Agnew et al 2000 landscape monitoring of semi arid rangelands in the kenyan rift valley by Lale'enok Resource Centre

Landscape monitoring of semi-arid rangelands in the Kenyan Rift Valley A. D. Q. Agnew1, C. M. Mwendia2, G. O. Oloo2, S. Roderick3 and P. Stevenson4 1

Biology Institute, University of Wales, Aberystwyth, U.K., 2Kenya Trypanosomiasis Research Institute, Kikuyu, Kenya, 3VEERU, Agriculture Department, Reading University, U.K., 4Zoology Department, University of Oxford, U.K.

Abstract A sampling system was devised and used from 1992 to 1996 in annual grassland and open woodland of the south-west Kenyan Rift Valley. The monitoring was designed to detect vegetation changes consequent on the attempted removal of the tsetse vectors of trypanosomiasis in cattle during a simultaneous trapping programme. The sampled sites covered most vegetationlandscape types, and have led to a better understanding of processes within some. Although fluctuations in pastoralist use and occupancy have been observed, no change in vegetation can be attributed to increased grazing or browsing. Although there is high climatic variability some vegetation types seemed stable over the period studied, and some trends appear to be predictable. Key words: range assessment, savanna, transect, vegetation, woodland

Re´sume´ On a conçu un syste`me d’ećhantillonnage qui fut utilise´ de 1992 a` 1996 dans les prairies annuelles et dans les foreˆts ouvertes du sud-ouest de la Rift Valley, au Kenya. La surveillance e´tait conçue de façon a` de´tecter les changements de ve´ge´tation qui re´sultaient des tentatives d’e´radication des mouches tse´-tse´, vecteurs de la trypanosomiase chez le be´tail, au cours d’un programme de pie´geage. Les sites d’ećhantillonnages couvraient des paysages correspondant a` la plupart des types de ve´ge´tation et ont conduit a` une meilleure compre´hension du processus dans certains d’entre eux. Bien qu’on ait obse-

Correspondence: A. D. Q. Agnew, Biology Institute, University of Wales, Aberystwyth, SY23 3DA, U.K.

rve´ des fluctuations dans l’occupation et l’utilisation pastorales, on ne peut attribuer aucun changement de ve´ge´tation a` une augmentation du paˆturage. Bien que le climat soit tre`s variable, certains types de ve´ge´tations semblent eˆtre reste´s stables pendant la dure´e de l’e´tude et l’on semble pouvoir pre´dire certaines tendances.

Introduction The Maasai farmers of the Rift Valley in Kenya occupy those grassland areas in which their stock are less susceptible to attack by tsetse flies (Glossina spp.) carrying the trypanosomiasis parasite. The flies breed and rest in thicker woodland, often along watercourses, and therefore the land suitable for stock is mainly open grassland, away from shaded water. Trees that provide suitable shade for breeding tsetse have a dense canopy and a high transpirative demand that can only be satisfied by perennial ground water in hot dry climates. Where such water is not available, that is over most of the Rift Valley, there is open vegetation of scattered trees and shrubs with underlying and interstitial grassland or chamaephytic shrubland. It exists in equilibrium with grazing pressure, which has been exerted by large domestic or wild herbivores at least since the last glacial period (Livingstone, 1996; Lovett, 1993). However, aridity in the Rift Valley of Kenya is associated more with high rainfall variability than with low mean annual precipitation, and ranchers utilizing such land must have a survival strategy for episodes of extreme drought. In the Maasai tradition this is accomplished by transhumance, naturally to more humid well-wooded areas, with a corresponding increased challenge from trypanosomiasis. There is therefore a limit to livestock numbers, because so many animals fail to breed or die

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either from low nutrition or disease during periodic drought (Roderick, Stevenson & Ndungu, 1998). Elimination of tsetse would reduce the impact of this constraint and, during the last decade, techniques have been developed for trapping Glossina flies, which seem to be effective so long as the trapping effort is maintained (Dransfield et al. 1990; Brightwell, Dransfield & Kyorku, 1991). The search continues for more sustainable and cheaper control methods than the large scale attempts of the past 50 years to clear tsetse from regions of Africa by removing wild herbivores or trees, or by aerial spraying (Jordan, 1986). Stocking rates may intimately affect the landscape and its biota, yet the interaction between pastoralist livestock and the dry landscapes of the Kenyan Rift Valley is poorly understood. To be sure, much work has been done on the dynamics of stock and perennial grassland (Belsky, 1992), but in the arid Rift Valley most grassland is composed of annual species and is therefore ephemeral; probably this again is a response to extreme climatic variability. This report is of a study carried out from late 1992 to 1996 on the annual grasslands and open woodlands of Ol Kiramatian Group Ranch, in response to the requirement for a long-term monitoring system that would assess landscape as well as the vegetational resource for pastoralists.

