Characteristics of Nighttime Sap Flow and Its Partition in a Mixed Forest in Jiuzhaigou Valley

YAN Chunhua, WANG Bei, ZOU Zhendong, YU Leiyu, HUANG Wanbin, QIU Guoyu†

School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055; † Corresponding author, E-mail: qiugy@pkusz.edu.cn

Abstract To investigate the characteristics of nighttime sap flow and its partition into nocturnal water refilling and transpiration, the Granier-type thermal dissipation probes were used to measure sap flux density in 2013 in three main species of a mixed forest in Jiuzhaigou Valley. The results showed that the ratio of nighttime sap flow to daily value mainly ranged from 0 to 30% for Betula albosinensis Burk. and Acer ginnala Maxim., from 0 to 25% for Pinus tabulaeformis Carr., and might excess 40% in some special occasions. It showed a “U-shaped” seasonal variation, with lower ratios in the mid-growing season and higher ratios in the early and late growing season. There was a significant linear relationship between nighttime sap flow and vapor pressure deficit during a clear day, indicating the occurrence of nocturnal transpiration along with refilling. The ratios of nocturnal water refilling and transpiration to the total nighttime sap flow were 80.7% and 19.3%, 81.4% and 18.6%, 63.9% and 36.1%, respectively for the three tree species. Key words nighttime sap flow; nocturnal water refilling; nocturnal transpiration; thermal dissipation probe; mixed forest; Jiuzhaigou Valley

植物夜间液流指夜间土壤水分进入根系后, 通过输导组织运送至植物冠层, 最终存储在植物茎干(即夜间茎干补水)或者通过气孔蒸腾到大气中(即夜间蒸腾)的过程。由于过去的液流技术难以准确地测低液流速率, 微气象技术无法准确地测量夜间湍

流数据[1], 因此之前的研究大多认为夜间植物气孔处于关闭状态[2], 植物不存在夜间液流现象。近年来, 随着液流技术和微气象技术的发展, 对不同物种、不同生态系统和不同生境条件下的野外观测表明, 植物夜间液流占全天液流的 5%~25%[3]。