Effects of defoliation on yield and reproduction of the dwarf shrub Indigofera spinosa

Acta I~Ecologica, 1997, 18 (4), 449-463

Effects of defoliation on yield and reproduction of the dwarf shrub Indigofera spinosa George A. Keya (1) National Arid Lands Research Centre, PO Box 147, Marsabit, Kenya. (1) Present address: c/o Dr B. Hornetz, Department of Geography and Goesciences,

University of Trio;, PO Box 3825, D-54286, Trier, Germany. Received: 26.9.96

Accepted: 3.2.97

Abstract

Indigofera sph~osa (Forsk.) is a key forage resource in the arid rangelands of northern Kenya inhabited by nomadic pastoralists. Because it is highly relished by livestock, it is subjected to heavy periodic and in some areas permanent browsing impact wherever it occurs. This study evaluated the effects of simulated herbivory on yield and reproduction of this species. Different levels of impact (light, moderate, moderately-heavy and heavy) were inflicted on individual shrubs at fortnightly and monthly intervals respectively. Results indicate that I. spinosa is highly resilient to browsing. Maximum regrowth compensation was achieved at the moderate level of impact. Heavy clipping reduced residual mass to an extend whereby the ability to compensate for lost vegetative organs was severely curtailed. A longer clipping interval tended to enhance regrowth, especially at the moderate intensity. Reproduction was significantly reduced by clipping impact. At the heavy clipping impact, reproduction was completely halted. Irrigation mitigated the impact of clipping by increasing the amount of regrowth and reproductive activity. To optimise the production and utilisation of L spinosa pastures during the growing (wet) seasons, it is recommended that grazing should be moderate to leave browse height of 10-15 cm. At this intensity, adequate reproduction will be stimulated while ensuring availability of residual mass critical for dry season feeding and long term sustainable production. Yield to browsers will also be optimised at this level of impact. Keywords: Indigofera spinosa, defoliation, yield, reproduction, northern Kenya, Marsabit, Rendille, pastoralists.

R6sum~

lndigofera spinosa (Forsk.) est une ressource fourrag~re clef des parcours afides du nord du Kenya, oO vivent des pasteurs nomades. Sa saveur 6tant tr~s appr6ci~e par le b6tail, L spinosa est souraise, p6fiodiquement et, par endroits en permanence, ~ un fort impact du broutement partout off elle pousse. Cette 6rude 6value les effets d'une herbivorie simul6e sur le rendement et la reproduction de cette esp~ce. Diff~rents niveaux d'impact (16get, modtrt, modtr6 ~ important, important) ont 6t~ inflig6s ~t des arbustes, fi des intervalles de 15 jours et un mois respectivement. Les rtsultats indiquent que L spinosa prtsente une forte rdsilience au broutement. La compensation maximale en repousse est atteinte pour le niveau d'impact modtrt. Une taille importante rdduit la masse rtsiduelle ~un point qui restreint stv~rement la possibilit6 de compenser la perte des organes vtgttatifs. Un intervalle plus grand entre les taffies a tendance ~t am61iorer la repousse, en particulier pour une intensit6 de taille modfrte. La reproduction est significativement rtduite par l'impact de la mille. Pour une taille imporActa ~Ecologica 1146-609 Xl97/04/$ 7.00 9 Gauthier-Villars

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G.A.

Keya

tante, la reproduction est complttement arratte. L'irrigation atttnue l'impact de la taille par une augmentation de la repousse et de l'activit6 reproductrice. Afin d'optimiser la production et l'utilisation des herbages ?l I. spinosa au cours des saisons (humides) de croissance, un pfiturage modtr6 est recommendt, qui laisse une hauteur de pgtture de 10 ~ 15 cm. A cette intensitt, une reproduction adtquate sera stimulte, et assurte la disponibilit6 d'une masse rtsiduelle, critique pour l'affouragement en ptriode s~che et pour une production durable ~ long terme. La production pour les troupeaux sera 6galement optimiste ~ ce niveau d'impact.

