Abstract:

The stable oxygen isotope, δ¹⁸O, is a very useful tracer tool to study the impacts of forests on the water cycle. The Wolong Natural Reserve is a key conservation area focusing on the protection of Panda, rare animals and alpine ecosystems. The Wolong Natural Reserve is located in the Mingjiang River headwaters and plays an important role in the conservation of soil and water in this watershed as well as the upper reaches of the Yangtze River. Although much research has been conducted on the water cycle, including changes in precipitation intensity, rainfall frequency, runoff dynamics and subsurface flow, little has been devoted to understanding rainfall water allocation in sub-alpine dark coniferous forests. Using a stable oxygen isotope technique, we studied changes in precipitation, throughfall and soilwater flow over time under different rainfall intensities in three plots in a sub-alpine dark conifer forest. The results showed that 1) there were no significant differences between rainfall δ¹⁸O and daily precipitation (r=0.612, p≥0.05,n=20). There was a significant linear relationship between the rainfall δ¹⁸O and throughfall δ¹⁸O defined by the following equations for the three plots: plot A: y₁=0.970 6x₁-2.254 2,R₁²=0.796 2, p<0.05,F=42.97; plot B: y₂=0.997 6x₂-1.632 1,R₁²=0.729 7, p<0.05,F=29.60; plot C: y₃=0.920 3x₃-2.728 9,R₃²=0.729 7, p<0.05,F=26.47. 2) The δ¹⁸O exceeded 0 when precipitation ≥ 3.20 mm, and the maximum δ¹⁸O value occurred when precipitation was 12.65 mm due to the interactions between canopy evaporation and precipitation. 3) There was limited soil water flow when rainfall intensity, daily precipitation or rainfall continuity was low resulting in a slow response of the precipitation δ¹⁸O signature on soil water. When soil water flow was strong and continuous, the precipitations δ¹⁸O signature showed up rapidly in the soil water flow. The δ¹⁸O of soil water showed the same rise and fall patterns of the rainfall δ¹⁸O when precipitation was less than 10 mm, but with 4-day time lag. When precipitation was between 10 mm to 20 mm, the time lag in the increase in the soil water δ¹⁸O occurred 2-3 days after raining, and, when precipitation was from 20 mm to 30 mm, the time lag was only 1-2 days after the rain fall event. The well-developed sub-alpine dark coniferous forest could effectively regulate rainfall by delaying the time, and rainfall became part of the subsurface water flow that helping to prevent downstream flooding. There were no significant differences in the δ¹⁸O between rainfall and throughfall (p=0.491>0.05), but there were a signficant differences between throughfall and soil waterδ¹⁸O (p=0.025<0.05). These results suggest that soil water flow was not directly supplied by the current rainfall event but was supplemented by a pre-rainfall event.

Key words: Oxygen stable isotope, Dark coniferous forest, Precipitation allocation, Sichuan