Assessing the impact of forest conversion to plantations on soil degradation and forest water conservation in the humid tropical region of Southeast Asia: Implications for forest restoration

Abstract

The deterioration of soil and water resources resulting from tropical rainforest (TR) conversion to monoculture plantations (e.g., rubber monoculture; RM) could be restrained and restored through intercropping. However, the response of soil properties and forest water conservation function to forest conversion, i.e., the conversion of RM to rubber rainforest (RR: derived from the invasion of wild native plants in abandoned RM), is still unclear. We involved four forests types, TR, RM, rubber-tea agroforestry (RTA), and RR, as transitional steps of forest conversion through a space-for-time substitution approach to examine the dynamic of soil physical, hydrological and chemical properties during the forest conversion (from TR to RM, RM to RTA, RTA to RR, and retransformation into TR). The results show that SOC, TN, TP, and TK decreased in the order of TR > RR > RTA > RM, which was followed by the trend of soil hydrological and physical properties among these forest types. The interrelation between soil physical and chemical properties was mediated by water flow behaviours. High macroporosity and related low Ks in TR favoured the occurrence of water flow behaviours. Water flow behaviours not only influenced the distribution of soil chemical elements but also played a crucial role in forming appropriate conditions for nutrient turnover. The co-occurrence of preferential and matrix flow was more prevalent in the rainy season than dry season due to the higher frequency and higher amount of rainwater. The preferential flow promoted the soil water flow in the water flow paths and enhanced water storage in the soil pores. In short, the soil properties and soil water supply decreased in the following order: TR > RR > RTA > RM, suggesting that the severe soil degradation that occurred after TR conversion to RM can be restored back to the extent of TR after a period of succession. The results provided new insights for understanding the forest water conservation function and soil properties in response to forest conversion and highlighted that the RR appeared as a transitional stage during the course of forest restoration from RM to TR under low rubber demand. These findings improve the current knowledge of the relationship among soil physical, hydrological and chemical properties in the rubber-growing humid tropical region of Southeast Asia.