INTRODUCTION

With the recognition that deterioration of rangelands through grazing leads to replacement of certain species by others which become abundant, researchers have focused efforts on understanding the effects of herbivory on individual plants and the underlying interactive mechanisms governing plant responses (Be,ANSON, 1953; JAMESON, 1963; McNAUGHTON,1979). Plant-herbivore interactions are characterised by short term effects such as modifications of primary productivity and long term effects such as changes in plant-community properties and in genetic properties of plant populations in response to varying grazing regimes (MCNAucm'ON, 1979). The ability of plants to withstand herbivory depends on several factors such as the intensity and frequency of defoliation, soil moisture status and compensatory growth responses (JONES, 1985; MCNAtJCrtTON, 1979). Although much of what is known about plant-herbivore interactions has concentrated on grasses, work on shrubs has only began to emerge. The persistence of grazing and subsistence pastoral ecosystems in arid and semiarid Africa depends on the ability of plants to cope with herbivory and water limitation (CotJcI~ENOtJR et al., 1990). In the pastoral arid rangelands of northern Kenya, shrubs contribute significantly to livestock diets. Indigofera spinosa is a multi-stemmed and spiny shrub belonging taxonomically to the leguminose family of the genus Papillionoideae. It has a funnel-like growth form, reaching to about 0.7 m tall and 0.3 m canopy diameter. Its leaves are small and trifoliate, with pinkish flowers occurring in racemes in dense clusters (PRATT& GWYNNE, 1977). It is one of the most widespread dwarf shrub species in the arid districts of northern Kenya. Its distribution extends into southern Sudan and Ethiopa (CotJCrtENOtYa et al., 1990). I. spinosa is common on sandy soils receiving less than 300 mm annual rainfall, but virtually absent on elevated, heavy soils with high rainfall. It is also common on old stabilised sand dunes, and gently sloped compact soils of low, broad ridges (HEPmOC~mR,1979). LAMPREYet al. (1980) and CoPPocK (1985) found that 2040% of the diets of goats and camel constituted of this shrub. LUSIGIet al. (1984) also found that 1. spinosa not only constituted a significant proportion of camel and goat diets in all seasons, but was also preferred by cattle and sheep. CoppocK et al. (1987) showed the nitrogen content ofL spinosa to be 3.5% and 1.5% in the wet and dry season, respectively. As a legume it could also fix nitrogen and therefore improve soil fertility. Despite its wide distribution and importance as a key forage resource, the scientific interest in this species is only recent. Interest has concentrated on understanding of its production dynamics in relation to browsing impact. BANBER~ (1986) and MUGAMB! (1989) in garden experiments concluded that L spinosa showed considerable persistence and tolerance to herbivory. MUCAMBI(1989) further suggested that the production of L spinosa was more closely linked to rainfall than to herbivory. He however concluded that although heavy grazing did not impact this species in the growing period, it could have a long term detrimental effect by reducing its competiActa ~Ecologica

Defoliation, yield and reproduction of L spinosa

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tive ability. O n the other hand, COUGHENOURet al. (1989) found conflicting results of defoliation on productivity of L spinosa done at different sites in the Ngisonyika ecosystem of N. W. Kenya. These authors concluded that, although some of the Indigofera plants responded favourably to defoliation, the trend was not e n o u g h to derive conclusions for the species or the region. There are no reported studies on the effect of browsing on the reproductive potential of this species. However, because reproductive plant parts are also eaten by herbivores, reproductive success and survival of this species is likely to be affected by herbivory. The objective of this study was to quantify the effects o f simulated herbivory on biomass yield and reproductive potential of I. spinosa under field conditions. I hypothesised that a) heavy and frequent clipping would negatively impact biomass yield and reproductive potential, and b) irrigation would mitigate the effects of clipping thereby resulting in higher yield and reproductive activity.

