摘要:

AbstractA degraded dwarf shrub, annual grass semidesert vegetation type in Turkana, northwestern Kenya, was protected from livestock grazing from October 1985 to December 1990. The effects of controlled grazing on plant cover, litter cover and bare ground were monitored to appraise the trend in range condition using grazed transects (GT) and ungrazed transects (UGT). Despite an initial increase, plant cover on the UGT deteriorated in a similar manner to that on the GT. At the conclusion of six years of protection, the cumulative mortality ofIndigofera cliffordianaon the UGT exceeded 60 per cent whereas on the GT it was 6 per cent. Furthermore the density ofI. cliffordianaon the protected area decreased whereas that on the GT remained substantially unchanged.Aristida mutabilisreplacedI. cliffordianaon the UGT but showed little change on the grazed area.Sedera hirsutadisappeared from the GT. On the UGT the percentage of bare ground, despite an initial decline (39.3 ± 7.3 per cent), increased substantially (68.9 ± 15.7 per cent) in parallel with the GT (77.3 ± 4.7 per cent). The results contradict a commonly held view that land degradation only occurs when plants are overused. The findings suggest that degradation can occur in the absence of grazing, especially if the plant species involved are dependent on perturbation. As over‐resting eliminatesI. cliffordiana, it seems judicious to allow a 2‐3 year deferral period followed by moderate grazing if it is to be optimally m

ISSN:1085-3278    

DOI:10.1002/ldr.3400030402

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractIn the Thar (Rajasthan) Desert of India, sand dunes and sandy plains dominate the landscape. Livestock raising and marginal land cultivation are the main occupations. Owing to the high growth rates of human and livestock populations and a decrease in pasture area and its primary productivity, these lands bear acute grazing pressure. Average rainfall is low and vegetation growth is restricted to the short rainy season of two to four months duration. Often there are years of lower than normal rainfall. In these circumstances, fenced areas, established to stabilize formerly active sand dunes, are increasingly opened to grazing to prevent high stock mortality during droughts. This study was conducted in four fenced and one open site, representative of the region's different vegetation types and grazing pressures. The impact of 2 and 5 per cent free grazing pressure on protected sand dune vegetation density and cover was tested in the two fenced sites. The vegetation data for grazed sites are compared with those for the two ungrazed fenced (protected) and one unfenced (open) site subjected to unlimited grazing pressure throughout the year. The vegetation sampling was carried out by the qaudrat and line‐transect method before and after grazing from November 1984 to October 1986. It shows a significant reduction in the density and cover of many palatable species and an increase of unpalatable plants. The effect of vegetation degradation is greater in the unfenced area. The low grazing pressure on the fenced sand dunes sites can still cause drastic changes in the vegetation density and cover. Protection resulted in reduced sand erosion and enhanced growth of palatable plants in the fenced sites under similar climatic and edaphic conditions to the unfenced sites. The grazing pressure in the unfenced areas remains high to extreme during the year, due to low land productivity and high feed demand. The regeneration rate is very slow under the constant (3rd to 5th degree) overgrazing. Low intensity grazing pressure on the fenced sites during droughts can cause heavy utilization and mortality of palatable vegetatio

ISSN:1085-3278    

DOI:10.1002/ldr.3400030403

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractVarious properties capable of neutralizing environmental pollution (aggregate stability, buffer capacity and ion adsorption capacity) were determined in spoil from a lignite mine in Galicia (northwest Spain) and in mine soils of four and seven years age developed from this spoil. Buffer capacity and the adsorption of copper, cadmium and phosphate increased markedly with soil age, whereas the dispersion ratios of both macro‐ and micro‐aggregates decreased sharply. These changes are attributed to the known increase in organic matter and extractable iron and aluminium. These components aid interparticle binding and create both exchange sites and specific adsorption sites. The ultimate causes are thought to be the successful establishment of an active field plant cover, the considerable colloid content of the parent material, the management of the mine soils and the climatic conditions of the area, which favour both the accumulation of soil organic matter and the weathering of the parent mater

ISSN:1085-3278    

DOI:10.1002/ldr.3400030404

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

摘要:

AbstractThroughout Rwanda, terracing was reintroduced in 1973 as the major conservation practice to minimize soil loss on its steep agricultural lands.1Terracing has been partially successful in reducing soil losses resulting from nonchannelized runoff, the goal of this practice. However, because of the widespread fragile environmental conditions in the highlands, soil fertility has decreased and soil acidity increased in numerous fields as a direct result of the terracing. To maintain sufficient agricultural yields, within the constraints of a low resource agricultural system, farmers have responded to the acidity and soil fertility problems by systematically removing a portion of the terrace berm during field preparation. This practice, in response to the changing environmental situation due to terracing, results in significant amounts of soil displaced downslope year after year. This human‐induced soil erosion process seriously counters many of the intended benefits of terrace construction and is contributing to the land degradation problem. The results of this study emphasize the need for both conservation strategies and the measurement of soil loss to be sensitive to human‐induced as well as natural erosional processes. By not considering the human response to terrace construction, the benefits of this conservation practice have been seriously off

ISSN:1085-3278    

DOI:10.1002/ldr.3400030405

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

ISSN:1085-3278    

DOI:10.1002/ldr.3400030406

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

ISSN:1085-3278    

DOI:10.1002/ldr.3400030407

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY

ISSN:1085-3278    

DOI:10.1002/ldr.3400030401

出版商:John Wiley&Sons, Ltd.    

年代:1992

数据来源: WILEY