Assessing hydrological responses to reforestation and fruit tree planting in a sub-tropical forested watershed using a combined research approach

Abstract

Fruit tree planting is often considered as one of reforestation practices in the past decades in many parts of China because it increases forest coverage while gaining short-term economic benefits. However, the effects of large-scale fruit tree planting on water resources at the watershed scale are rarely assessed, especially in subtropical China. In this study, the forested watershed (Jiujushui watershed, 261.4 km2), located in Southeastern China, was selected to examine the responses of major flow components including baseflow, surface runoff, and total streamflow to both reforestation (e.g., Cunninghamia lanceolata (Lamb.) Hook and Pinus massoniana Lamb.) and fruit tree planting (e.g., Citrus reticulate Blanco) practices. The study period (1961–2016) was split into three sub-periods including reference, reforestation and fruit tree planting. The combined approach including pair-wise method and separation of relative contributions was adopted in this study. The former was used to test the statistical relations between forest changes and flow components, while the latter (sensitivity-based and time trend methods) was applied to quantitatively separate the relative contributions of forest changes and climatic variability to each flow component so that the effects of forest changes (reforestation and fruit tree planting) on flow components can be quantified and compared. Results from the pair-wise technique showed that reforestation and fruit tree planting had no significant effects on total streamflow. However, reforestation significantly increased base flow by 15% (p = 0.03), while fruit tree planting did not. In contrast, surface runoff was significantly increased by fruit tree planting (p = 0.048), while it was not by reforestation. The results from the separation technique showed that in the reforestation period, the relative contributions of reforestation to surface runoff, baseflow and total streamflow were 23.9%, 67.3% and 50.9%, respectively, in comparison with 76.1%, 32.7% and 49.1% from climate variability, highlighting that reforestation had greater effects on base flow. However, fruit tree planting accounted for 54.4%, 48.8% and 57.3% of total streamflow, baseflow and surface runoff variations, while climatic variability were responsible for 45.6%, 51.3% and 42.7%, respectively in the fruit tree planting period, demonstrating that fruit tree planting had greater effects on surface runoff. Thus, the results from both techniques clearly demonstrated that forest recovery through reforestation effectively increased baseflow, while fruit tree planting caused increasing of surface runoff. We also conclude that the combination approach provides a robust and complementary research framework for assessing the hydrological effects of reforestation in a single watershed.