Abstract:
Mountain forests play major ecological and environmental roles. This study
evaluated the roles of the Eastern Arc Mountain forests of Tanzania in conservation of
biodiversity, water resources, and net contribution to atmospheric carbon. The major
objectives were three-fold: (1) the classification and description of plant community
composition, diversity patterns, and their environmental correlates, (2) assessment of
biomass and carbon pool in the phytomass and soils, (3) assessment of rainfall interception
throughfall, stemflow, streamflow, and their correlation with rainfall.
Data on vegetation, topography, soils, and hydrology were collected from the
Usambara and Uluguru ranges. Using cluster analyses, Non-metric Multidimensional
Scaling (NMS) ordination, and indicator species analyses, five different plant communities
were identified on each range. These communities were associated with two topographic
and fourteen edaphic factors. Elevation was the strongest correlate of community
composition in individual ranges, followed by several edaphic factors. Landform index and
soil sodium concentration [Na] play major roles in separating plant communities between
different mountain ranges. The proportions of rare species were high raising a conservation
concern as to whether this is evidence of species’ declines or a biological characteristic. The
forests have tremendous capacity for C storage both in the phytomass (517 ± 17 t ha’1 in the
Usambaras and 384 ± 10 t ha’1 in the Ulugurus), and in the soil (420 ± 100 t ha’1 in the
Usambaras and 290 ± 53 t ha’1 in the Ulugurus). Phytomass C was higher in mid elevation
communities while high elevation communities had higher soil carbon, and total carbon.
Rainfall interception was 23% in the Usambaras and 20% in the Ulugurus. Throughfall was
more than 76% in both forests and stemflow was less than 2%. Streamflow was best
modeled using three or more months running mean rainfall. The results suggest that plant
community patterns in the Eastern Arc are associated with a complex of topographic and
edaphic factors. This complex of factors is an important consideration in restoration and
conservation programs. Attention to rare species is especially important. The forests have substantial capacity for carbon emission mitigation. The slow response in streamflow to
rainfall events shows the efficiency of the forests to store water, mitigate storm water
impacts by reducing runoff, delaying onset of peak flows, and ensuring constant water
supplies.
