Nature Conservation Research. Заповедная наука 2020. 5(4): 55–64 https://dx.doi.org/10.24189/ncr.2020.056 HERBACEOUS VEGETATION COMMUNITIES AROUND LAKE MANYARA, TANZANIA: RESPONSE TO ENVIRONMENTAL GRADIENTS Gideon A. Mseja, Beatrice B. Furael, Gastor J. Lyakurwa, Emanuel H. Martin College of African Wildlife Management, Tanzania e-mail: [email protected], [email protected], [email protected], [email protected] Received: 24.07.2019. Revised: 02.09.2020. Accepted: 08.09.2020. Healthy populations of wildlife require quality forage sufficiently available in both space and time. The herba- ceous vegetation around rift valley lakes in Tanzania varies along productivity, disturbance and stress gradients. We have analysed herbaceous vegetation communities and ascertained the nature of their association with en- vironmental variables around Lake Manyara in Kwakuchinja Wildlife Corridor. The transect method was used for sampling vegetation data. Environmental variables including soil pH, available soil phosphorus, distance from the lakeshore, bare soils, soil water, soil salinity and soil total nitrogen were used as possible explanatory variables. Ordination was used to assemble the herbaceous plant communities and relate with the determined environmental variables. The Shannon-Wiener Index was employed to determine species diversity in each plant community. The results revealed three distinct grassland communities named after the dominant and subdomi- nant species: Sporobolus spicatus, Sporobolus ioclados – S. spicatus and Cynodon dactylon. The Monte Carlo Permutation test revealed that soil pH, available soil phosphorus, distance from the lakeshore and bare soil significantly correlated with herbaceous plant community assemblages. The indices of plant species richness and diversity reflected the influence of soil saturation gradients on plant communities. This study contributes to understanding spatial patterns of herbaceous plant communities around alkaline-saline lakes of the Gregory rift valley in East Africa. It also reveals an association between plant community structures with varying edaphic gradients. We conclude that our study forms the basis for monitoring herbaceous plant community change based on determined environmental variables in the corridors of wildlife conservation areas. Key words: CANOCO, eigen-value, Kwakuchinja Wildlife Corridor, Monte-Carlo, simulation Introduction 2006; Li & Pennings, 2019) and interactions be- Wildlife management and pastoralism are tween productivity factors and disturbances (Wil- prime economic activities in semi-arid grasslands son & Tilman, 2002; Aarrestad et al., 2010). The around eastern rift valley lakes in East Africa relationship between environmental variables and (Yanda & Madulu, 2005; Homewood et al., 2012). species diversity and richness can be explained Persistence of these grasslands as foraging areas by intermediate disturbance and productivity hy- for ungulates requires land use planning for live- potheses (Grime, 1973; Huston, 1979; Cornwell stock rearing and/or wild herbivores management. & Grubb, 2003). These hypotheses predict a uni- There is a need to analyse the herbaceous vegeta- modal pattern with stress and competitive exclu- tion in these areas to be able to answer some im- sion reducing diversity at extreme environmental portant ecological questions. These questions are conditions. Maximum diversity and richness oc- whether there are spatial variations in herbaceous cur at intermediate levels of disturbance and pro- species composition and the determinants of such ductivity. Rangeland fire, grazing and browsing variation. Gradient analysis is an important ap- are examples of disturbance whereas soil nutri- proach to spatially and temporally describe large- ents represent productivity (Holdo et al., 2009). scale and local variations in plant community Thus, co-existence of herbaceous plant species composition. Such patterns of variation in vegeta- is dependent on plant community productivity, tion have been attributed to variation in resource levels of disturbance and stressors as they regu- availability, such as soil nutrients, water and light late the vigor of competitively dominant species (Connel & Orias, 1964; Scholes & Walker, 1993; (Cornwell & Grubb, 2003). Gaston, 2000; Golodets et al., 2013), disturbanc- Soil properties such as nutrients, water con- es such as herbivory and trampling (Augustine & tent, pH and salinity also determine the variation McNaughton, 1998; Wüest et al., 2018), stress in plant species richness, diversity and plant com- such as extreme pH and salinity (Begon et al., munity composition (Scholes & Walker, 1993; 55 Nature Conservation Research. Заповедная наука 2020. 