Canopy transpiration is an important component of the water balance in forest ecosystems. Quercus acutissima and Cunninghamia lanceolata are two important, fast-growing and commercial tree species that have been extensively used for vegetation restoration, water conservation and building artificial forests in important regions of southern China. The primary objective of this study was to characterize sap flow densities of the two species by comparing diurnal, nocturnal and seasonal sap flow patterns and their relationships with environmental factors. Sap flow densities (Sd) were measured between September 2012 and August 2013 using the commercially-available thermal dissipation probes. Hourly meteorological data were measured in an open field, located 200 m away from the studied stand. Standard meteorological data were logged hourly at this site, including photosynthetically active radiation (PAR), air temperature (Ta), relative air humidity (RH), Vapor pressure deficit (Vpd) and precipitation (P). Soil water content (Swc) data were logged hourly in different layers at Q. acutissima and C. lanceolata forests.
Results/Conclusions
The mean Sd of Q.acutissima in summer was significantly higher than in spring and autumn, and was lowest in winter. The mean Sd of C. lanceolata showed no significant difference among seasons. The Sd of Q. acutissima showed distinct diurnal rhythms during the growing season (between May and October), whereas C. lanceolata followed similar sap flow patterns in all months except February. Nocturnal sap flow density (Sdn) was noticeable and both species followed similar patterns during the growing season, in which Q. acutissima followed a power function from April to November and C. lanceolata followed similar patterns in all months except February. Principal component analysis (PCA) and Pearson correlation analysis suggested that the leading environmental factors that influenced the Sd were difference in different growing stages. The diurnal sap flow density (Sdd) was more sensitive to environmental factors than Sdn. The Sd during the growing season was more sensitive to environmental factors than in the dormant period. Ta, Vpd and PAR were significantly correlated with Sdd. The leading environmental factors that influenced the hourly Sdn of Q. acutissima were Vpd and Swc of 60cm soil layer in growth period and Ta and Swc of the top 10cm soil layer in dormant period, respectively. The hourly Sdn of C. lanceolata in growth and dormant period all corresponded closely to Ta and Vpd. The results of this study can be used to estimate the transpiration of Q. acutissima and C. lanceolata.