**1. Introduction**

The tropics encompass a wide variety of environmental conditions sharing high radiation and high temperatures, whilst the timing and annual amount of the rainfall and the seasonal moisture pattern enable the distinction between humid tropical, seasonal wet tropical and arid tropical zones and of the savannah and rain forest environments [40]. As a result of its great areal extent, the tropical zone encompasses a wide range of tectonic regimes, structural and lithological settings and landscapes [38].

The understanding of environmental changes in the tropics appears to be of particular im‐ portance as this zone encompasses 35 to 40 per cent of the land surface of the earth and in‐ cludes about 50 per cent of the world's population [67]. Tropical countries are characterized by a rapid growth in the population and a rapid development of urban areas [6, 45]. This has resulted in increasing demands on fresh water, food, arable land and energy and miner‐ al resources, leading to an increase in per capita consumption and severe environmental degradation.

Tropical ecosystems have been subjected to human interference for thousands of years in the form of traditional land use of many and varied kinds [87, 38]. However, rapid growth in the population and the technical advances of the last 100 years have increased the human impact on physical environments to a much higher degree than the thousands of years of human activity before that. Human interference and environmental change have been rapid‐ ly increasing since the mid-twentieth century. Gupta quotes a mean annual loss of rain for‐ est of 174,000 km2 during the decade 1980 to 1990 [38]. Agriculture and urbanization have modified and transformed large parts of the physical environment and have altered the op‐ eration of the geomorphic process-response systems [36]. According to [2], the annual defor‐ estation rate of rain forests ranges from 0.38 to 0.91 per cent in Latin America, Africa and

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Southeast Asia with extraordinary high rates of 5.9 per cent in Sumatra and 4.9 per cent in Madagascar. More recent estimates of gross forest-cover loss in the first decade of the 21st century indicate no reversal of these trends [39]. Recent studies indicate an increase in haz‐ ards in many regions in the tropics. These appear to be linked to changes in global climate, an accelerated and disorderly process of urbanization, deforestation and the associated loss of hydrological storage capacity, particularly in mountainous domains and to the concentra‐ tion of settlement activity in potential high-risk areas [44].

However, the severity of their impact varies spatially, and the intensity and course of the response to environment changes varies in the different physiographic domains depending on the nature and severity of the change and the sensitivity of the landscape. Landscapes can be viewed as systems consisting of various interconnected components or subsystems [15]. As the subsystems tend to interact on different spatial and temporal scales via different feedbacks, they may dampen or reinforce the effects of environmental changes depending on the coupling strength existing between the system components. The crossing of thresh‐ olds, on the other hand, causes a sudden change in the landscape or in the geomorphic proc‐ esses, and the mutual operation of feedbacks and thresholds within the geomorphic system tends to induce a complex response to changes in environmental conditions. A consequence of these interactions is that the rate of change of landscapes as well as the severity of the ge‐ omorphic response to environmental changes is extremely variable.

#### **1.2. Purpose and objectives**

The understanding of developmental patterns in respect of the diverse and complex envi‐ ronmental controls and geomorphic responses in the tropics appears to be an essential pre‐ requisite for the assessment and distinction of climatically-driven and humanly-induced environmental changes as well as for the planning of technological, social and political measures and a sustainable development. The objective of this paper is to demonstrate the role of the geomorphic response to environmental changes on a variety of temporal and spa‐ tial scales. However, a comprehensive and balanced view of the wide range of geomorphic process responses to environmental change, their causes and functional relationships is be‐ yond the scope of this study. Instead, this study attempts to concentrate on the response of hillslope processes and their specific controls in the humid tropics, and, in particular, on the stability of hillslopes, on the role of surface wash processes in accelerating soil erosion, and on the role of weathering processes from the point of view of the availability of nutrients in the soils and the geotechnical properties of the weathered materials. A further objective of this study is to highlight some aspects of the role of the long-term development paths of the landscapes as this factor may provide some indication of susceptibility and of a potential re‐ sponse to environmental changes on the part of larger-scale landscape units.

The second chapter encompasses a discussion on the various factors which determine rapid mass movements in humid tropical mountains, and provides an overview of the role of ex‐ treme rainfall events in triggering landslides. As the responses of hillslopes are often predis‐ posed by virtue of long-term evolutionary processes, the chapter also includes some case studies on the role of long-term hillslope development and of the effects of susceptibility on landslide hazards in rural and urban areas.

The third chapter is focussed on different aspects of soil erosion, land degradation and soil fertility. The fourth chapter highlights some factors which determine the intrinsic com‐ plexity of geomorphic response, interaction between human interferences, the role of changes in the frequency and magnitude of external events and the importance of inter‐ disciplinary approaches.