The study site The Ranch (Fig. 1) occupies about 325 sq km north of 1°55?S, and east of 36°25?E, mostly between 600 and 700 m above sea level. It abuts the main Rift Escarpment at Nguruman on the west and ends in the minor Oleikonet Escarpment above Lake Magadi in the east. The northern and southern boundaries are featureless. Vegetation facies have been mapped from air and satellite imagery (Voss, 1986; Trump, 1991) but there has been no published survey of vegetation communities. The Ranch is entirely controlled by a group of Maasai families, with an elected Group Ranch Committee. Stock numbers were estimated to be in the region of 10,000 bovines, 43,000 sheep and goats, about 880 donkeys and a small but increasing number of camels. The numbers are an average of two surveys made by the Ministry of Livestock Development (MoL) and the Arid and Semiarid Lands Programme (ASAL), only the first of which has been published (Mol & ASAL, 1988). The published

figures were 6705, 18,153, 19,639 and 401 for cattle, sheep, goats and donkeys, respectively. The most recent, in 1996, showed numbers doubling during the intervening 8 years. Monthly rainfall at the Ol Kiramatian Ranch has a percentage variability of 35% (Fig. 2) and a mean of around 550 mm over 10 years. Most rainstorms occur in the hot summer season (November–May) and are ineffective because they are brief and small in quantity; the majority are from isolated adiabatic clouds crossing the Rift Valley and deliver less than 50 mm of precipitation. In addition they are mostly followed by hours or days of near-zenith sun with maximum daily temperatures of 30–35°C when surface moisture is quickly lost by evaporation. The Ranch is accessible along a road built by the Magadi Soda Company for servicing a fresh water pipeline from the streams descending the Nguruman Escarpment, and, during the earlier years of operation, for harvesting fuel wood for the manufacture of purified Sodium Carbonate (soda). Until 1940 therefore, open woodland trees were being harvested and this has some bearing on the tree population structure observable today. The water abstraction might have affected the seasonally flooded groundwater forest below the escarpment or deep soil water in the alluvium to the west of the river Ewaso Nyiro. However, no decrease in the riverine forest or bushland is visible from aerial photographs spanning the period 1950–1993.

Methods The aims of the monitoring programme were to set up a representative permanent sampling system, capable of being continued by technicians who, although without botanical education, could recognize or be trained to recognize most of the species encountered. The objective of the system was to provide evidence for change or stability in the landscape and vegetation. A preliminary ground survey using aerial photographs, guided by local farmers, showed that the vegetation and soil facies could be divided into six strata of pastoral usage. They are listed below in order of increasing distance from the Nguruman Escarpment and aridity. 1 Open woodland on stony soils on the outwash slopes of the Nguruman Escarpment. 2 Groundwater forest, either riverine or on regularly deeply flooded water-gathering areas.

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Fig 1 Ol Kiramatian Group Ranch and its vegetation zones. Transect sites are numbered 1–12, with Site 3 omitted because it coincides with Site 4 and was used for another monitoring investigation

3 Scattered thicket of Salvadora persica on deep alluvium near water. 4 Acacia tortilis open woodland on alluvium. 5 Treeless grassland and shrubland. 6 Acacia tortilis (with Grewia spp. and Boscia) bushland on perched soils with a covering of pyroclastic rocks, often called stonefields.

These zones were similar to outline vegetation facies maps provided by previous workers (Voss, 1986) and could be related to those described for Africa as a whole by Monod (1963) with the exception of the Salvadora persica thicket. It is not intended, nor is it necessary, to describe them in this publication. Our classification was utilitarian and allowed us to cover the area with six

