Relationship between tree species abundance, slope and soil properties in the Udzungwa scarp forest reserve, Tanzania.

Relationships between tree species abundance, slope and soil properties were studied in the Udzungwa Scarp Forest Reserve. Vegetation data and soil samples were collected from plots established in three landscape positions (ridge tops, valley sides and valley bottoms) from October 1996 to February 1997. Vegetation data were used to estimate the tree species abundance (stem density and basal area) and soil samples were analyzed for pH, water content, texture, bulk density, total nitrogen, organic matter, available soil phosphorus, exchangeable bases and cation exchange capacity. Single factor analysis of variance (ANOVA) was used to test the differences in the stem density, basal area, tree height, slope and soil properties between the three landscape positions. There were statistically significant differences in terms of stem density anal basal area between the three landscape positions but no significant differences in terms of most of the soil properties in the three landscape positions. However, some soil property values appeared to be higher in the top-soils than the sub-soils. Such properties include soil organic matter, water content and available soil phosphorus which are possibly caused by the accumulation and decay of litter and roots within the top-sails. Shannon's Diversity Index indicated the highest species diversity in the ridge tops, as with basal area and stem density which were also high. Low diversity of tree species > 10 cm dbh) in the valley bottoms was associated with disturbance and with the soil properties of the site. Thus, topographic position is the major factor which influences the tree species abundance and diversity. Multiple regression analysis was used to test the relationships between stem density, basal area, tree height, slope and soil properties in each landscape position. Multiple regression showered that there were no significant relationships between tree species abundance (stem density and basal area) and most of the soil properties in each landscape position.