MATERIALS AND METHODS The study site The research site was located at Kargi (2~ 31"N 37 ~ 34"E), 60 km from Marsabit on the eastern fringes of the Chalbi desert, -600 rn a.s.1. Geologically, it lies in the region of unconsolidated sediments; geornorphologically it is a plain with relief slope 1.0-2.0% (HoRNeTZet aL, 1992). Mean annual rainfall is -250 rnm occun'ing in two seasons. The first wet season, generally refered to as the "long rains" occurs between March and May. The second wet season, the "short rains" begins from late October ending in December, but sometimes in January. A dry season prevails from June to September. Mean potential annual evapotranspiration is 2300-2500 rnm (BAKE, 1983; HORNETZet al., 1992). Monthly average temperature lies between 27-29~ with mean diurnal minimum and maximum of 20~ and 35~ respectively. High wind speeds (> 4 m/s) are common. Soils at the site are Cambic Arenosols (Petrocalcic phase) with a sandy to sandy loam texture and water holding capacity of 54 rnm in the upper 30 crn (VANKEKEM, 1986; HOR~TZ et al., 1992). Vegetation is principally dwarf shrub type with Indigofera spinosa as the dominant species. Pastoralist Rendille inhabit the Kargi area. The nomads of this area practice seasonal and yearlong grazing strategies depending on rainfall, nearness to water sources and security considerations.

Field m e t h o d s

a) 1990/91 short rains study This study was carried out during the 1990 second wet season (short rains). A subplot measuring about 50 • 50 rn2 in the Kargi experimental exclosure was selected for clipping experiments. This plot was fenced in 1981 to exclude grazing/browsing from animals. Eighty individual mature L spinosa plants of similar phenology and physiognomy (size: 40-50 cm tall), were selected and randomly assigned to the defined clipping and control treatrnents. Defoliation was achieved by clipping plants at 0-1 crn, 5 cm, 10 cm and 15 cm from ground level (2) to simulate heavy, moderately-heavy, moderate

(2) These clipping levels were chosen based on the author's own observationsand measurements of browsed lndifogera spinosa plants along differentbrowsing gradients on commanalityused rangeland.They are therefore, a realistic representationof browsingintensitieson the permanentlyand seasonallyused rangeland. Vol. 18, n~ 4 - 1997

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G.A. Keya

and light browsing intensities respectively. Each of the treatments was further divided into two replicates, each consisting of 2-week and 4-week clipping intervals. The 5 cm clipping treatment was applied only at the 4-week interval in order to gain further insight about plant reactions at this level. These clipping treatments simulated camel and goat browsing. Each treatment consisted of twenty plants, except the 5 cm clipping level, which consisted of ten plants. Of the twenty plants from the 0-1, 10 and 15 cm clipping treatments, ten plants were assigned to each of the two and four-week clipping intervals. Ten plants from the original sample were included as controls. Subsequent regrowth (shoots including leaves) from each treatment was harvested at the respective intensities. Clipped material was air dried to constant weight. Biomass data were analysed in four categories as follows (McNAUGHTON, 1983; MUGAMBI, 1989; COUCHENOURet al., 1990): "yield to browsers"- this is total biomass removed at each harvest (including the initial harvest); "regrowth yield" - sum of biomass collected following initial harvest; "residual yield"- biomass remaining after the final regrowth harvest; "total yield"- sum of total production from the plant (sum of yield to browsers and residual yield). Observations and records of reproduction capacity were made weekly. It involved counts of reproductive parts viz. flower buds, flowers and pods per plant. The study was done between 10.10.1990 and 19.1.1991; a total of six and four clippings for the fortnightly and monthly sampling, respectively, was achieved. Counts of the reproductive plant parts were accomplished for clipped and control plants. A one way analysis of variance for a completely randomised design was used to analyse the data. Tukeys Multiple Range Test was employed for mean separation (STEEL & TORRIE, 1960). STATIGRAPHICS version 3.1 software package (STC Inc., 1986) was used in the statistical analysis. Significant differences were tested at 95% probability level. Statistical analyses were computed on logarithmically transformed data to stabilise variances (STEEL & TORRIE, 1960), but the data are presented as arithmetic means. Daily rainfall was measured using a standard raingauge. Daily temperature and relative humidity were continuously recorded three times daily using a thermo-hygrograph placed inside a Stevenson screen, while daily wind speed was measured by a wind anemometer at 2 m height aboveground (HORNETZ et al., 1992). Potential evapotranspiration (ETo) was calculated according to the formula of MCCULLOCH (1965).

