|dc.description.abstract||Nebkha dunes, formed by the trapping of sand around the body of a plant, provide habitats for small fauna and are thus, important in the conservation of biodiversity in desert ecosystems. Many authors have reported results on investigating of nebkha dunes in the Kalahari, Sahara and Namib deserts of Africa. In contrast to this, scant information exists on the nebkha dunes of the Chalbi semi-desert region of Northern Kenya. Besides, land degradation in arid rangelands resulting from sand encroachment has to be evaluated within the framework of climate-vegetation-livestock interactions. However, no study has particularly focused on the dual role of vegetation for economic (livestock-production) and ecological functioning in nebkha dune stabilisation in Kenya. A study was therefore, designed to investigate theoretical and empirical issues related to sand encroachment and nebkha-dune stabilisation in North Horr, Northern Kenya.
The study focused on soil and vegetation degradation as the main biophysical indicators of land degradation in North Horr. Satellite images taken at four points in time (1976, 1986, 1996 and 1998) were evaluated and complemented with field vegetation surveys. This enabled the assessment of the origin, history and extent of the process of sand accumulation in North Horr. Growth performance of different indigenous and exotic tree and shrub species was also evaluated in terms of height, relative growth rates and mortality. These were collectively used in assessing the suitability of exotic and indigenous trees and shrubs in dune stabilisation. Soil chemical and physical analyses were carried out and related to sand encroachment. Wind erosivity indices that give an indication of potential wind erosion hazard and sand movement potential were derived from wind speed data collected between July 1999 and September 2000.
The Gabra camel production system involves involuntary culling of the mature breeding females at 300 months while both fattening male and female camels are culled at 58 months. These parameters were incorporated into a range utilization model and a bio-economic model-PRY for the assessment of ecological and economic trade-offs in the utilization of Suaeda monoica for dune stabilisation and camel browsing. The results of the simulation modelling were explored and linked to the problem of sand encroachment.
Sand dunes in North Horr were predominantly of the nebkha type and were preferentially aligned with the predominant wind direction during the dry season. The morphonetry, orientation and textural examinations of the nebkhas suggested that fluvially deposited sand in North Horr, had in the recent years been reshaped by aeolian activity. Empirical evidence showed a positive correlation between wind speed and sand movement potential, which varied between month and time of the day. Average monthly wind speed of 3.99±0.06 m/s, with absolute maximum wind speeds of 10 m/s, implied that wind erosivity and sand-movement potential in North Horr were high, indicating increased soil degradation.
Nebkhas showed heterogeneity in their morphomettry attributed to differences in plant species cover. For instance, in terms of length, there was a decreasing order of Hyphaene coriacea (16.36±3.23m), Acacia tortilis (5.10±0.20 m) and Suaeda monoica (3.20±0.68 m). Large nebkhas were, therefore, found in the southern and western parts of North Horr settlement dominated by Hyphaene coriacea (doun palm) and Acacia tortilis. Hyphaene coriacea is, therefore, the most important tree trapping large volumes of sand. However, due to the less compact (diffuse) growth form of Hyphaene coriacea, the trapped sand is loose and therefore, highly mobile when compared with that of Suaeda monoica with a more compact growth form and a denser canopy.
Percent soil organic carbon was used to evaluate fertility status. The decreasing order of fertility beneath the different trees/shrubs were: Acacia tortilis (0.24%). Hyphaene coriacea (0.21%), Suaeda monoica (0.14%), Prosopis chilensis (0.12%), Acacia goetzei (0.11%) and bare ground (control) (0.09%). Although there were no significant differences (P<0.05) in soil organic carbon between vegetation types, it nevertheless demonstrated the differential abilities of plant species for land rehabilitation in North Horr. In additional, nebkhas could probably represented a large and significant storage of nutrients within the dryland system of North Horr.
Multi-temporal analysis of statelite imageries (1976, 1986, 1996 and 1998) showed that the area (in ha) covered by bushland, degraded bushed grassland, degraded grassland, denuded bushland and open bushland decreased significantly (p<0.05) by 11.38%, 0.19%, 3.22%, 0.39% and 7.54% respectively. Over the same period, the area (in ha) covered by grassland and exposed land covered with sand, gullies and rocks increased significantly (p<0.05) by 15.25% and 8.51% respectively. This indicated progressive increase in area covered with sand. The greatest increased in the are covered with sand was recorded between 1976 and 1986 (7.08%) and the lowest between 1996 and 1998 (3.34%). There were corresponding dynamics in coefficient of variation (CV) in annual rainfall betwen 1976 and 1998 with the highest and lowest variability being recorded for the period 1976-1986 (56.42%) and 1996-1998 (33.64%) respectively. Correlations between land cover and rainfall were significant (p<0.05). Empirical evidence presented in this study suggested that exogenous stochastic factors (climatic) were primarily responsible for land cover dynamics and the concomitant localized degradation in North Horr.
Simulation modeling with PRY showed that camel population in Stuaeda monica vegetation complex was not viable. Therefore, stochastic forces could also be responsible for the imbalance in supply and demand for Suaeda monoica forage by camels. However, with appropriate culling policy, a sustainable balance in the economic and ecological functions of Suaeda monoica could be achieved. It was concluded that participatory nebkha-dune stabilisation would only be possible if economic aspects of the Gabra pastoralists were taken into consideration. These findings have important implications for policy in the sustainable management of natural resources in arid rangelands for both economic and ecological functions.
To rehabilitate saline soils and stabilise sand dunes in North Horr, emphasis should be placed on the used of indigenous tree and shrub species. Although fencing and enclosures encourage natural regeneration of vegetation, the viability of fencing for large-scale land rehabilitation in a nomadic pastoral system is questionable. Natural vegetation regeneration could, therefore, be the most cost effective, fastest and easiest method for stabilizing dunes in North Horr. The management of important dune stabilizing plants such as Suaeda monoica, an important camel forage, through natural regeneration is, therefore, significant in ensuring that it performs both the economic and ecological functions. This option is compatible with the pastoral land use system and appears to be the most viable for nebkha-dune stabilization in the eastern part of North Horr settlement||en_US