The multiple land degradation effects caused by land-use intensification in tropical steeplands: A catchment study from northern Thailand

The strongly incised mountain landscape of northern Thailand has changed dramatically during the last few decades due to increased population pressure, agricultural commercialization, limitation to use old fallows and reforestation of upper catchments. The traditional shifting cultivation with fallow periods of 7 years and longer was gradually replaced by 1 to 4 year fallow periods. As a result, in high population areas the landscape became dominated by fields planted to rainfed upland crops, wetland rice terraces, fallow vegetation, and patches of secondary forest. This new land-use system seems to have triggered new land degradation processes that are easy to observe when travelling through this landscape.     

Landslide densities associated with rainfall,stand age,and topography on forested landscapes,southwestern Washington,USA

Elevated landslide rates in forested landscapes can adversely impact aquatic habitat and water quality and remove and/or degrade soil resources required for forest regeneration. As a result, understanding the associations between management actions, natural factors, and landslide rates is important information needed for land managers. An unusual and powerful storm in early December, 2007, caused record flooding and thousands of landslides across southwest Washington and northwest Oregon, USA, and provided a rare opportunity to examine the effects of both natural factors and forest management practices on landslide density. Landslide inventory data were collected from both aerial photos and systematic field surveys to provide a broad survey database that was used to develop estimates of landslide density and to examine associations between landslide density, precipitation, topography, and forest stand age across a 152,000 ha forested landscape in the Willapa Hills, Washington. We estimated the probability of detecting landslides on aerial photos for six strata defined by forest stand age and a broad range of rainfall intensity, expressed as percent of the 100-year, 24-h, maximum rainfall. Key findings are that landslide detection probability decreased with increasing stand age, but was similar across rainfall intensities. The overall fraction of field-detected landslides that were not detected on 1:12,000-scale aerial photos was 39%. Very few landslides occurred in the 0–100% of 100-year rainfall category, regardless of stand age or slope gradient class. At higher rainfall intensities, significantly higher landslide densities occurred on steep slopes (>70% gradient) compared to lower gradient slopes, as expected. Above ∼150% of 100-year rainfall, the density of landslides was ∼2–3 times larger in the 0–5 and 6–10 year stand age categories than in the 11–20, 21–30, 31–40, and 41+ categories. The effect of stand age was strongest at the highest rainfall intensities. Our results demonstrate that ground-based landslide inventory data are required in order to correct for detection bias from aerial photos, develop reasonable estimates of landslide density across environmental gradients such as rainfall magnitude and topography, and make unbiased interpretations of relationships between forest management associations and landslide occurrence.     

Root reinforcement in plantations of Cryptomeria japonica D. Don: effect of tree age and stand structure on slope stability

The role of vegetation in preventing shallow soil mass movement is now fairly well understood, particularly at the individual plant level. However, how soil is reinforced on a larger scale and the influence of changes in vegetation over time has rarely been investigated. Therefore we carried out a study on the temporal and spatial changes within stands of Cryptomeria japonica D. Don, growing in the Sichuan province of China, an area where shallow landslides are frequent.     

Landslide occurrence as a response to land use change: a review of evidence from New Zealand

Vegetation cover is an important factor influencing the occurrence and movement of rainfall-triggered landslides, and changes to vegetation cover often result in modified landslide behaviour. However, it is difficult to relate the occurrence of landslides directly to variations in land use, especially in some European countries. In contrast, New Zealand provides a good opportunity to investigate geomorphic responses to anthropogenic land cover changes. Before European settlers first arrived in the 1840s, hilly regions were only marginally influenced by human activity. The Maoris, New Zealand's first settlers, lived largely on coastal plains or near lakes and rivers. They influenced general vegetation cover only through localized burning practices. In contrast, European settlers moved into the back country and converted extensive hill areas from native forest and bush to pasture. This reduced the strength of the regolith and rendered the slopes more susceptible to landslides. Pulses of natural sedimentation in the pre-European period have been related to volcanic activity, climatic variability, including changes in frequency of cyclonic storms and wind erosion and fluvial erosion following forest fires initiated either by volcanic eruptions or lightning strikes. Since European deforestation began, sediment production has largely been determined by landslide events. On unstable slopes, thousands of landslides were triggered by high-magnitude/low-frequency climatic events during storms with estimated return periods in excess of 50 years. In contrast, low-magnitude/high-frequency rainfall events have caused gully and channel erosion. Examples from different parts of New Zealand indicate changes in sediment-generating processes following land use modifications. After deforestation, landslides have contributed significantly to sediment sequences in depositional basins such as lakes, swamps, estuaries, coastal wetlands and the nearshore and offshore zones of continental platforms.     

Environmental change, hillslope erosion and suspended sediment: Some observations from Hong Kong

Evidence of environmental change in Hong Kong, including vegetation, climate and the frequency of hillfires is presented. Hillfires may be an important influence upon slope degradation in the territory. Field observations made in the Northwest New Territories provide evidence that hillfires increase soil erosion on slopes and result in higher storm-period concentrations of suspended sediment. Monitoring carried out in a small drainage basin reveals that landslides deliver sediment to an upland stream giving very high concentrations and affecting sediment properties, with the C and N content declining under the influence of landslides. Should environmental change alter the frequency of landsliding and hillfire on natural terrain in Hong Kong this will have implications for hillslope degradation and sediment transport in upland streams.     

