Sokoine University of Agriculture

Influence of terrain, soil and water inter-relationships on the distribution of plant communities in Jozani Groundwater Forest, Zanzibar, Tanzania

Show simple item record Said, M. S. 2018-05-09T07:45:03Z 2018-05-09T07:45:03Z 2018
dc.description.abstract On a coastline, tidal water and air constantly reach land surface and/or aquifer. Tropical forests before or adjacent to this line are usually having a wide range of habitats. Jozani Groundwater Forest (JGWF) is one of the tropical coastal forests with a wide range of floral and faunal habitats. The forest (JGWF) has a significant role in biodiversity conservation and ecotourism. International bodies including UNESCO recognize JGWF as the most important part of Jozani-Chwaka Bay Bio-Sphere Reserve (formerly Jozani-Chwaka Bay National Park). The Zanzibar Wood Biomass Survey found that JGWF area has a biomass of above 50 tonnes ha-1 compared to the surrounding areas having a biomass of about 10 tonnes ha-1. Plant species found within JGWF area exceed 300 counts and vary in their assemblages. Thus, there was a need of ditermining the underlying conditions that made JGWF to be so rich in biomass and biodiversity. Plant growth is favoured by soil and water characteristics, so those characteristics favoured JGWF area over its surroundings were to be revealed. Again, it is known that plant species existence, richness, assemblage and spatial distribution are influenced by biotic and abiotic factors. So, it was important to determine environmental factors influencing plant species assemblage and distribution in JGWF. The aim of the current study therefore, was to determine the underlying soil and water inter-relationships influencing plant species communities’ distribution in Jozani Groundwater Forest area. Specifically, this study aimed to i) determine the elevation of Jozani Groundwater Forest (JGWF), ii) characterize soil depth to coral bedrock and bedrock roughness in JGWF, iii) determine seawater tidal trends and magnitude at Uzi and Chwaka bays as a proxy of the sea water intrusion into JGWF, iv) determine the extent of seawater intrusion, v) characterize the effects and implication of rainwater - seawater interaction on physical and chemical soil properties in JGWF and vi) map-out distribution of plant species assemblages in relation to abiotic environmental variables including elevation, salinity, soil and water characteristics. The study was conducted in Jozani Groundwater Forest area of about 590 km2 located between Chwaka and Uzi bays within the Jozani-Chwaka Biosphere Reserve (former Jozani-Chwaka Bay National Park), Zanzibar, Tanzania. Generally, seawater intrusion occurs along low elevated coastal lands and/or aquifer. As JGWF is a low elevated coastal forest, it is likely being intruded by seawater from Chwaka and Uzi bays. Hence, it was hypothesized that if there was seawater intrusion into JGWF land and aquifer, such intrusion has direct relation to tidal characteristics on the two bays. Masika (long rain season) and Vuli (short rain season) are a good source of freshwater into JGWF aquifer. The aquifer continuously interacting with seawater and to some extent reduces the effects of intrusion. The water interaction and its spatial trends is therefore influenced by tidal trends and wet (Masika and Vuli) and dry (Kiangazi and Kipupwe) seasons. Seawater intrusion processes are also affected by land elevation, soil type and vegetation conditions. Effects caused and/or brought by abiotic factors such as terrain, soil and water conditions significantly affect forest plant assemblage and distribution. Plant communities’ assemblage and spatial distribution often imply the species response to the abiotic factors. This study therefore, intended to determine terrain, soil and water characterictics of JGWF, examine tidal trends and magnitude as proxy of intrusion into JGWF, reveal the extent of the intrusion, and how far these conditions influenced plant communities’ distribution in JGWF. With the help Height of Instrument/Collimation method of reduction in levelling survey, the reduced levels (RLs) of the north-end and south-end of JGWF were identified. Two benchmarks (no. 205 and 210) of the Zanzibar Department of Survey (DOS) were used for RLs determination at and inbetween the north and south ends of JGWF. During the survey, the level of water table in local (shallow) wells was measured. Water table was used as a wide benchmark to produce Digital Elevation Model (DEM). Thereafter, with the help Height of Instrument/Collimation method, benchmark no. 205 was used to validate elevation of ten points in the southern part of JGWF. ArcGIS 10.1 was used to produce DEM, soil depth to the coral bedrock (SDCB) and bedrock roughness (BR) maps for JGWF. Data were collected from soil auger boreholes in a 320 grid system made from 32 northern and 10 eastern gridlines.The data were depths of water table and coral bedrock from the ground surface. To determine tidal trends and magnitude (elevation of reach) of seawater from Chwaka and Uzi bays into JGWF, two sets of three observation wells (OW) were drilled at the north and south ends of JGWF. The OWs were the data collection points. At OW, a water level change recorder was installed from which levels of water table (above mean sea level (AMSL)) were measured during low water of spring tides. Water from OWs was tested in situ to determine salinity level (total dissolved solids (TDS)) round OWs. The data of water and salinity levels were computed and used to describe the seawater tidal trends and magnitude of seawater intrusion into JGWF. Data on water salinity to determine the extent of seawater intrusion, soil characteristics and plant species composition were collected from second grid system. The second grid system had a total of 44 gridpoints made from 11 northern and 4 eastern gridlines. Data on soil characteristics were collected from soil auger probing. Soil profile description and sampling were done in two profile pits based