Background Stand transpiration accounts for 50%-90% of the total forest evapotranspiration. Stem sap flow directly determines transpirations of individual tree and stand. Study on stem sap flow is helpful to scientifically understand variation of forest transpiration and accurately evaluate the water conservation service of forest. Pinus massoniana forest is the most important forest type in the shelter- forest system of the Three Gorges Reservoir Area, which plays an important role in maintaining the ecological security of the Three Gorges Reservoir.

Methods Taking Pinus massoniana forest at Tieshanping of Chongqing in the Three Gorges Reservoir area as the research project and using thermal diffusion probes to long-term continuously observe the sap flow of different individual trees in 2010—2012, temporal and radial variation of sap flow from Pinus massoniana were studied by methods of drawing the scatter plots and fitting the regression equations, and difference of sap flow among individual trees and its influencing factors were analyzed by methods of the univariate analysis of variance and the Pearson correlation analysis.

Results 1) The diurnal variation of sap flow velocity for Pinus massoniana was different in different seasons and weather. The diurnal variation of sap flow velocity having a "one-peak" pattern in sunny days in each season also presented a significant change of "peak-valley" in cloudy days in spring and summer, while it presented a low level straight line (< 0.10 g/(cm² · min)) in rainy days. 2)The annual variation of daily sap flow velocity for Pinus massoniana with obvious interannual variation generally presented a "one-peak" pattern, which generally reached the peak in July and August (>0.15 kg/(cm² · d)). The sap flow velocity presented an obvious radial variation and individual difference, which was a quadratic function of one variable (P < 0.01). 3)Dominance and leaf area index of individual Pinus massoniana had significant effects on sap flow velocity (P < 0.01), and the leaf area index was an important canopy factor affecting sap flow velocity, but tree species had no obvious effect on sap flow velocity during the monitoring period (P>0.05). 4)Sap flow velocity of most individual Pinus massoniana was positively correlated with air temperature, potential evaporation, total solar radiation, max of solar radiation intensity and wind speed (P < 0.01), and negatively correlated with relative humidity and rainfall (P < 0.01). There was a multivariate function relationship between sap flow velocity of most individual trees and meteorological factors (P < 0.01), of which the total solar radiation was the main meteorological factor affecting sap flow velocity.

Conclusions The temporal and spatial variation of sap flow velocities of individual trees in Pinus massoniana forest is obvious and comprehensively affected by dominance, leaf area index and meteorological conditions, which may provide a scientific basis for distributed evaluating transpiration of Pinus massoniana forest in the Three Gorges Reservoir area.