b) 1991 long rains study This study further investigated plants reactions to clipping under rainfed and irrigated conditions during the 1991 first wet season (long rains). The study was also conducted in the exclosure at Kargi. The design was a randomised complete block design with two treatments (defoliation and irrigation) and four levels of clipping (0-1 cm, 10 cm, 20 cm, control). Each treatment had five replications. Irrigation was done during weeks with less than 15 mm of rain (3) in order to achieve this amount. This amount of moisture was found to be the minimum necessary to trigger and sustain any significant plant growth in the study area (LITsc~KO, 1989). Irrigation was done by surface application using overlapping, fine spray nozzles at a discharge height of about 45 cm above the soil surface. Each plot measured 1 • 2 m. Clipping frequency was maintained at a 2-week interval for a total of six clipping (12.3.91-28.5.91). Clipped samples were bagged, airdried and weighed for dry matter determination. Biomass data were analysed as the preceeding section. Reproductive plant parts were investigated as already described, on a weekly basis ( 19.3.91-4.6.91 ).

(3) In other words, if in any week less than 15 mm fell, the difference was made up for by irrigation to achieve this amount.

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Data was subjected to a two-way of variance (STEEL• TORR1E,1960). Data presentation, transformation and mean separation tests were done as already explained. Climatic data was recorded as described in the preceeding section. RESULTS

a) Short rainy season 1990/1991

Climatic parameters D a i l y rainfall at the study site did not differ significantly f r o m that at the Kargi recording station, 5 k m away. Total rainfall received at both sites was 161 m m and 169 mm, respectively. Trends of daily rainfall, w i n d speed, and potential evapotranspiration (ETo) and their relationships for the Kargi recording station are presented in figure 1.

Rainfall(mm):

ETo(mm/day)

Wind speed(l(m/day)

207

, 350 300

15

250 200

10 150 I00

5

50

O

0 1.10

9.10

17.10

25.10

2.11

I0.Ii

18.11

26.11

4.12

Date Rainfall

I -wind

speed

~

ETo

FI~. 1. - Rainfall, wind speed and potential evapotranspiration (ETo) during the 1990 short rainy season at Kargi.

Biomass yield Total yield was not significantly different at moderate and light clipping c o m pared to control plants under the t w o - w e e k clipping regime. H e a v i l y clipped plants Vol. 18, n~ 4 - 1997

454

G.A. Keya TABLE I. -- ANOVA results showing which measured variables o f l n d i g o f e r a spinosa were affected

by defoliation during the 1990 short rainy season. Variable

Defoliation intensity

Defoliation frequency

Interaction NS

Regrowth yield

***

NS

Yield to grazers

***

***

*

Residual yield

***

NS

NS

Total production

**

**

*

Flowerbud production

***

NS

NS

Flower production

***

NS

NS

Pod production

***

NS

NS

* p < 0.05; ** p < 0.0l; *** p < 0.0001; NS = nonsignificant effect (p > 0.05).

yielded significantly less total biomass (table I; fig. 2a). Although moderate clipping intensity produced more regrowth than the lightly defoliated plants, this difference was not significant. However, heavily defoliated plants produced significantly less regrowth compared to all other treatments. Although yield available to browsers was slightly higher at moderate and heavy clipping compared to lightly clipped plants, this difference was not significant. Clipping significantly reduced residual yield although no significant differences were found between moderate and light clipping. Heavy clipping produced insignificant residual mass. Yield i(g/plant) o

b

14 b

12 10

8 b

6

4 2 0

Heavy

Moderate

Light

No

clip

Deto] t a t l o n i n t e n s i t y

[~

Yield

for browsers

~

Residual

~

Total yield

FIG. 2a. - Effect of defoliation intensity at a t w o - w e e k interval on biomass yield of L spinosa during the 1990 short rains, Note: bars of similar shade having the same letter(s) are not significantly different a t p < 0.05.

