Abstract:

Thermal dissipation probe (TDP) was applied to measure the sap flow dynamics of Robinia pseudoacacia in Ansai County in semi arid region of the Loess Plateau from April 26 to May 31, 2008. Soil water content, photosynthetic active radiation (PAR), air temperature, relative air humidity, water vapor pressure deficit (VPD) and wind speed were measured at the same time. The results showed that the leafflushing period in a R. pseudoacaciaplantation was divided into four stages: bud swollen, the beginning of leafflushing, the middle period of leaf flushing and the fully expanded leaf period. Sap flow velocity of R. pseudoacaciagradually increased along with the leaf flushing phenology and became stable untill the fullleaf period. Diurnal variation of sapflow velocity displayed a singlepeak curve with fastrising and slowdeclining trends, peak value in the full leaf period of leafflushing is about 0.002 7 cm·s^-1. Sap flow velocity was significantly correlated with PAR, air temperature, VPD and wind speed, and was negatively correlated with air relative humidity during the monitoring period. The correlation coefficients declined following an order of PAR, air temperature, VPD, relative humidity, and wind speed. Simple linear equations were tightly fitted with the variables of PAR and air temperature, and the regressive coefficients were highly significant. Soil water content was gradually declined around the leafflushing period, but its' influence on the sap flow was not significant. Diurnal variations of sap flow velocity for four individual trees had the same trend, and total water transpiration of each individual tree was correlated with its stem diameter. A close linear relationship was detected between total water consumption for transpiration of R. pseudoacacia tree and its diameter at breast height (DBH), which may be used to estimate total water consumption for transpiration of R. pseudoacacia plantation around the leafflushing period.

Key words: Loess Plateau, Robinia pseudoacacia, leaf flushing period, sap flow, thermal dissipation probe (TDP), climatic factors