**2.1. Description of the study area**

The West Usambara Mountains are 31 km wide and narrowed to the east 19 km wide and approximately 90 km long. The study area is a rectangle comprising part of adjoining plain, escarpment and part of plateau. The area is selected because it has diverse and unique characteristics that may help to understand how abiotic factors are influencing spatial distribution of plant and animals' species from hot dry plains across the escarpment to the cool plateau (**Figure 1**).

The study lies between latitude 4°30′ and 4°45′S and longitude 38°00′ and 38°45′E. It is located in a cold and warm dry zones of West Usambara Mountains. The plateau section receives annual precipitation of 1200 mm and less than 1000 mm for cold and warm plateau, respectively. The study area extends from the plain across a steeper escarpment both located in a

**Figure 1.** Location of biogeography study area, in West Usambara Mountains, Tanzania.

rain shadow side of the Usambara Mountains (plateau). The plain and escarpment receives annual precipitations of 400 and 800 mm, respectively. The average annual temperatures for plateau cold and warm zones are 14°C and 27°C, respectively, whereas the relative humidity is 70%. Temperatures in the plain range from 25 to 34°C per annum. The plateau grows diverse tropical crops and temperate fruits, while the plain has limited farming activities except for sisal (*Agave sisalana*).

#### **2.2. Geology of West Usambara Mountains**

West Usambara Mountains were formed by block faulting and repeated uplifting of Precambrian basement rocks between 180 and 290 million years ago [27, 28]. The geologic composition and variability in West Usambara Mountains depicts differences in terms of geology with regards to geomorphic position. The **plain** is mainly composed of duricrust calcareous yellow grey sand, whereas the geological rocks in the **escarpment** are gneisses mixed with undifferentiated granulites and distinctive bands of hornblende and pyroxenes. The geology in the **plateau** is mainly gneisses with leucocratic quartzo-feldspathic granulites and khondalites. There are few areas composed of recent alluvial materials like depressions and valley bottoms which are composed of mixed alluvial-fluvial materials [29].

#### **2.3. Determination of landform and soil characteristics**

Visual observation of satellite images, stereoscopic aerial photographs and orthophoto maps (numbers 9480410, 9480400, 9470410, and 9470400) was done. Also, visual analysis of topographic and geologic maps was done. The interpretation base-map was digitised to produce the georeferenced base-map used to guide field. Transect were made for augering for mapping soils and landforms by the methods by Dent and Young [30]. At each observation site, data on landform and soil morphological characteristics were examined and recorded. Landform units similar in parent material, relief, topography and soil morphological characteristics were considered to be similar and were accorded as mapping unit. Vegetation habitats and associated characteristics were determined and mapped. Identification of vegetation was according to the FAO Guidelines [31] in grids of 20 m × 20 m. Each observation site was geo-referenced by Global Positioning System (GPS). Representative soil profiles were dug in major soils, where description was done, and then soil samples were collected from natural horizons for laboratory analysis.

Also, landform analysis was done using ASTER Digital Elevation Model (DEM) which was carried out using ArcGIS 9.3 to derive continuous surfaces for elevation (m a.s.l.) (slope gradient (degrees), slope aspect (radians), slope length (m), and slope types (straight, convexity and concavity) and different types of land surface curvatures.
