for soil stabilization in regions where soils are sandy and rainfall levels insufficient for perennial grass growth «350-400 mm/year). These combinations of traits are ideal in an environment where pastoralism is an important subsistence mode and where rainfall is erratic. Development activities and ecosystem studies in arid and semi-arid sub-Saharan pastoral ecosystems will be improved by increased understanding of dwarf shrub ecology and its role as a forage resource base. Dwarf shrubs are often under-represented in studies of forage resources, although they appear to be more important forage than larger shrubs and trees in general and they surpass herbacous forage in importance during critical periods of low rainfall. This work was supported by grants Foundation. Numerous technicians Gregg, Paul Bovitz, Robert Popp, Coppinger. We thank Roger Ruess experiment.

BSR-8206864 and DEB-8004182 from the National Science and field assistants contributed, including Christy ProctorMohammed Bashir, Lopayone, Eliud, Joseph, and Karen for help in the design and establishment of the defoliation

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