Does forest conversion to tree plantations affect properties of subsoil horizons? Findings from mainland Southeast Asia (Lao PDR, Yunnan-China)

While forest conversion to tree plantations in northern Laos is ongoing since the 1990s, its impact on soil properties is yet to be assessed. Our objectives were (i) to assess changes in soil properties, including subsoil horizons down to 160 cm, in <14-year-old teak and 21-year-old rubber tree plantations compared to forest soils, in Luang Namtha province, Laos; (ii) review the recently published studies on the impact of tree plantations in neighboring Xishuangbanna, China; and (iii) to provide, based on (i) and (ii) broad recommendations for sustainable soil management in areas of northern Laos converted to tree plantations. We analyzed selected chemical and physical properties of soil collected at 6 depths, from surface to a depth of 160 cm, along hillslopes, at the locations of 4 paired sites. All layers (0–160 cm) were very acidic pH (≤ 4.3) and depleted in exchangeable cations. Compared to their levels in the surface layer (0–10 cm), organic carbon content (21 g kg⁻¹) and specific pore volume (0.68 cm³ g⁻¹) decreased by 62% and 44%, respectively at 40 cm, and by 81% and 53%, respectively at 110 cm. Forest conversion to tree plantations induced a decrease in pore volume and carbon content in the topsoil, but the subsoil remained unaffected. In rubber tree plantations, the nitrogen content dropped in all layers, including subsoil. In teak tree plantations, pH and phosphorus content increased in the deepest layers. Studies from Xishuangbanna reported comparable degradations, also limited to the upper layers, following conversion to rubber tree plantations. Some isolated studies mentioned some apparent improvements of soil properties, related to textural differences. Overall, the only significant subsoil degradation consisted in N depletion (0–160 cm) under rubber plantations. Even though the impact of forest conversion on soil properties, up to two decades following tree plantation, appeared limited to the uppermost soil layers, the observed alteration of soil porosity may have an impact on runoff and erosion, inducing irreversible degradation of soil functions and productive potential. The subtle nature of such changes hampers adequate perception of their actual importance by farmers and stakeholders who likely have the false impression that soils are unaffected by forest conversion to tree plantations. Consequently, an enforcement of soil conservation practices is recommended when establishing commercial tree plantations on the naturally nutrient-depleted and low porosity Acrisols of this region.