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

摘要

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. The objective of this research was to assess the multiple effects of land-use intensification in a tropical steepland environment on land degradation processes. A case study was conducted at Pakha village (located in Thailand's northern most Chiang Rai province), which is dominated by steepland with average slope gradients ranging from 30 to 70%. Soil erosion processes were monitored in a selected catchment for 2 years, and informal interviews were conducted to elucidate farmers' perceptions regarding land degradation processes. The rapid land-use changes at the Dze Donglo catchment (164 ha) resulted in severe and accelerated land degradation, including tillage erosion (386 ton/year), inter-rill and rill erosion (502 ton/year), gully erosion (423 ton/year), and landslides (7572 ton during 1994). Water erosion is most common in intensively farmed areas. The combination of runoff-generating areas, runoff-concentrating features and connectivity led to extensive gully erosion. Landslides were most common in steep fallows and in wetland terraces along incising streams. Many of these steepland degradation processes interacted with each other (i.e. rills with gully erosion, tillage erosion with water erosion, gullies with landslides). The observed land degradation processes matched very well with farmers' perceptions. This study enabled to identify potential land degradation hotspots and indicates the necessity to analyse steepland degradation processes in a holistic way.