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Although the four-week clipping interval did not alter significantly the yield at all defoliation intensities, moderately and heavily clipped plants produced slightly more total yield than the lightly clipped and the nonclipped controls (fig. 2b). Clipping Yield

(g/plant)

20-r~ 18 16

W

V

w

!

E~

yv

14 V

12

l0

H,

0

Heavy

Moderate~-heavy

Moderate--

DeloI iatton

l

Regrowth

~

Yield for browsers

Light

No clip

Intensity

~

Residual

~

Total y i e l d

Ftc. 2b. -Effect of defoliationintensityat a monthlyintervalon biomass yieldof I. spinosa duringthe 1990 short rains. Note: bars of similar shade having the same letter(s)are not significantlydifferentatp < 0.05. at t0 cm height produced more total biomass than all other clipping treatments. Regrowth at 5, 10 and 15 cm clipping intensity was not significantly different although slightly more biomass was produced at the 10 cm intensity. Heavy defoliation (0-1 cm) inhibited the regrowth potential of the plants. At this four-weekly clipping interval, the yield available for browsers was not significantly affected across all intensities, although it steadily declined with decreasing intensity of clipping. Although residual yield was significantly reduced by clipping at the 4-week interval, no significant differences were found between 10 and 15 cm intensities. Total biomass yield of I. spinosa was affected more by clipping intensity than frequency. Similarly, regrowth and residual biomass were significantly more affected by intensity of clipping than by frequency. Interaction effects of clipping intensity and frequency were evident for total yield and yield for browsers.

Reproduction Table I and figure 2c present the impact on reproduction of I. spinosa. Clipping significantly reduced the reproductive activity of this species at all the clipping freVol. 18, n~4 - 1997

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G.A. Keya

(No/plant)

300 250 200

150 100

b

5~[ 0

E Heavy

L. . . . . a. . . . . . . . . . . Moderate

Light

- - - -

No clip

Deloliation intensity.

1 Flower b u d s ~ Flowers Pods F,G.2c. - Impactof defoliationintensityat a two-weekintervalon reproductionof/. spinosa duringthe 1990 short rains. Note: bars with similarshade havingthe same letter(s) are not significantlydifferentat p < 0.05. quencies. At the two-week interval, production of flower buds dropped by 65%, 83% and 100% at the light, moderate and heavy clipping, respectively, compared to control plants. Flower production dropped by 69%, 94% and 100% respectively, while pod formation fell by 80%, 97% and 100% respectively across the clipping intensities. Similarly, the four-week clipping interval significantly reduced the reproduction of L spinosa (fig. 2d). Flower bud production declined by 75%, 85%, 99% and 100% at the light (15 cm), moderate (10 cm), moderately-heavy (5 cm) and heavy (0-1 cm) intensities respectively compared to control plants. Flower production dropped by 70%, 90%, 100% and 100% with increasing intensity of defoliation. Pod production fell by 85%, 97%, 100% and 100% respectively. Clipping intensity was more important that clipping frequency in influencing reproductive capacity of I. spinosa during this study.

b) Long rainy season 1991

Climatic parameters Rainfall distribution in relation to wind speed and potential evapotranspiration (ETo) for this season is shown in figure 3. Total rainfall during the 81-day observation period (10.3.91-24.5.91) was 52.7 and 58.7 mm at the Kargi recording station and the experimental site respectively. Daily rainfall at both sites was not statistically different Acta CEcologica

Defoliation, yield and reproduction ofL spinosa

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No/plant)

3OO 250

200

150

100 wx

50

Heavy

Moderate-heavy

/

Moderate Detoltatton tntensttu

Flower buds

~

Flowers

Light

~

No c l i p

Pods

FI~. 2d. - Impactof monthlydefoliationon reproductionof L spinosa duringthe 1990short rains. Note: bars with similar shade havingthe same letter(s)are not significantlydifferentatp < 0.05. (p < 0.05). Irrigation provided an additional 107.3 mm. Rainfall distribution was very uneven; 10 and 12 rainy days at the Kargi recording station and experimental paddock respectively. Potential evapotranspiration (ETo) amounted to 557.3 mm during the measurement period 10.3-13.5.91. In the same period only 45.5 mm of rain fell thus leading to a moisture deficit of 511.8 mm.

