Irangi Hills Research Articles

Changing land use/cover patterns and implications for sustainable environmental management in the Irangi Hills, central Tanzania

This article examines the changes in land-use/cover types in the Irangi Hills, central Tanzania during the last 45 years and how such changes have influenced environmental and agricultural sustainability in the area. The spatial and temporal changes of land-use/cover were analysed through aerial photographs interpretation. Local perceptions and experiences of changes were addressed through household interviews and field observations. Results from this study show that during the last 45 years open and wooded grasslands, and other tree-cover types covered about 40% of the land area, ranging from 29% in 1960 to 43–45% between 1977 and 1992. Also, during the same period both the total area and spatial distribution of cultivated fields varied greatly. The cultivated area increased from 31% in 1977 to 35% in 1992, mainly due to agricultural expansion into areas formerly used for grazing and in sandy watercourses that shrunk by 55% between 1977 and 1992. The spatial distribution of the different land-use/cover types is influenced by variations in the scale of soil erosion and soil-conservation initiatives implemented in the Irangi Hills since the early 1970s. However, with increasing pressure on the land, and the declining capacity of the soil conservation authority, sustaining agricultural production in the area remains a major challenge.

Seed longevity of dominant plant species from degraded savanna in semi-arid Tanzania

Artificially buried seeds were followed by periodic exhumation and germination during 21 months in Kondoa Irangi Hills, central Tanzania. Viable seeds from thirteen selected species were buried in polythene envelopes at a depth of 15 cm below woodland vegetation. Lethal germination, fungal decay and insect infestation were singled out as the most important contributors to loss of viability among seeds. Hierarchical clustering based on a seed’s mean viability, seed shape and presence or absence of dispersal appendages, produced three distinct groups: (1) non-dormant seeds (2) seeds with enforced dormancy and (3) seeds with seed coat imposed dormancy. The low decay constant of some species is an indication of their abilities to form persistent seed banks. The intermittent and extended germination of seeds of the same species from the same batch shown by some species may be regarded as an ecological adaptation to prevent synchronous germination in unpredictable harsh environments, whereas prompt germination of some Acacia seeds may be viewed as a strategy to avoid seed predation in the soil. Seed dormancy, which predicts seed longevity, is strongly dependent on seed moisture content and partly on other environmental factors, notably low temperatures, light and increased carbon dioxide levels.

Land use/cover changes and their implications on rural livelihoods in the degraded environments of central Tanzania

AbstractThis paper examines the changes in land use/cover types in the degraded environment of central Tanzania over the last 45 years, and how such changes have influenced agricultural and livelihoods sustainability, especially in the Irangi Hills. Changes of land use/cover were measured through aerial photographs interpretations, while local perceptions and description of change were addressed through household interviews and field observations. The results of this study show that there have been variations over the years in terms of both the areas and spatial distribution of cultivated fields. The total land cultivated increased from 31% in the late 1970s to 35% in early 1990s, mainly because of agricultural expansion to sandy watercourses and former grazing areas. Water courses shrank by 55% between late 1970s and early 1990s providing new areas for cultivation. Over the last 45 years, the open and wooded grasslands, and tree‐cover types covered about 40% of the total land area, ranging from 29% in 1960 to 43–45% between late 1970s and early 1990s. Spatial and temporal distribution of the cultivated fields and other vegetation cover types were influenced by differences in the scale of land degradation, and the soil‐conservation initiatives that have been implemented. With increasing pressure on the land, however, sustaining livelihoods through agricultural production in the area remains a critical challenge.

