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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. |
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