**4. Conclusion**

ter on the soil surface is slowed down whilst the breakdown of the organic matter is acceler‐

Although the physico-chemical processes controlling the productivity and fertility of the soils and the turnovers of the nutrients are not completely understood, several lines of evi‐ dence suggest that the degree of weathering and textural characteristics of the soil play an important role in the nutrient cycle. High weathering rates result in excessive base-leaching and a low pH, creating a decline in base saturation, loss of major cations and a decrease in the cation-exchange capacity [66]. This promotes the occurrence of free iron and aluminium either in the clay complexes or as amorphous iron and aluminium oxides or hydroxides in the weathering layers. As amorphous iron and aluminium oxides readily absorb, phospho‐ rus tropical soils with a low pH are often characterized by a high phosphorus fixation ca‐ pacity, resulting in a phosphorus deficiency [85, 59]. According to studies in the Amazon of Brazil the fixation of phosphorus rather than the overall nutrient decline appears in many

Soil erosion and intense leaching in soils are responsible for several problems concerning productivity in agricultural land use. In a case study carried out in Rwanda several green farming methods applied to highly degraded soils failed to restore the fertility of the soils [77]. Improved fallow, mulching, green manure and the use of compost and cow dung were not sufficient to maintain the nutrient levels in the soil as the rapid decomposition of the or‐ ganic matter at the start of the rain season resulted in a release and leaching of high amounts of nitrogen and a rapid reduction in the fertility of the soils [77]. From the point of view of the sustainability any agricultural strategies being considered, the materials used for fertiliz‐ ing the soils have to be inexpensive and available from regional or local resources. The im‐ provement of the physico-chemical properties of highly degraded soils, on the other hand, depends on several-site specific and soil-specific factors, and additional information is fre‐ quently required on the dynamics of the soil. Important improvements usually involve in‐ creasing the pH. This reduces phosphorus fixation, the disintegration of chlorite structures

However, any application of material has to maintain the slow dissolution of cations from dissolved minerals and has to inhibit silica dissolution which often involves an increase in pH and results in an increase in the disintegration rate of chlorite structures [77]. In rela‐ tion to the requirements specified in Rwanda, several tests with calcium carbonate, traver‐ tine and volcanic tephra indicated that the combined application of cow dung and tephra represents a measure capable of improving the agricultural capacity of the degraded soils [77]. However, soil erosion, nutrient cycles and soil fertility are highly interrelated and depend often on specific local and regional factors. Although quantitative data of soil ero‐ sion rates and depletion rates are important for the implementation of effective soil conser‐ vation measures, socio-economic factors and the understanding of the traditional/cultural background appear to be of equal importance because many conservation strategies may be impractical or too expensive or are rejected as a result of limited access to the technolo‐

cases to be the cause of the decline in pasture productivity [69].

and reduces antagonistic effects in cation exchange.

gies required.

ated by solar radiation [87, p. 277].

18 Environmental Change and Sustainability

Over the period of the last fifty years, most tropical mountains have experienced marked changes in their environmental conditions due to the high rate of deforestation, rural land use and urban growth. These changes have often reinforced hillslope processes such as soil erosion and landsliding and have also resulted in an increase of geomorphic hazards, even in areas with a previously lower susceptibility to soil erosion or landsliding. Increased rates of soil erosion and landsliding have been documented from regions where large areas are affected by human intervention and hillslope processes are highly interdependent and tend to reinforce each other. The changes have not only affected the hillslope system but have also influenced other subsystems of the geomorphic/ecological system, which has result‐ ed in the coupling of different responses similar to "chain reactions". Such "chain reac‐ tions" appear to occur frequently when the urban fringe expands into mountainous terrains [38]. Urbanisation and deforestation increase the runoff and hence induce soil erosion whilst the increase in storm runoff results in the undercutting of hillslopes and landsliding, there‐ by increasing the supply of material to the rivers, which in turn, increases the likelihood of flooding.

However, the geomorphic response displays a high degree of spatial and temporal variabili‐ ty. Under similar geologic and bioclimatic settings, some landscapes tend to react rapidly to ongoing environmental changes whilst others tend to absorb the effects of environmental change, as the reaction is delayed or dampened in the various interconnected geomorphic/ ecological subsystems. Several factors contribute to the differences in the geomorphic re‐ sponse. The current state of the landscape, the degree of human modification of the land‐ scape, the magnitude of climatically-driven events and the differing coupling strength between the long-term evolution of the hillslope system and the current hillslope processes. As geomorphic processes are triggered by separate events, the response to changes is a func‐ tion of the magnitude and frequency of exogenous or endogenous events. With respect to rainfall-triggered events, the incidence of hillslope processes is often controlled by thresh‐ olds. However, these thresholds are continuously altered by human interference in the land‐ scape, thereby increasing the risks of soil-erosion hazard and landsliding, though this interference is often necessary in that it benefits economic progress and advancement.