Biomass yield Clipping intensity reduced regrowth yield of both irrigated and non-irrigated plants. The drop in yield was however statistically insignificant (fig. 4a). This was consistent with 1990 short rainy season results, where defoliation did not significantly affect regrowth at light to moderate defoliation intensity. Although heavy defoliation reduced regrowth in both studies, the reduction was more significant in the 1990 short rains study. On average, irrigation increased regrowth yield by slightlty over 20%.

Reproduction As in this short rains study, increasing clipping intensity significantly reduced reproduction of I. spinosa both on irrigated and non irrigated plots (figs. 4b-d; table II). Flower bud production on rainfed plots dropped by 47%, 78% and 98% respectively with increasing clipping intensity in comparison with control plots. Flower production similarly dropped by 96.2%, 99.1% and 100% while podding Vol. 18, n~ 4 - 1997

458

16

G.A.

Rainfall(ram); EYo(mm)

Keya

Wind s p e e d ( k m / d a y ) , 500

14

400

12 10 8

/

6

?

300

200

4

[ iii!lll !

100

2 0

I~111[[1111111 10.3

18.3

llJl[l[~rTllill[ .

19.4

27.4

5.5

Date /Rainfall

ETo

*

Wind

speed

FIG. 3. - Rainfall, wind speed and potential evapotranspiration (ETo) during the 1991 long rainy season at Kargi.

dropped by 98% and 100% respectively. Flower bud production on irrigated plots declined by 40%, 88% and 100% respectively with increasing clipping intensity. The corresponding decline in flowering was 86%, 97% and 100% while podding dropped by 78% and 100% respectively with increasing clipping intensity under irrigation. On average, irrigation enhanced flower bud, flower and pod production by 45%, 67% and 57% respectively over rainfed treatment. Although both clipping and watering treatments significantly affected reproduction of L spinosa, interaction between the treatments was evident only during podding. DISCUSSION The adaptive survival strategy of pastoralists require herds to be highly mobile to cope with the temporal and spatial variability of rainfall and forage availability common to this arid environment. Grazing periods on any given range are therefore normally of short durations, say, a maximum of two wet months, except on those ranges that are utilized on a yearlong basis. In some cases, a given area may be utilized for one to four weeks depending on rainfall and pasture conditons. In such situations, pastures are subjected to heavy grazing pressure for short durations. The present study demonstrated that total yield of lndigofera spinosa was not significantly affected by light to moderate clipping intensities. The resilience of this species to clipping was demonstrated by the fact that even under the most intense clipping, Acta G~cologica

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Yield ( g / p l a n t )

27

1,5

0,5

Heavy

Moderate

Light

Deloliatton Intensity

1

Non-irrigated

~

Irrigated

FIG.4a. - Effects of defoliation and irrigation on regrowth yield of lndigofera spinosa during the 1991 long rains. Note: bars of similar shade having the same letter(s) are not significantly different atp < 0.05. regrowth was still stimulated. However, clipping significantly r e d u c e d residual mass in proportion to the b r o w s i n g intensity inflicted. These findings are in a g r e e m e n t with those reported in the literature (BAMBERG, 1986; MUGAMBI, 1989; COUGHENOUR et aL, 1990; OBA, 1994). The ability o f L spinosa to withstand even severe c l i p p i n g levels is attributable to its quick growth responses that enable it to c o m p e n s a t e for loss o f vegetative organs. OBA (1994) found c o m p e n s a t o r y growth o f L s p i n o s a to be positively associated with residual b i o m a s s following clipping and that m o d e r a t e levels o f defoliation led to higher c o m p e n s a t o r y growth. COUGHENOUR et al. (1990) suggested that plants achieve partial b i o m a s s c o m p e n s a t i o n through alterations in shoot activity and continued allocation of photosynthates to roots. T h e s e authors o b s e r v e d that tolerance o f I. spinosa was in large part due to m o r p h o l o g i c a l responses. Firstly, budding TABLEII. -- ANOVA results showing which measured variables oflndigofera spinosa were affected by defoliation during the 1991 long rainy season. Variable Flowerbud production Flower production Pod production

Defoliation *** *** ***

Irrigation *

Interaction NS

** **

NS **

* p < 0.05; ** p < 0.01; *** p < 0.0001; NS = nonsignificanteffect (p > 0.05).