on auger probing results. From auger bore holes (temporary wells) water salinity was tested in the middle of Kiangazi (hot, dry), Masika (long rainy), Kipupwe (cold, shower) and Vuli (short rainy) seasons, respectively arranged from January to December. For vegetation characterization, plant species with diameter at breast height (DBH) ≤ 5 cm were measured in a quadrat of 4 m2, > 5 but < 10 cm in quadrat of 100 m2, > 10 but < 25 cm in quadrat of 400 m2, and > 25 cm in quadrat of 900 m2 respectively. This study found that the elevation of JGWF ranges from 0.7 to 2.5 m AMSL. Level of water table in North and South fluctuated between 0.586 and 1.206 m AMSL and between 0.820 and 1.586 m AMSL, respectively, during dry and rainy seasons and during high and low water of spring tides. Soil depth to coral bedrock and bedrock roughness varied from 0.3 to >1.4 and 0.1 to > 0.4 m, respectively. The levels of TDS in water ranged between 0. 5 and 2 x 103 mg L-1 in central part of JGWF and exceed 14 x 103 mg L-1 in areas close to Chwaka and Uzi bays. The soil was covered by 0.10 to 0.23 m thick layer of moist decomposing plant residues. Beneath this layer, found a humic horizon of about 0 – 0.12 m which consisted of moist, slightly compact, dark well decomposed material mixed with topmost soil mass of greyish brown (10 YR 3/2) colour. Soil horizons beneath the humic horizon comprised a brown (10 YR 4/3), slightly compact horizon of about 0.1 to 0.15 m overlying a thick water saturated olive gray (5 YR 4/2) clay-paste like loose horizon resting on an unevenly undulating coral bedrock. The thickness of all the horizons increased as the probing points approached south-eastern of JGWF. The soil had a considerably high percentage of available moisture (AM) that increased with depth and ranged from 28 to 36%. The soil had low bulk density (BD) which increased with depth from 0.23 - 0.54 g cm-3. The values of AM and BD in JGWF were mainly contributed by a thick layer of plant residues and paste-like clay material. More than 69 plant species were found in probed areas. Five plant communities were identified from Redundancy Analysis (RDA) analysis and more than 3 specific communities were observed as patches during survey. About 60% of the plant species community assemblages consisted of about 19 ± 5 plant species of 15 ± 3 Genera. However, the community assamblages were in complex distribution making less distinct and smoothly diffusing ecotones in between. A clear ecotone of about 40 m wide dominated by Acrostichum aureum (Mangrove fern) stands was inbetween the area of pure Paspalum vaginatum (Seashore couch) stands (with TDS > 20 x 103 mg L-1) and the area of mixed plant species dominated by Nephrolepis biserrata (Tropical fern), Terminalia boivinii (Terminalia) and Guettarda speciosa (Beach gardenia) (with TDS of about 15 x 103 mg L-1). Based on the findings, this study concludes that: i) in multi-storey high canopy groundwater forest, the method of using water table for levelling is feasible, applicable and an alternative method, ii) Jozani Groundwater Forest (JGWF) has shallow water table that seasonally fluctuates and slightly sloping from the south towards the north, iii) JGWF area has elevation ranges between 0.75 and 2.5 m above the mean sea level (AMSL), iv) Chwaka and Uzi bays are the only sources of seawater and rainwater apparently is the only source of freshwater which dilutes and partially drains out seawater that intruded into JGWF, v) seawater intrusion caused by surface movement through Chwaka and Uzi creeks takes place frequently and on large area at and beyond North-end towards JGWF during high water of spring tides, vi) the values of TDS fell to a minimum range of 0.7 x 103 to 4.9 x 103 mg L-1 during the rainy seasons and rose to a maximum range of 25.5 x 103 to 34.1 x 103 mg L-1 during dry seasons at South-end and North-end, respectively, vii) high densities of mangroves and Paspalum vaginatum (Seashore couch) stands pulled down seawater elevation of reach into JGWF soil surface by 0.9 and 0.5 m for Chwaka bay and Uzi bay respectively, viii) seawater abrasion forces acted on JGWF area resulted into the formation of Jozani trough like terrain, ix) so long as Masika and Vuli are the only sources of freshwater, seasonal trend of the rise and fall of water table in JGWF remains constant and the northern part of JGWF shall be more affected by seawater intrusion than the southern part, x) water salinity levels in about 70% of JGWF area favour most of the plant species found in the JGWF xi) water salinity fluctuations will remain constant if JGWF biomass conditions remain intact, no water pumping off JGWF aquifer, no extremely wet or extremely dry conditions, xii) soils of JGWF are peat soils overlying gluey clays which were formed under partially anaerobic conditions and were classified as Limnic Histosols (Gleyic) (FAO WRB) and Haplosaprists (USDA Taxonomy) and rainwater and marine water appeared to be involved in the formation of the soil, xiii) plant species in JGWF enjoy soil wetness throughout the year and their spatial distribution is influenced much by salinity. This study recommending the following: i) levelling with the use of water table method, ii) reforestation with mangroves to reduce surface seawater intrusion, iii) proper utilize of information from this study and iv) further studies on spatial environmental factors affecting JGWF plant species. en_US
dc.language.iso en en_US
dc.publisher Sokoine University of Agriculture en_US
dc.subject Terrain influence en_US
dc.subject SoiL inter-relationships en_US
dc.subject Plant communities en_US
dc.subject Jozani Groundwater Forest en_US
dc.subject Zanzibar en_US
dc.subject Tanzania en_US
dc.subject Soil profile description en_US
dc.subject Temporary wells en_US
dc.title Influence of terrain, soil and water inter-relationships on the distribution of plant communities in Jozani Groundwater Forest, Zanzibar, Tanzania en_US
dc.type Thesis en_US

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