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Fig 2 Monthly rainfall at Nguruman, KETRI Station 1990–1997

strata, as shown in Fig. 1. White’s description of the vegetation of Africa (White, 1983) includes a chapter on ‘The Somalia–Masai regional centre of endemism’ within which our area falls. However, his vegetation delimitations rely solely on floristics and are not applicable to a fine grained study of derived landscape such as Nguruman. Indeed he designates our area as ‘AcaciaCommiphora bushland’, a regional vegetation type that is absent from this part of the Rift Valley except in rare patches along the base of the Escarpment. For the purposes of permanent monitoring, the time and techniques available were not applicable to the forest (Stratum 2), although it constitutes a grazing resource and a focus for trypanosomiasis infection. It depends on permanent groundwater and is flooded to a depth of a foot or two every rainstorm event of over 120 mm. It is a mosaic of Acacia (Faidherbia) albida, Ficus sycamorus and Trichilia roka, separated by glades of herbland and some perennial grassland of Echinochloa haploclada. Traps for tsetse flies were in place and could be monitored by vehicle, but wild buffalo are common and vegetation recording on foot was considered too dangerous. In any case the best way of monitoring tree canopies is through air photography. Stony bushland with Grewia species (Stratum 6) was considered to be under low threat from cattle increase and was used for sampling standing crop only, which is not reported upon here. In any case, when the sampling system was set up we considered that the stony outwash slope at the foot of the Nguruman Escarpment (Stratum 1) could also be used to characterize the stony bushland.

Permanent monitoring sites were located in each of the other strata (Fig. 1). Each site was set up using a standard plan. A northsouth baseline of 200 m subtended three parallel transects, 100 m apart, running from east to west (set by hand-bearing compass). Because the geological pattern of the landscape runs north-south in the Rift Valley, east-west transects were appropriate. The lengths of the transect lines were varied according to the heterogeneity of the vegetation traversed; in uniform grassland 60 m or 90 m was used, but 200 m was required in the Salvadora persica thicket mosaic. Thus the plan of each site resembled a large E on the ground, if viewed looking south, with the upright as the north-south baseline. The ends of each 100 m length were marked more or less permanently on the ground by white-painted angle-iron posts anchored in concrete blocks. White-painted stones marked the east-west transect ends. This system was designed to provide a consistent geometry which overruled observer bias in site choice and could be reinstated if the ground marks were removed by the farmers, who were understandably wary of interference in land demarcation. In fact, after some initial disturbance, the marker posts and stones were accepted. These transects were used only for intercept records, the intercepted feature being relevant to the vegetation traversed, and measured along a surveyor’s tape laid out along the ground. Thus tree, shrub and low shrub maximum canopy intercepts were always measured, while tussock intercepts were recorded only for the basal

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mounded rhizome mass. These were only recordable in perennial vegetation. The edges of clear changes in vegetation structure were also recorded, although, for reasons given below, this was difficult to do consistently. The monitoring programme at first required observations at set times of year. However, once the observers appreciated the objectives of the study, they were encouraged to set their own timetable which could take into account the variability of local rainfall events.

Results When the first observations were made in 1992, the vegetation on the Ranch appeared to be in excellent condition, and there were numerous stands of the large facultatively perennial grasses Cynodon plectostachyus and Sorghum arundinaceum scattered along river banks and in the alluvial areas. This must have been the result of the series of high rainfall years that ended in 1990, because the 1991/1992 precipitation was adequate but not exceptional (Fig. 2). At this time all the potential types of grassland had developed on the Ranch and sampling decisions were based on these. But these conditions were not repeated at any time in the ensuing 5 years, so that the first data set is not compatible in some ways with the later ones. There was also the expected problem of missing (lost) and corrupted data, so that the record is incomplete, and no site has a truly consistent data set. A secondary objective of the sample site geometry was to provide three replicates at each site for use in statistical treatments, but this has proved impossible with the first 4 years of data because of difficulty in identification of points of origin on the ground and some initial observer error. These arose when permanent position markers were disturbed, and until the excellent site technicians attained proficiency. Therefore the results used here are based on summations over the three transects at each site, in which the fine scale of the original observations has been lost. The sample system was set up to monitor change, and therefore we are not surprised that some sites have been disturbed by changes in human use and occupancy. For example Site 4 is adjacent to Maasai homes, while a new settlement was built near Site 7 during the 4-year period. These are traditional groups of houses within a thorn fence (called bomas or ‘enkang’ in the Maa language) built from local materials and occupied often intermittently while families move from one grazing area to

another. The disturbance imposed on the transect sites is minor and is part of the land use pattern and change on which we report: the sampling system has accommodated it. Results are presented below for each of the vegetation strata outlined in the Methods section.

Open woodland Site 1, at Oloibortoto (Fig. 1) was located in this vegetation type which is subject to rainfall enhancement from runoff down the scarp slope. The overall mean canopy cover recorded varied from 40 to 60% over the 4 years. This transect line includes patches where several species’ canopies overlap in a complex way, and a record often does not conform with subsequent or earlier ones. The general trend, however, is of little change and this is demonstrated in Table 1. The death of one Acacia tortilis and two Balanites aegyptiaca trees account for the slight decline in cover over the period. The slight increase in species diversity of shrubs accords with the observations of local people and their changing use of the area: pastoral settlements have been abandoned and permanent dwellings based on trade and agriculture are increasing.