5(4): 55–64 https://dx.doi.org/10.24189/ncr.2020.056 Begon et al., 2006). Soil nitrogen and phosphorus tance from the shore of Lake Manyara, soil total are soil macronutrients, which play a major role nitrogen, available soil phosphorus and soil water in primary productivity. They are important nutri- concentration, and (ii) negatively related to soil ents and often the limiting factors for productivity pH, density of ungulate faeces, soil electric con- in the tropical grasslands. Nutrients` concentra- ductivity (EC) and bare soil. We also predicted tion also influences the impact of herbivory on that soil-related predictors would be more influ- plants, and plant responses to herbivory (Bryant ential than herbivory-related predictors. et al., 1983). In the dry tropics, where evapora- tive loss of water is high, soil water content is Material and Methods an important factor in determining primary pro- Study area ductivity (Milchunas & Lauenroth, 1993). Soil KWC is located in Northern Tanzania with- water determines mineralisation rate and soil nu- in the Maasai steppe in East Africa. It is located trient mobility, hence making nutrients available between 3.593889°–3.800556°S and 35.80583°– to plants (De Neve & Hofman, 2002). Water also 35.99028°E. It covers 240 km2 and shares a border influences physical structure and plant communi- with Manyara Ranch to the north, Mswakini vil- ty composition of the arid and semi-arid savanna lage to the northeast, Tarangire National Park to (Scholes & Walker, 1993). the east, Burunge Wildlife Management Area to Begon et al. (2006) explained extreme soil pH the southeast, Vilima Vitatu village to the south- and salinity as environmental stressors, which in- west and Lake Manyara to the west. A large pro- fluence plant species composition, richness and di- portion of this area was gazetted to form Burunge versity. Extreme pH affects soil nutrients and min- Wildlife Management Area, a type of Protected eral mobilisation reducing plant productivity and Area where wildlife management is under the hence influencing plant community assemblage. custodianship of villages. The area is located in High acidity can increase the concentration of an inland drainage basin with an altitude range of Mn2+, Fe3+ and Al3+ to toxic levels; whereas high al- 960–1200 m a.s.l. Lake Manyara is a saline alka- kalinity fixes Fe3+, PHO43+ and Mn2+ in a relatively line rift valley lake with solute concentration lev- insoluble state, and hence unavailable to plants els influenced by the seasonal weather variations. (Larcher, 2003). Extreme levels of salinity are ex- The area is bisected by the Great North Highway. pected in arid and semi-arid areas due to water loss The focal area for this study was on the eastern from the soil through evaporation, which leads to side of Lake Manyara (Fig. 1). an accumulation of crystalline salts on the soil sur- KWC is located in the semi-arid climatic face. Flooding of banks of sodic lakes and shallow zone with mean annual rainfall of 600 mm. The ground water tables may be other factors contrib- annual mean monthly temperature is 27°C with uting to high soil salinity adjacent to such lakes. rainfall from November to mid-May and a long High soil salt concentration interferes with plant dry season from June to October (Pratt & Gwyn- osmosis, resisting nutrients and water absorption ne, 1977). The area is characterised by plain soils from the substrate (Van Langevelde et al., 2003; with light colour, in association with black, poor- Begon et al., 2006). It is nutritive costly to toler- ly drained clay soils. ate such harsh conditions and hence only adapted KWC connects Lake Manyara National Park herbaceous plants tolerate and survive in the men- and Tarangire National Park, forming part of the tioned areas influencing diversity, richness, domi- Tarangire – Manyara ecosystem. The high pH nance and other plant community characteristics. level of Lake Manyara’s water is caused by alka- The aim of this study was to assess the varia- line volcanic bed rocks commonly produced by tion of herbaceous vegetation in relation to gradi- many volcanoes in East Africa (Yanda & Madulu, ents of environmental variables in grasslands ad- 2005). Since human settlement and agriculture jacent to Lake Manyara in Kwakuchinja Wildlife are prohibited close to Lake Manyara and inside Corridor (KWC). Specifically, we aimed to (i) as- Burunge Wildlife Management Area, common sess variation in herbaceous plant species diversi- wildlife such as Phacochoerus africanus ma- ty, species richness and plant community compo- saicus Lönnberg, 1908, Connochaetes taurinus sition, and (ii) determine the best environmental Burchell, 1823, Aepyceros melampus Lichten- predictors of herbaceous species variation. We stein, 1812, and