ABSTRACT
Land use in Xishuangbanna, SW China, has been dramatically changed over the past 30 years. Rubber plantation combined with tea cultivation boosted from 1.3% to 11.8%, while deforestation decreased the forest cover from 69% to 45%. The major factor affecting hydrological processes was land cover change overarching the climate change impact. The conversion of tropical rain forest to rubber plantation may markedly affect quantity and quality of surface runoff, which serves as the most important source for drinking water in the mountainous area. Therefore this land use change can further threaten water security of local villages. In order to reduce impact of rubber expansion, the action guidance of Twelfth Five Year Plan of Xishuangbanna government has limited rubber planting area to elevation below 950 m and slope smaller than 25 degree. However, this threshold was proposed by considering growth of trees instead of improving ecosystem services, e.g. soil conservation. This study aims at 1) evaluating the effect of rubber plantations on erosion at plot and watershed level; 2) enhancing ecosystem function of rubber plantations by improving soil conservation at plot level and ameliorating water quality by better landscape planning at watershed level e.g. establishing buffer zone; 3) balancing land use between economic benefic and environmental protection. We evaluated sediment yields and runoff production in different plantation ages within one lifespan: young rubber with open canopy (4 year), mid-age with closed canopy (12 year, 18 year) and old rubber with dense canopy (25 year and 36 year). Driven factors like rainfall intensity, canopy interception, surface cover, soil properties, fine root density were measured simultaneously. Different herbicide applications schemes were applied to assess impact of improved management on decreasing soil loss at plot level. Rubber-dominated and forest-dominated (as reference) watersheds were selected for continuous measurement of water discharge and turbidity in the outlet position to evaluate the water yield and export of suspended solids from rubber plantation at watershed scale. Based on field survey we found that highest soil loss at plot level (277 g m–2) was in mid-age rubber plantation, which implicated potential water pollution (threshold 100 g m–2). Reduced herbicide rate (2 times per year V.S bimonthly application) can efficiently decrease sediment yields to 50 g m–2. Land use change from forest to rubber increases stream turbidity at watershed level from 350 to 1300 nephelometric turbidity units (NTU). Soil loss in rubber dominated watershed was estimated as 0.3 t ha–1 y–1 while high frequency (over 15% time in rainy season) of medium high turbidity (> 30 NTU) and turbidity over 1200 NTU appeared in storm events still strongly threatened security of local drinking water resource. Measures at plot (reduced herbicide) and watershed level (introduction of riparian buffer zones) should be taken to supply better ecosystem service. Size and location of the buffer zones serving as effective filter strip needs to be determined. Therefore, the Land Use Change Impact Assessment (LUCIA) model is used to simulate integrated land management (reduced herbicide + buffer zone) effect on erosion and sediment transport. LUCIA model simulates the erosion process in a spatially explicit dynamic way, depending on development of plant canopy and terrain properties. Data collected in field are used to parameterize and validate the baseline scenario. Meanwhile, hotspots of deposition and erosion along the stream can be identified. Plot (reduced herbicide) and landscape (buffer zone) controls on erosion are simulated by the model to identify best management practices for improved ecosystem function (better soil conservation and decreased stream turbidity) as well as maintained economic profitability (total latex production in study region). The results can further assist the cost-benefit analysis and decision-making in land management and landscape planning.
