Sap flow of Robinia pseudoacacia and Quercus variabilis in the city west mountain area of Beijing City was systematically measured from May to September of 2008 using thermal dissipation sap flow probes

combined with environmental factors.Diurnal variation of sap flow displayed a wide-peaked or double-peaked curve on sunny days and a multiple-peaked curve on overcast and rainy days.In May and June

the averaged sap flow velocities of R.pseudoacacia were 1.58 and 1.08 times slower than those of Q.variabilis

respectively

and in July

August

and September

1.391

1.408

and 1.450 times higher

respectively.In May and June

sap flow velocities of Q.variabilis were significantly faster at upper heights than median and lower heights and from July to September

faster at lower heights than median and upper heights on typical sunny days.During the season of rapid growth

sap flow velocities of both tree species were positively correlated with solar radiation intensity

atmosphere water potential

and vapor pressure deficit and negatively correlated with soil water potential.Multiple linear regression equations for environmental factors with sap flow velocity were established

which all met the significance at 0.01 levels with F test.