Soil erosion control in tree plantations on steep slopes: Runoff water and sediment trapping efficiency of riparian grass buffer in mountainous humid tropics

Riparian grass buffers reduce the velocity of water flowing over the soil surface during storms, capturing surface runoff (SR) and trapping soil particles eroded from cultivated slopes. Rarely quantified under steep slope conditions (>45 %), this phenomenon probably occurs in many mountain agroecosystems in the humid tropics. In Southeast Asia, teak plantations are often established on steep slopes where they can lead to significant soil loss (SL), particularly when the understory is removed. Therefore, we aimed to: quantify the effect of riparian grass buffers on SR and SL downstream of a teak plantation; and estimate the trapping efficiency of riparian grass buffers for water (WTE) and sediment (STE). Field measurements were carried out in Northern Lao PDR during the2014 rainy season, considering riparian zones with contrasted ground covers: (1) uncovered (URZ - 7-year-old teak trees with mean grass and litter densities of 7 % (SD 2 %) and 4 % (SD 3 %), respectively; (2) transitional (TRZ - 7-year-old teak trees with mean grass and litter densities of 19 % (SD 10 %) and 56 % (SD 9 %), respectively; and (3) covered (CRZ - grassed areas without teak trees with mean grass and litter densities of 46 % (SD 13 %) and 47 % (SD 21 %), respectively). WTE and STE were estimated based on measurements carried out from 6 July to 6 September2014 under natural rainfall conditions, using pairs of triplicate Gerlach troughs installed at the upper and lower margins of 5- and 10-m riparian sections (encompassing areas of ∼25 and 50 m 2 , respectively). Runoff coefficient (Rc), SL, and soil surface features were measured on the occasion of 20 rainfall events in 1-m 2 microplots. Rc and SL were higher in URZ (56 %, 5791 g·m −2 ) than in TRZ (13 %, 250 g·m −2 ) and CRZ (19 %, 159 g·m −2 ). Median WTE and STE were the highest along the 10-m TRZ + CRZ riparian grass buffer at 85 % and 97 % respectively. Partial Least Square Regression (PLSR) modelling yielded a good agreement between observation and prediction for WTE and STE at seasonal scale. Overall, the results of this work indicate that 5 to 10 m riparian grass buffers limit the export of surface water and sediment downstream during small (24-h rainfall ≤ 20.9 mm·d −1 , return period ≤ 1 year) to large storms (40.0 mm·d −1 < 24-h rainfall ≤ 84.5 mm·d −1 , 1.01 year<return period ≤ 2 years). Water and sediment trapping efficiency and main characteristics of riparian zones in a headwater catchment with teak plantation (Northern Laos, Houay Pano Critical Zone Observatory, https://mtropics.obs-mip.fr/ ): (1) 10-m wide riparian zone with teak and without understory (URZ); (2) 5-m wide transitional riparian grass buffer with teak (TRZ); (3) 5-m wide riparian grass buffer without teak (CRZ); (4) 10-m wide riparian zone combining TRZ and CRZ (TRZ + CRZ). • Runoff from teak-grown steep slopes trapped by riparian grass buffers was quantified • 10-m grass buffers abated runoff water and sediment export by 82 and 89 %, respectively • 5–10 m riparian grass buffers limit soil loss for storms of <2-year return period • Riparian grass buffer trapping efficiency was less effective during extreme events