*3.2.1. Effects of slope gradient*

characterised by comparably broad crests with few localised rock outcrops at summits and/or upper slopes) and well covered with diverse dense vegetation where human did not clear for cultivation. There is a strong correction between plant species distribution, landform characteristics and soil characteristics particularly soil depth and quantity of gravel and/or stoniness

The soils in the plateau are diverse but are congruent with the landforms position on which they occur. The soils found on upper slopes and on their ridges' crests of PTI and PTII are dominantly **Regosols,** and **Lithic Leptosols.** The mid and lower slopes of PTI ridge crests are complexes of **Cutanic Acrisols** and **Cutanic Alisols**. The PTII plateau soils are **Ferralic Cambisols** on the upper slopes and **Cutanic Acrisols** and **Ferralic Cambisols** on the mid slopes [39]. The soils on ridge crests, upper and mid slopes of plateau PTIII are dominantly complexes of **Cutanic Alisols** and **Haplic Regosols**. The dominant soils in the lower slopes of PTIII are **Luvic Ferralic Phaeozems** while the dominant soils of the very narrow valley bot-

The entire plateau is composed of aggregated micro and macro watershed with high potential for soil loss through erosion. The erosion hazard is attributed to the steep slopes; weak soil structure and poor agronomic practices whereby farmers cultivate at very steep slopes of over 45° without conservation measures. The soils of the area had overall poor fertility. One of the macronutrient phosphorus is very low below 4 mgP/kg soil which may affect uptake of others. Also, Ca, Mg and K are low in most soils. Micronutrients Fe and Mn are in very large quantities whereas Cu and Zn are within recommended critical levels. These soils are good for establishment of most vegetation and habitats. However, for food crops, which most small mammals are depending upon as food, the poor soil fertility which is leading to poor crops and in dry years no crops will soon bring in natural selection especially to animal species whereby those which will not be able to scramble for small amount of food will perish and those which will adapt to smaller amount and new food will survival. From residents of the area, there are already several species of gazelle and wild pigs, which are no longer, found in the Usambara because of poor habitats and possibly availability of food. Furthermore, it is important to note that due to the influence of elevation on temperature the plateau is colder than the low plains. There are even variations between valley bottoms, higher ridges and Mountains in the Plateau, and congruent to soil variation, there are vegetation distribution and hence forest dwellers. The explanation agree well with reported by Cottle [24] and research work by Valencia et al. [25] and Baltzer et al. [41] that soil type have a strong influence on spatial distribution of

Escarpment geomorphic unit indicates three levels of uplift, indicating tectonic cycles and it's characterised by steep slopes, canyons, cliffs and rocks with slope gradients of over 72°. There are colluvial foothills, and slope complexes with varied slopes from 3 to 60°. In certain locations, steep slopes over 60° with deep, shallow and rock soils were observed. Escarpment rises from the plain at 600 m a.s.l., to over 2000 m a.s.l. (**Figure 2**). Lower escarpment is characterised by colluvial/alluvial foot slopes, scattered foot ridges and talus slopes. Dominant soils in escarpment are complexes of **Mollic Leptosols**, **Lithic Leptosols, Cutanic Luvisols** and

toms of plateau are **Mollic Fluvisols, Gleyic Fluvisols** and **Antrosols** [40].

or rockiness.

150 Pure and Applied Biogeography

plant species.

Slope gradient a measures of steepness of the landform surface [42], is presented in **Figure 3** in degrees, and it varied with geomorphic units. **Figure 3** shows variability from the plain, escarpment to the dissected plateau. It varies from <1° in the plain to over 64° at the escarpment.

**Figure 3.** Slope gradient (degrees) variation along the landscape of West Usambara Mountains, Tanzania.

Very steep slopes are found in the escarpment and young landscape in the plateau. Gentle slopes are in valley bottoms and in the low plain. Due to the steep slope gradient, it is convincing that slope gradients and their types (**Figure 4**) are the strongest topographic attributes contributing significantly to landform forming processes by influencing speed, pathways of surface and subsurface water movement in the area. This conforms to report by Moore et al. [43] and Hutchinson [44] who indicated slope gradient to be among major factors of ecohydrology that influence overland flows. Similar results [45–47] indicate that slope influences water movement and landform forming processes including landslide in different parts of the globe. For instance, Zhou et al. [48] reported that most of the landslides that occurred in Hong Kong in 1993 took place on slope angles between 25 and 30°. Similarly, Mulders and Alexander [49] reported that areas with slope gradient of 35° and above have high likelihood of shallow landslides, whereas Fernandesa et al. [50] reported that in Brazil landslides are very common in slope angles between 37.1 and 55°.

Therefore, the west Usambara Mountain slopes of over 35°, mainly in escarpment and in plateau terraces PTI and PTII suggest that landslides could been one of the landforming processes in the past and still active given observable landslide scars in the area. The major slope forms are convex with excessive eroding and washing power hence exposing rocks to the surface, and/or concave with deposition manifested with deeper topsoils mainly none rocky or stony.

**Figure 4.** Dominant slope types in West Usambara Mountains, Tanzania.

## *3.2.2. Slope forms*

Very steep slopes are found in the escarpment and young landscape in the plateau. Gentle slopes are in valley bottoms and in the low plain. Due to the steep slope gradient, it is convincing that slope gradients and their types (**Figure 4**) are the strongest topographic attributes contributing significantly to landform forming processes by influencing speed, pathways of surface and subsurface water movement in the area. This conforms to report by Moore et al. [43] and Hutchinson [44] who indicated slope gradient to be among major factors of ecohydrology that influence overland flows. Similar results [45–47] indicate that slope influences water movement and landform forming processes including landslide in different parts of the globe. For instance, Zhou et al. [48] reported that most of the landslides that occurred in Hong Kong in 1993 took place on slope angles between 25 and 30°. Similarly, Mulders and Alexander [49] reported that areas with slope gradient of 35° and above have high likelihood of shallow landslides, whereas Fernandesa et al. [50] reported that in Brazil landslides are

Therefore, the west Usambara Mountain slopes of over 35°, mainly in escarpment and in plateau terraces PTI and PTII suggest that landslides could been one of the landforming processes in the past and still active given observable landslide scars in the area. The major slope forms are convex with excessive eroding and washing power hence exposing rocks to the surface, and/or concave with deposition manifested with deeper topsoils mainly none rocky

very common in slope angles between 37.1 and 55°.

**Figure 4.** Dominant slope types in West Usambara Mountains, Tanzania.

or stony.

152 Pure and Applied Biogeography

Apart from slope gradient, slope forms (**Figure 4**) also vary in the area with convex slope form characterised with severe soil erosion (landslides, mass movement), hence has thin topsoils and in place stony and/or gravely surfaces and poor vegetation cover. Concave slope form are characterised as recipient sites with deep topsoils of over 50 cm in some places. Straight slope form although not apparent in the area, resembles convex in soil forming processes as they highly erodible, mainly washing by water. The complex slopes exhibit diverse properties depending on local area characteristics.

Slopes gradient and slope form have been found to strongly influence vegetation in Usambara Mountains. Dense vegetation is found in concave slopes with relatively deep soils and where moisture collects for longer times. This is in contrast to convex slopes with gravelly and shallows soils where there sparse vegetation and in many incases they harbour woody shrubs. It has been observed from this study that there are associations between vegetation establishment, and animals' species, which comply with studies by Njaka et al. [6] and Meliyo et al. [51]. This is in agreement with a work by Valencia et al. [25] who reported the influence of slope gradient and forms to trees distribution in South America.