Changing land-use patterns and farming strategies in the degraded environment of the Irangi Hills, central Tanzania

The approaches adopted by local farmers to put the degraded landscape of the Irangi Hills in central Tanzania to productive agricultural use are analysed. The area has been extensively affected by severe soil erosion, thereby reducing its potential for agriculture. While soil erosion in the upper and middle reaches of the slopes has resulted in extensive gullies, sedimentation in the lower reaches has created extensive sandfans and buried some of the fertile, clayey soils. The changes in the present land-use practices were assessed by means of group discussions and transect walks, household interviews, field observations, and by archival research. The results of the study indicate that farmers have responded to the evolving land degradation by using more intensive and more productive forms of land-use. Crops are grown in diverse mixtures, aiming at increasing farm productivity and avoiding the risk of crop failures. Many farmers have also responded to land degradation by employing on-farm conservation practices such as ridge cultivation and tree-planting. There has also been a general shift from cultivation and settlement on hillslopes to less steep, middle and lower pediments and footslope areas. Land-use patterns have constantly changed over the last few decades. One major intervention to try to rehabilitate the worst degraded areas came in 1979, when all livestock were evicted. The quarantine still remains, but since the early 1990s free-grazing livestock have gradually, but illegally, been brought back into the area. Although the return of livestock has increased the availability of manure, it is likely to reverse the trends of ongoing land recovery. To complement the limited availability of animal manure more than 85% of farmers make and use compost to fertilise fields close to homesteads. Evidence is also presented to demonstrate that farmers have been quick to grasp whatever chances they had to make use of new land-use opportunities. When some of the sandfans in the area stabilised, following soil-conservation initiatives, farmers immediately realised that there were new niches in the landscape that could be utilised. Today the total cultivated area has increased considerably, compared to the early 1970s, when intensive conservation efforts were begun.

Changes in the fluvial system of the Kondoa Irangi Hills, central Tanzania, since 1960

AbstractUsing evidence from aerial photographs, supported by field checks, changes in the fluvial systems of three catchments in the Kondoa Irangi Hills, Kondoa District, central Tanzania were mapped. This area is known for its severely eroded landscape and, today, also for the drastic measures introduced to deal with the soil erosion problem. In the early stages these included mechanical construction of contour bunds, but later emphasis was placed on tree planting and planting of elephant grass on sand fans and dry, sandy riverbeds. Restrictions were introduced on clearing land for cultivation and on felling of trees for construction material and fuel wood. The most dramatic conservation measure was the eviction, in 1979, of all livestock from 19 villages to halt the severe overgrazing.Since the different conservation measures have now been in effect for more than 20 years, their impact can be assessed. The sand rivers, conspicuous features of the study area, have in many places decreased in width. Their total surface area in the three catchments decreased by about two‐thirds between 1960 and 1987. Previously unvegetated sand fans have been converted to crop production. Natural vegetation is now establishing itself on formerly barren areas. However, the badlands, which cover some 25% of the study area and which take a very long time to recover, still seem to be relatively unaffected by the conservation efforts, although they decreased in extent by about 10% between 1960 and 1987. In the present report, the biophysical landscape changes and their implications for the drainage system in parts of the Kondoa Irangi Hills are discussed. Copyright © 2003 John Wiley & Sons, Ltd.

Sustaining water resource use in the degraded environment of the Irangi Hills, central Tanzania

The Irangi Hills in the semiarid central Tanzania are known for their severe land degradation, particularly through soil erosion. This study aims at investigating the dynamics in the local resource management and adaptive farming strategies. A combination of conventional survey methods, participatory analysis, soil description and aerial photo interpretations were used in the analysis of spatial and temporal land-use patterns. Results from the study showed that farmers utilised all possible niches in the landscape with the agricultural practices varying considerably, depending on spatial variations in soil conditions, water availability and retention capacity of the soil. The hard clayey soils were often ploughed at the end of the rainy season, before the soil dried out, to facilitate timeliness of planting in the following growing season, and to enhance water infiltration at the onset of the rainy season. Seasonally waterlogged soils were intensively cultivated and planted late during the growing season, with crops maturing only from groundwater resource. To make effective use of available groundwater resources during the dry season, the droughty and often infertile sands in ephemeral watercourses are used for growing vegetables or sugarcane in sunken holes without additional fertilisers/manure. This suggests that the crops grown obtain sufficient nutrients only from groundwater resources. However, the sustainability of cultivation in these ephemeral waterways is still questionable.