Table 1 Mean canopy cover percentage measured by intercept length along three transects of 100 m for Site 1, Oloibortoto Open Woodland, for 1992 (total canopy cover 43.1%) and 1996 (total canopy cover 40.8%). The increased species representation in 1996 is accounted for by increased layering in the woodland rather than increased canopy cover Species

1992

1996

Acacia mellifera Acacia tortilis Balanites aegyptiaca Boscia coriacea Cadaba farinosa Commiphora campestris Cordia sinensis Grewia bicolor Grewia tenax Grewia villosa Maerua endlichii Total

15.4 6.5 3.0 0.8 – 8.9 1.2 – 4.7 1.2 – 41.7

18.9 12.2 2.5 2.0 0.8 4.7 0.3 0.4 14.7 0.3 2.0 57.8

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Rhaetophytic Forest As stated in the Methods section, this vegetation was not sampled.

Thicket on deep alluvium Only one site (Site 7) was on this vegetation type which produces stands of the tall grass Sorghum arundinaceum when the soil water has been sufficiently replenished. This usually comes about through surface run-in from neighbouring collection areas. Sorghum arundinaceum has the greatest production of any annual grass on the Ranch and, although said to be low in nutritional value, it is used as a survival fodder during the dry seasons. It is therefore an important resource to the cattle farmers and our sampling should monitor its availability. The area under Sorghum declined over the first year of monitoring, and was only partially reinstated after the heavier rains of late 1994. The adjacent annual grassland, characterized by Urochloa panicoides although dominated by Sporobolus cordofanus, is more ephemeral, and is not closely correlated with presence of Sorghum. The vegetation of Site 7 is similar to that found along the flood plain west of the river, occupying only about 4% of the ranch area. It is a mosaic of low spreading evergreen shrubs of Salvadora persica up to 3 m tall with intervening patches of hard-packed alluvium bearing annual or no vegetation. There are associated trees of Cordia sinensis, shrubs of Cadaba farinosa, and a clear zonation of woody herbs and short-lived perennial grasses (Cynodon plectostachyus, Sorghum arundinaceum, and Urochloa panicoides) around the Salvadora thicket patches. When fully grown after wetter-than-average seasons, this is the focus of breeding sites for the quelea finch (Quelea quelea), and so has wider economic importance in Kenya (Voss, 1986).

Open woodland sites The most reliable data came from monitoring trees in open woodland vegetation (Sites 2, 4, 5, 8, 9 and 10) by their canopy intercepts (Table 2, Fig. 3c). The canopy cover in these wooded grasslands is from 5 to 25%, and is increasing; there is certainly no evidence for a decline in tree presence during the 4 years. There is no statistical

difference between any 2 consecutive years, although, naturally, there is between sites. Site 3 is in open woodland vegetation (Fig. 1), but during the nearby establishment of new settlements all available Acacia tortilis trees were severely cut back for constructing perimeter barriers. Site 5 spans a patch of thorn trees and an open area of Sporobolus tussocks and Indigofera bushes. Thus the Site contributes to records of change in open woodland and in treeless shrubland. The species involved in most changes in cover is Acacia tortilis which is increasing in Sites 4, 5 and 9, but the death of large individuals accounts for the apparent decline in canopy at sites 2 and 10. The annual grassland characterized by Urochloa panicoides, Sporobolus cordofanus or the forb Tribulus terrestris, in order of decreasing rainfall demand, is found as the ground flora between Acacia tortilis trees in the open woodland sites. In Site 9, however, woodland has developed in an area receiving considerable run-in onto shallow alluvium over a calcium carbonate (calcrete) pan. The bushy perennial grass Pennisetum mezianum has become dominant under these conditions. Fig. 3(c) shows the course of change in cover of Acacia tortilis and the grass over the 4 years at this site.

Treeless shrubland (dwarf shrub steppe of Monod, 1963) Transect Sites 5 and 6 covered this vegetation type, both probably underlain by calcrete pan and both with mixed stands of the legume Indigofera cliffordiana and the dropseed grass Sporobolus ioclados (Fig. 3a). One line at Site 5 crosses a patch of trees and shrubs along a line of apparently increased water availability. The intercept count apparently has yielded more consistent results than has total intercept length. Clearly this is because an intercept length will vary with the growth and spread of the individual plant and plant edges are horizontally fuzzy. On the other hand counts of tussocks intercepted are unequivocal. Note also that the data of November 1993 are suspect because they include no tree canopies, which should have been present in every record.

