Spatial pattern of landslides due to heavy rains in a mixed Dipterocarp forest, North-Western Borneo

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During January and February 1963, there was some of the heaviest rainfall on record, causing disastrous flooding and numerous landslides in Sarawak. In this paper we assess the damage that this heavy rainfall caused to the mixed Dipterocarp forests of the Lambir Hills in northwestern Borneo. Using aerial photographs taken four months after this period of heavy rainfall (June 1963) and another taken 18 years later (July 1981), we evaluated its impact by examining the spatial patterns and scale of the landslides, as well as the processes of recovery in a 1905.5 ha area. In addition, the relationship between the occurrence of landslides and topography was analyzed in part of the sample area (843 ha). The occurrence of landslides was found to be strongly affected by the monoclinal structure of Tertiary sediments on an undulating, cuesta-like hilly terrain composed of dip and back slopes. Most of the 1963 landslide scars occurred on the back slopes and had an aggregated distribution. The area disturbed by the landslides was 2.5-2.91% of the area of the forest canopy [Landslide Formation Rate (LFR): 0.029 or 0.058% · year-1]. Mean landslide area is 0.45±0.37 ha (max: 1.93 ha). By 1981 (after 18 years), 2.43% of the total area identified in 1963 as landslide area had been colonized with vegetation and 0.32% of the total landslide area was newly created [mean area=0.23±0.23 ha (max: 1.1 ha)] and mostly expanded from old landslides detected. The area class distribution of the disappeared landslide scars for the period skewed to smaller classes. Calculation of Revegetation Time (RevT) by a transition probability matrix model showed smallest landslide area class (0.4 ha) had re-vegetated 3.86 times faster than largest class (2.0 ha), and the total mean RevT is 16.6±13.2 years. The recurrence of large landslides during a catastrophic event depending on topography, therefore, has important ecological consequences of the accumulation of post-landslide vegetation leading to an increase in successional and non-climax phases in the forests, at the regional level. Thus the catastrophic landslides in 1963 caused disturbance on a considerable scale in the region, but the Total Area Disturbed [TAD=LFR · RevT (or Recovery Time (RecT))] values greatly fluctuated depending on degree of erosional landslide recurrence on particular topography and RevT (or RecT) after landslides.

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  • Tropics

    Tropics 16 (1), 59-70, 2007

    日本熱帯生態学会

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