[Evapotranspiration of natural Quercus litaotungensis and Tilia paucicostata secondary stands in Liupan Mountains of Ningxia].
With heat dissipation probe technique and combined with microlysimeter and hydrological methods, this paper studied the evapotranspiration of secondary Quercus liaotungensis and Tilia paucicostata stands, and its relationship to forest structure from August to September 2004. The results indicated that the stem sap flux density (SFD) of Quercus liaotungensis and Tilia paucicostata changed regularly from day to night in later growth season. In relatively still period (nighttime), the SFD kept low values continuously, usually below 0.05 microl x cm(-2). min(-1), while in active period (daytime), it increased quickly, usually below 0.25 microl x cm(-2) x min(-1). The daily transpiration of whole-tree was estimated based on the calculation of daily cumulative SFD, which showed that there existed a significant difference between two tree species. The daily transpiration of Quercus liaotungensis reached 5.31 and 2.48 L x d(-1) in sunny- and cloudy-days, 2.35 and 3.75 folds as that of Tilia paucicostata, respectively. There was no significant difference in daily leaf transpiration rate between these two shrub species. During measurement periods, the average daily stand evapotranspiration was 1.45 mm x d(-1), including transpiration (0.72 mm x d(-1)), soil evaporation (0.19 mm x d(-1)), and canopy interception (0.54 mm x d(-1)), which accounted for 49.6%, 13.3% and 37.1% of the total evapotranspiration, respectively. The results showed that the difference of whole-tree transpiration between tree species contributed significantly to the difference of tree canopy transpiration, while the difference of shrub canopy transpiration was on account of the leaf amount in the canopy. The effects of vertical layers in the stand on total stand evapotranspiration also differed, with a percentage of 65.8%, 20.9% and 13.3% for tree canopy, shrub canopy and floor, respectively, which meant that the transpiration and interception of tree canopy contributed most to the total evpotranspiration, followed by shrub transpiration, soil evaporation, and grass transpiration.