Disastrous landslide events are often closely associated with the expansion of the urban fringes into hilly and mountainous areas, and settlement activity in these areas has often re‐ sulted in the unsuitable modification of hillslopes, which, in turn, has increased susceptibili‐ ty to mass movements [38, 53]. Although most of the recent landslide disasters are primarily controlled by geological, structural and environmental factors as well as by human interfer‐ ence, slope failure is often predisposed as a consequence of long-term evolutionary process‐ es on the hillslopes. The dynamic coupling of existing controls and long-term evolutionary processes may result in the lowering of crucial thresholds. This includes the reduction of the shear strength by weathering processes, and the increase of shear stresses on the hillslopes caused by small subtle changes in slope angle and hydrology.

The intensity of human impact on tropical environments is documented in the large areas that have been subjected to deforestation. The impact has affected the geomorphic processresponse system, the nutrient cycles, and biodiversity. Recent studies have shown that there is no reversal in the overall trend of tropical deforestation, though the rates of deforestation vary strongly from one decade to another and from one country to another, depending also on the methods used to assess deforestation [2, 28, 79, 39]. Estimates of the world-wide con‐ tribution of deforestation in the tropics to carbon emissions indicate a total emission of 810 \*106 metric tonnes/year (period 2000 to 2005) excluding carbon emissions from logging, peat‐ lands drainage and burning, and forest recovery [39]. However, the contribution of carbon emissions from tropical deforestation to global climatic change remains obscure as the turn‐ over rates and recovery rates are related to various factors and the interaction between these factors is not completely understood [35, 38]. This applies also to the effects of global climat‐ ic change on the geomorphic process-response system as changes in magnitude and fre‐ quency of geomorphic processes depend also on all other environmental changes. Predictions on future climatic development trends in the tropics suggest an increase in summer monsoon and a decrease in summer rainfalls in Central America and Mexico and an increase in the number of cyclones, tropical storms and hurricanes [44]. However, human interference and climatic change often act simultaneously. This complicates predictions of crucial thresholds and the establishment of relationships between landsliding and large soil erosion events and the spatial distribution and the seasonal and annual variability of rain‐ fall. The temporal clustering of landslide events in some regions, on the other hand, appears to indicate some associations. In Kenya landsliding was closely associated with the occur‐ rence of El Niňo circulation [52]. In southern America, on the other hand, the temporal pat‐ tern of landslide events appears to coincide with the ENSO climatic cycle. However, hillslope processes are characterized by an intrinsic complexity. Many factors appear to be capable of causing changes in both frequency and magnitude on different spatial and tem‐ poral scales.

Studies on deforestation rates in several countries of humid tropical Africa have shown that the rate of forest destruction is not only a result of the growth in population but depends also on macro-economic changes. Apart from dependence on international market prices, the extent of the agricultural area appears to depend directly and indirectly on factors such as public investment, monetary policy and exchange-rate policy, urban income levels, fertil‐ izer subsidies, and rural-to-urban and urban-to-rural migration [50, 64, 65]. With respect to the issue of sustainable development, socioeconomic factors must also be considered.

A significant statistical relationship has been determined between the decline of the cocoa and coffee prices and subsiding governmental input, which has forced farmers in Cameroon to expand their food and crop cultivation into forested areas [51]. International prices and demands on agricultural resources, on the other hand, often result in an expansion of agri‐ cultural areas at the expense of rain forests. An example is the expansion of agricultural areas for soybean production and the increase in cultivated pastures in Brazil, which result‐ ed from the growing importance of cattle ranching. The expansion of soybean cultivation re‐ sulted in extensive clearance of savannah forests and of tropical forests and is noted to be the second most important driver of deforestation after ranching [43].

Socio-economic factors also play an important role in establishing new methods in agricul‐ ture to improve environmental quality. Financial aspects, work expenditure, the availability of resources necessary for carrying out improvements and the consideration of traditional agricultural techniques may determine the success of sustainable developments. Socio-eco‐ nomic aspects are also important in the mitigation of hazards in urban areas. Hillslopes prone to landsliding are often occupied as a result of the unplanned growth of cities and in‐ creases in rent and the declining availability of land for building in the cities to house the growing population [26, 4].

The complex interaction of socio-economic, biological, geological and geomorphic aspects indicates that sustainable development requires an interdisciplinary approach. With respect to environmental planning and sustainable development in the tropics, geomorphic studies of hillslope processes may contribute to unravelling the intrinsic complexity of various hill‐ slope hazards. This takes in a multitude of objectives, which range from assessment of the severity of influences impacting on the environment, determination of the dominant proc‐ esses and hazards, assessment of the vulnerability of specific sites, determination of external triggers and predictions of events which have no historically recorded precedent.