Vol. 18, n~ 4- 1997

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G.A. Keya (No/plant)

70

y

60 50 X

40 30 20

10 0

Heavy

Moderate

Light

No clip

Defoliation intensity

1

Non-irrigated

[~

Irrigated

FJG. 4b. - Effects of defoliation and irrigation on flower bud production of I. spinosa during the 1991 long rains. Note: bars of similar shade having the same letter(s) are not significantly different at p < 0.05.

from auxiliary meristems compensated for clipping removal of apical meristems. Without this morphological change in growth response, production of new shoots would have been impaired with a consequent loss of yield. Secondly, residual leaves were longer lived and hence, contributed a greater quantity of photosynthetic activity. Both of these morphological responses were observed in my study. A third morphological factor concerns the size of the growing residual leaves. Leaves of clipped plants were generally larger (see also HORNETZet al., 1992) and more pigmented than those of the controls. Larger leaves having more chlorophyll pigment are able to achieve higher photosynthetic capacities than otherwise. The enhanced photosynthetic capacity would result in faster growth and subsequently more yield. Poor regrowth following intense defoliation was perhaps due to undercompensation following excessive removal of photosynthetic tissues. Under natural grazing conditions morphological alterations (e.g. "rosette" forms) also help to prevent excessive browsing. Reproduction of I. spinosa was significantly reduced by defoliation in this study. Defoliated plants tended to remain in vegetative mode for longer periods than controis. In other words, time to reproductive maturity was delayed due to clipping, thus resulting in fewer reproductive elements. At heavy defoliation, regrowing shoots never reached reproductive maturity. Under optimal conditions, I. spinosa reproduces by seed (generatively). This study therefore suggests that to optimise production and utilisation of L spinosa pastures during the growing season, browsing should be moderate to leave browse of 10-15 cm high. This intensity of browsing will allow adequate reproduction Acta (Ecologica

461

Defoliation, yield and reproduction ofL spinosa

(No/plant)

300 250

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~

......

200

150 I00

50

1a a a Moderate

Heavy

b

Detoliat ion Jnlensily

1

Flower b u d s

~

~ Light

Flowers

~ Noclip Pods

FIG. 4c. - Effects o f defoliation a n d irrigation o n f l o w e r p r o d u c t i o n o f L spinosa d u r i n g the 1991 l o n g rains. Note: b a r s o f similar s h a d e h a v i n g the s a m e letter(s) are not significantly different at p < 0.05.

while ensuring enough biomass yield to the browsing animal. Since I. spinosa is drought deciduous, browsing at moderate levels would be advantageous because new growth from defoliated plants remains green for a longer period than unbrowsed plants which shed their mature leaves. Moderate browsing would ensure adequate residual biomass so critical for dry season feeding. Since moderate browsing leaves adequate residual mass necessary for regeneration and reproduction, long term sustainable use of the browse would be assured. Studies on communal rangeland (I~VA, 1996) have indicated that I. spinosa pastures browsed over long periods and more intensively (heavy defoliation) had less standing net biomass production than those browsed on a seasonal basis and less intensively (moderate defoliation). More long term studies are however needed to clarify the long term effects of different clipping treatments on yield and reproduction of L spinosa. Water availability is the main driving force of plant productivity in arid environments, having effect on several physiological processes. Changes in these physiological processes are manifested in leaf growth and senescence, reproductive activity, root growth and yield (Jo~Es, 1985). However when biotic factors (i.e. herbivory) are superimposed on abiotic factors (i.e. water), the results may not be as predictable and straightforward as one may imagine. For example, COUCHENOURet al. (1990) pointed out that drought effect can reduce the net effect of herbivory by increasing tissue transfer to litter before grazer removal. These workers observed that the least water stressed I. spinosa plants were most negatively affected by defoliation. They sugVol. 18, n ~ 4 - 1997