Soil erosion, iron smelting and human settlement in the Haubi Basin, north-central Tanzania

The Haubi Basin, situated in the Irangi Hills of Kondoa District, Dodoma Region, Tanzania (FIGURE 1), exhibits some of the most extreme examples of erosion and associated sedimentation in the region (Christiansson 1981; Payton et al. 1992) (FIGLIRE 2). The severity of the problem, with its consequent loss of productive land, has been known since the beginning of the 20th century (Kannenberg 1900; Obst 1915). Recognition of the extent and severity of the issues encouraged the British colonial government to introduce a variety of measures aimed at soil conservation in the 1930s (Fosbrooke 1950). These included the construction of check-dams, contour bunds and contour planting with sisal. After independence, a second programme of soil conservation measures was launched. These were initially similar to those employed under colonialism, but later included de-stocking, resulting in the removal of some 90,000 livestock from the most severely eroded areas. As in the colonial period, recent overexploitation of land resources by local populations was regarded as the primary explanation for the continuing soil erosion.

Soil erosion history in central Tanzania based on OSL dating of colluvial and alluvial hillslope deposits

The Irangi Hills in Kondoa District, central Tanzania, are severely degraded by sheet, rill and gully erosion. Using recently developed optically stimulated luminescence (OSL) dating techniques, and a detailed study of the hillslope stratigraphy and soils, we have determined the sequence of events that gave rise to this highly degraded landscape. Two major colluvial deposits have been identified on the slopes. The oldest colluvium gave OSL deposition dates of 14,700±1600, 14,200±1500 and 11,400±1300 years ago. These dates coincide with the climatic change from dry to wet conditions, which took place during the Late Pleistocene. It is possible that the erosion and deposition of the old colluvium occurred as a response to this change. This phase of erosion and deposition was followed by a long phase of stability and pedogenetic alteration of the old colluvium. A phase of accelerated soil erosion began not later than 900 years ago, as indicated by a dated alluvial fan, the presence of which indicates that some deeper gullies already existed on upper pediment slopes at that time. The second, more recent colluvial deposit gave OSL dates of 460±40, 590±70 and 660±50 years. A major period of gully formation and incision, with subsequent fan development, occurred sometime between 600 and 300 years ago. The recent phase of erosion (<1000 years), which is still continuing, is probably a result of the introduction and/or intensification of agriculture, livestock husbandry and iron smelting practices in the Irangi Hills.

Floristic, structural and seed bank diversity of a dry Afromontane forest at Mafai, central Tanzania

Flora, vegetation and seed bank were studied in a dry Afromontane forest in the Kondoa Irangi Hills, Tanzania with the objective to obtain information necessary for the promotion of conservation of this forest, which is rapidly degenerating due to human pressure. A preliminary checklist of 104 vascular plants occurring in the forest is provided. Based on 27 plots, each of 400 m2, the forest had the following ecological features: mean tree density 408 stems · ha−1; basal area 66.56 m2 · ha−1; Shannon and Wiener diversity 2.6406; evenness 0.7585 and species richness 31 tree species per 1.08 ha. The seed bank was dominated by woody species, including mature forest tree species, and had several species in common with the standing vegetation. The forest contains species that are phytogeographically linked to Madagascan, Western and Southern African floras and also species disjunctly distributed in this forest and the Eastern Arc Mountains of Tanzania. Correspondence Analysis of tree species based on size classes showed that the timber species had declining populations, indicating the possibility of genetic erosion, while other species showed expanding or interrupted populations. Some DBH-size classes (e.g. 145.0–189.9 cm) have disappeared from the forest, presumably due to their selective removal. We recommend detailed integrated floristic and faunistic studies of the forest, targeting the ecologically sensitive indicators of habitat change such as orchids, birds, reptiles and amphibians. Finally, a proposal to conserve the forest under a community-based forest conservation scheme is put forward.

Late Pleistocene colluvial deposits in central Tanzania; erosional response to climatic change?