Stony bushland As explained in the Methods section there was no transect site placed in this landscape type.

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Table 2 Total tree canopy cover at open woodland sites, measured by intercept and summed for sample dates within years and the three transects at each site; nr, inconsistent record

Site Year

Mean

1992 1993 1994 1995 1996

17.6 16.9 19.6 15.1 14.4

10.9 11.6 12.9 13.9 13.8

6.5 5.2 5.7 5.7 9.0

13.8 nr 15.3 11.8 14.7

4.2 nr 5.3 7.7 10.9

22.0 nr 22.9 24.1 21.7

12.5 11.2 13.6 13.1 14.1

Fig 3 Perennial species cover change at three sites. (a) Site 5, Ndigya area, percentage intercept in 200 m. Black lozenge symbols track Indigofera cliffordiana, grey square symbols track Sporobolus ioclados. (b) Site 6, Empaash, total number of intercepted individuals in 3 × 90 m of transect. Black lozenge symbols track Indigofera cliffordiana shrubs, grey square symbols track Sporobolus ioclados tussock bases. (c) Site 9 near Ol Kiramatian Village. Lozenge symbols track Acacia tortilis, square symbols track Pennisetum mezianum

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Discussion The transect system has proved moderately resilient. The consistency of records shown in Fig. 3a–c attest to this. In these results the cumulative effect of the two good rainfall events of 1995 can be evaluated: soil water recharge is clearly a possible cause of the increased availability of prime grazing, enhanced tree growth and increased area occupied by shrubs and tussock grasses. On the other hand persistent human error in data acquisition and transcription has made statistical analyses difficult because the data set can be evaluated only at the coarsest scale at present; fine scale interpretation may be possible if monitoring is continued for another decade and if the records show more internal consistency. Nevertheless, the results fulfil some of the objectives of the study. The increasing tree canopy cover is explicable in a landscape that is still recovering from the clearances for fuelwood by the Magadi Soda Company in the mid 20th century. There is no evidence of tree loss during the present period of observation except from natural causes: even those sites that have been currently disturbed (Sites 4 and 7) have not shown marked loss of tree cover. There is evidence of recruitment only in Site 9. Tree canopy cover is an important part of the resource mix for pastoralists in open woodland. Belsky (1994) and co-workers (Belsky et al., 1993a; Belsky, Mwonga & Duxbury, 1993b) describe the system in which shaded forbs and grasses remain living and photosynthetic for longer and often contain more nutrients than adjacent exposed herbaceous vegetation. Therefore the evaluation of trends in open woodland tree populations is important not only to biodiversity and landscape relations, but also for the practical development and management of pastoralism. There seems to be no alternative to the measurement of canopy dimensions along a line transect in small areas of important resource grassland such as Sorghum arundinaceum. Occlusion angle canopy counts have been tried (Agnew, 1968) but the canopy cover is too patchy for the method to yield reasonable results except in large scale surveys. Finally, we should deal with the predictability of change in the ephemeral grasslands encountered on the Ranch. Ellis (1995) suggests that with percentage variation of annual rainfall of well over 33% there is little chance of vegetation and grazing systems being predictable in our area. The results we give here do not

support his contention. Certainly there is considerable yearly variation in vegetation facies displayed (Fig. 3) but there is no evidence that this may not be eventually predictable with rainfall as the forcing function. Perhaps the deep soils and runoff/run-on patterns of the Ol Kiramatian Ranch damp down the effects of climatic variability. Nevertheless, unquantified observations of the much drier stony bushlands over the past 4 years, as well as anecdotal evidence from farmers, give no cause to suspect that rapid or stochastic change is taking place there.

Acknowledgements The most essential help in this project was given by families dwelling in the Ol kiramatian Ranch. We had much help and understanding from Charles Mpessi, James Meiposo and Michael Kapoli, the Group Ranch Chairman and his Council. But the major field worker and information source was Mr Julius Ole Saiboku, who tirelessly accompanied the authors on field work, and ran the monitoring system in their absence. We are also grateful to the Kenya Trypanosomiasis Research Institute and to the Overseas Administration of Britain, through the Natural Resources Institute, Chatham, for the provision of facilities and project funding for the first author to undertake this work.

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(Manuscript accepted 25 June 1999)