Mass movement mapping for geomorphological understanding and sustainable development: Tigray,Ethiopia

Mass movement topography characterises the escarpments and piedmont zones of the tabular ridges in the western part of the Mekelle outlier, Tigray, Ethiopia. Several types of mass movements can be distinguished. The first type is rockfall produced by 357 km rocky escarpments and cliffs during the rainy season. In the study area, every current kilometer of Amba Aradam sandstone cliff annually produces 3.7 m³ of rock fragments. However, this is an under-estimation of the actual cliff and escarpment evolution, which is also characterised by debris slides and small rock slides.     

瑞丽市滑坡灾害调查

对云南瑞丽市6个乡镇地质灾害进行调查。结果显示:该区滑坡总数为51个,形态以舌形、半圆形为主;滑坡规模以小型居多,占82.35%;滑坡主要发生在岩层倾向与坡向大致斜交的斜向坡部位。同时,对滑坡的危害程度、形成条件、稳定性与发展趋势进行了全面分析。     

The impacts of logging on landslide activity at Clayoquot Sound, British Columbia

The objective of this study was to investigate the impacts of logging on landslide activity in Clayoquot Sound on the west coast of Vancouver Island, British Columbia. A total of 1004 landslides were documented in order to test the hypothesis that areas affected by logging activities show different density, frequency and magnitude characteristics of landsliding than areas unaffected by logging. The frequency of landslides in logged terrain was found to be nine times higher than in undisturbed forest. An exponential increase in landslide frequency within the area logged was observed on a large watershed scale. Failures in logged terrain occur on gentler slopes than in natural terrain, partly because fewer slopes steeper than 40° have been logged. Debris slides and debris flows are the most frequently occurring mass movements, initiating mostly from road fill failures and from within cutblocks. Most landslides initiated on SE-facing slopes, with the highest landslide frequency near the coast and diminishing exponentially further inland. This pattern of activity most likely reflects the greater impact of severe storms arriving from the Pacific with winds dominating from the southeast. Concave and straight slopes are most susceptible to landslide initiation. A preliminary examination of geologic control on the frequency of landslides suggests that the Quatsino Formation, the Island Plutonic Suite and the Sicker Group are significantly above the average rate. Further work is needed to identify and quantify factors confounding the trends and correlations identified in this overview study. Results obtained in this study will be used to implement changes in forest management.     

Periglacial cover beds and soils in landslide areas of SW-Germany

At the slopes of the Jurassic cuesta scarp in SW-Germany, pedological and mineralogical investigations were carried out in landslide areas. So far, it is possible to distinguish two well-defined landslide areas, one of them belonging to the Pleistocene, the other one characterised by Holocene movements. In general, the distribution of soils and sediments is strongly linked to the age of landslide deposits. In Pleistocene landslide areas, the parent material of the studied soils is formed by periglacial cover beds of Late Pleistocene age, consisting of Jurassic, aeolian and volcanic components. The Upper Periglacial Cover Bed was recognised as the most important marker horizon in the studied slope areas. There, the existence of minerals originating from the eruption of the Laacher See volcano, dated to 12,900 yr BP, could be demonstrated for the first time. Most of the Pleistocene landforms are characterised by well-developed soils, like Clayic and Vertic Cambisols, whereas relic soils exclusively occur in the oldest parts of landslide deposits. Landslide areas affected by Holocene slope processes do not exhibit periglacial layers, as mass movements removed periglacial sediments and former soils extensively. As a consequence, the parent material is different from those of Pleistocene landslide areas. Therefore, sediments in these areas are characterised by Jurassic components and only initial soils are present.     

Frequency and spatial distribution of landslides in a mountainous drainage basin: Western Foothills, Taiwan

Maps from 1904 and 1915 and air photographs from 1963, 1980, 1985, 1993 and 1996 provide a record of landslide incidence in a 92.1-km² drainage basin, a headwater tributary of the Cho-Shui River in Taiwan. Interpretation of landslide patterns from the early maps indicate that in four sub-basins (36 km²) structural geological factors control chronic landsliding regularly reactivated by intense rains. Within these four sub-basins, all later air photographs reveal a continuing high incidence of landslides (with landslide densities of 3–11 ha/km²). Air photographs taken in 1963, following extensive logging, in 1985, following highway construction, and in 1996, following the very large typhoon Herb event demonstrate the short-term effects of disturbance in these structurally weak sub-basins. Air photographs from 1980 and 1993 demonstrate recovery of the land surface from logging and highway construction impacts, respectively. For the adjacent sub-basins (56 km²), two modes of response to perturbations were identified: six sub-basins (48 km²) showed direct response to logging, road construction and typhoon Herb and five sub-basins (8 km²) were more buffered and showed some lagged responses. Even this last category of sub-basins is more active than the average for Taiwan, where the mean landslide density is 0.84 ha/km². It is proposed that, for the 92.1-km² Hoshe basin, the ‘formative event’ sensu Brunsden [Z. Geomorphol. Suppl. 79 (1990) 1] is one that produces approximately 200 ha of landslides, a value that has been equaled or exceeded in each of the periods of study (1963–1980, 1980–1985, 1985–1993 and 1993–1996). Logging activity, major road construction, and extreme typhoon and earthquake events produce short-term acceleration of landslide incidence. In principle, recovery rates of the land from pulsed perturbations of about 20 years for logging activity and about 8 years for major road construction may also be suggested.