Two major colluvial deposits have been identified on the slopes of the Irangi Hills in north central Tanzania. The oldest of these occurs in shallow depressions on the pediment slopes. The time of deposition has been dated by the use of optical stimulated luminescence (OSL) dating techniques to Late Pleistocene. The OSL dates were 14,700 ± 1600, 14,200 ± 1500, and 11,400 ± 1300 years. In Late Pleistocene time the climate changed from dry to wet conditions throughout much of the tropics and this time period is frequently reported as a period characterised by enhanced soil erosion. It is likely that the older colluvial unit in the Irangi Hills was formed as a result of climatic change when rainfall increased ahead of a vegetation change and thereby provided conditions favourable for soil erosion. The younger colluvial unit yielded deposition ages of 400–600 years and is part of a number of successive erosion and deposition periods during the last millenium, probably as a result of anthropogenic impact on the landscape.

Soil seed bank and regeneration potential on eroded hill slopes in the Kondoa Irangi Hills, central Tanzania

Abstract. The soil seed bank of the severely eroded Kondoa Irangi Hills, Tanzania was studied in order to determine the seed density and composition and to establish the relationship between seed bank and standing vegetation. The area had not been grazed for 15 yr prior to the study. A recently grazed area was used for comparison. The seed bank density (at 0 ‐ 5 cm depth) ranged from 344 to 915 8 seeds/m2 in the dry season and 172 to 5107 seeds/m2 in the wet season. The seed bank was very heterogeneous, both spatially and temporally, and it showed significant variation in size and composition in both sampling periods. The species similarity between the seed bank and the above‐ground vegetation in all plots was low (Sørensen's index = 0.00–0.44). The highest similarity was found in the recently grazed area. The seed bank was dominated by annuals and by early successional species. It is concluded that re‐vegetating the hill slopes with woody vegetation by using the seed bank will be difficult because seeds of woody species were not found in the soil.

Influence of crustal movements on landforms, erosion and sediment deposition in the Irangi Hills, central Tanzania

Abstract The influence of crustal movements on landforms, erosion and sediment deposition has been analysed in the severely eroded Irangi Hills in Kondoa District, central Tanzania. Field observations, in combination with satellite imagery, aerial photographs, and topographical and geological maps have been used. The Irangi Hills, which are uplifted relative to the vast Maasai Plain, consist of several tectonic blocks, with each block being tilted to the west. Frequent earthquakes indicate that relative uplift, faulting and tilting of the Irangi Hills are still taking place. The drainage pattern of the area and the deposition of river sediments are directly influenced by the morphotectonics. Where fault scarps occur, large rivers have cut into the bedrock across the fault scarps and follow antecedent stream courses, while small streams have been dammed, forming swamps or lakes. The denudation rate for a small catchment (33.4km 2 ) has been calculated to 2.7mm a −1 for the years 1836–1992 by using a dated sediment record. This is considered to be an accelerated denudation rate. The soil erosion of the Irangi Hills is a long-term natural stripping of regolith, accelerated by anthropogenic causes, probably in combination with crustal uplift and the tilting of tectonic blocks.

Accelerated soil erosion in Central Tanzania during the last few hundred years

The Irangi Hills of Kondoa District in central Tanzania are severely affected by soil erosion. Exceptionally large gullies dissect the slopes and in many locations badlands have developed. Through analysis of a 3.6 m sediment record from the small Lake Haubi, reconstruction of the erosion and sedimentation sequences are attempted. A sediment chronology has been established based on analysis of 210Pb and 137Cs, which dates the record to the beginning of the 19th century. The formation of the present lake took place around the turn of the century and was possibly initiated by accumulation of erosion products at the outlet of the basin. The first phase of gully incision must have taken place before the 19th century. It is not known what initiated and triggered the processes, although it is likely that clearing of land for cultivation, grazing and charcoal burning in precolonial times have played a decisive role.

Landform, Soils and Erosion in the North-Eastern Irangi Hills, Kondoa, Tanzania

Landform, Soils and Erosion in the North-Eastern Irangi Hills, Kondoa, Tanzania

Landform, Soils and Erosion in the North-Eastern Irangi Hills, Kondoa, Tanzania

Landform, Soils and Erosion in the North-Eastern Irangi Hills, Kondoa, Tanzania