462

G.A. Keya (No/plant)

40 3836 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2

X

W

Deloliatlon intensily

1

Non-irrigated

~

Irrigated

FIG. 4d. - Effects of defoliation and irrigation on pod production of 1. spinosa during the 1991 long rains. Note: bars of similar shade having the same letter(s) are not significantly different atp < 0.05.

gested that clipping of water stressed plants increased their photosynthetic activity. Significant interactions between clipping and water stress on residual aboveground biomass ofL spinosa in their study, seemed to suggest ameliorative effect of one stress factor upon the response of another under dry environmental conditions. In this study, regrowth yield of both irrigated and rainfed L spinosa were both reduced by increasing clipping intensity, although the reduction was not statistically significant. However, irrigation increased regrowth yield indicating that watering mitigated against the deleterious effects of defoliation. The reduction in reproductive potential following clipping was basically due to delayed sexual reproductive maturity and the initial investment in vegetative organs compared to the controls (KEYA,1996). Similarly, irrigation enhanced reproduction of this species. Since there were no statistical interactive effects of irrigation and clipping, it was concluded that these two factors acted singly in this study to influence regrowth yield, and that watering reduced the negative effects of heavy defoliation. ACKNOWLEDGEMENTS I acknowledge the technical assistance provided by the staff of the K~I-National Arid Lands Research Centre, Marsabit, in the data collection. In particular, the services of Diba Gvu Ahmed SHEIKH,Adano WARIO and Dibayo ILJALAare greatly appreciated. The watchmen who guarded the research plot daily are also acknowledged. Dr. Habil. B. HORNETZof the Department of Geography, University of Trier, gave valuable advise on

Acta O~cologica

Defoliation, yield and reproduction ofL spinosa

463

methodological aspects. Two anonymous reviewers provided valuable comments on the original draft. This research was undertaken with the financial support of the Kenya Agricultural Research Institute (KARl). For this, I am grateful to the Director of KARl.

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A beda. LITSCHKO T., 1989. - P a s t u r e : A water balance model for the estimation of growing periods" in arid lands. Report prepared for the Kenya Range Management Handbook Project, 93 p. LUSIGI W. J., NKURUNZIZAE. R. & MASHETI S., 1984. - Forage preference of livestock in the arid lands of northern Kenya. J. Range. Manage., 37, 542-548. McCULLOCrt J. S. G., 1965. - Tables for the rapid computation of the Penman estimation of evaporation. E. Agric. For. J., 43, 403-222. McNAUrHTON S. J,, 1 9 7 9 . - G r a z i n g as an optimization process: Grass-ungulate relationships in the Serengeti. Am Nat., 113, 691-703. McNAuGa'rON S. J., 1983. - Compensatory plant growth as a response to herbivory. Oikos, 40, 329-336. MUGAMBIM., 1989. - Responses of an African dwarf shrub lndigofera spinosa to competition, water stress and defoliation. PhD. Dissertation, Colorado State University, Fort Collins. 103 p. OBA G., 1994. - Responses of Indigofera spinosa to simulated herbivory in a semi-desert of North-west Kenya. Acta Oecologica, 15, 105-117. PRATT D. J. & GWYNNE M. D., 1977. - Rangeland management and ecology in East Africa. Hodder and Stoughton, 310 p. STATISTICALGRAPHICSCORPORATION,1986. - Statigraphics Package Vers. 3.1. STSC, USA. STEE'~R. G. D. & TORmE J. H., 1960. - Principles and procedures of statistics. McGraw-Hill, 481 p. VAN KEKEM A. J., 1 9 8 6 . - S o i l s of the Mt. Kulal Marsabit Area, Ministry of Agriculture and Livestock Development (MOLD), Nairohi Kenya, 268 p.

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