Leaf-level Ecophysiology And Ecosystem Carbon Flux Of A Poplar Plantation On A Sandy Soil

Popla(rPopulus×euramericana cv."74/76")is the most important plantation species in northern China. To fully understand the changes in photosynthesis at different temporal scales, respiration and transpiration under water stress are also needed. We installed a field experiment in a 3-year plantation (2x2 m) in DaXing District of Beijing in 2005. From May to October, we measured several physiological characteristics using a Li-6400 Portable photosynthesis system between 6:00 and 20:00 hours on 5-7 mature leaves in the upper part of the canopy. We found that the diurnal change in net photosynthetic rate (Pn) appeared as a single-peak curve, with daily averages decreased from May to October. The correlation analysis with environmental and physiological variables suggested that photosynthetic active radiation (PAR) and stomata conductance (Gs) were the two most significant factors influencing Pn, especially in May and August. Gs appeared more important in July, while PAR was the most important factor in September and October. Vapor pressure deficit– a combined indicator of air temperature and humidity -- was also important from August through October but not in May through July. When the underline mechanisms for diurnal changes of Pn in May were explored based on changes in stomata conductance, it appeared that Pn reached the peak value at 8:00 hour and decreased dramatically because of lowered Gs until 16:00 hour. But low PAR level seemed to be the limiting factor after the hour. The above regulations were different in June and July when stomata were closed at around 14:00 hour as indicted by high leaf respiration and low transpiration rate (Tr). From August through October, the primary factor regulating Pn was PAR. We also found that R changed in a similar pattern with air temperature, showing a strong temperature controlling mechanism. The diurnal change in Tr also showed a single-peaked pattern, with the maximum valued reached between 12:00-14:00 hour. Many factors were found to be influential on Tr, but PAR seemed to the...