[Stem photosynthesis and its main influencing factors of Haloxylon ammodendron and Tamarix ramosissima.]

Stem photosynthesis (Pg) is an alternative and significant carbon source, playing a crucial role in plant survival under extreme environment. The main aims of this study were to quantify stem and leaf photosynthesis, find out the main drivers of Pg, and estimate the contributions of Pg to plant individual carbon balance of two dominant species Haloxylon ammodendron and Tamarix ramosissima in Gurbantunggut Desert. A Li-Cor 6400 portable photosynthesis system and a special chamber were used to measure leaf and stem photosynthesis. Ancillary measurements included leaf/stem functional trait (chlorophyll content, water content, leaf/stem area, carbon/nitrogen content, etc.) and environmental factors (air temperature and humidity, photosynthetically active radiation, soil temperature, and soil water content). Our results showed that Pg of H. ammodendron and T. ramosissima was 2.37 and 0.98 μmol·m-2·s-1, Pg refixation CO2 of stem respiration by 65%-76% and 57%-77% in H. ammodendron and T. ramosissima. Pg was influenced by photosynthetically active radiation, air temperature, soil temperature and water vapor deficit. Pg assimilation CO2 accounted for 8.2%-16.6% and 3.6%-8.3% of CO2 assimilation of H. ammodendron and T. ramosissima, respectively. The maximum value appeared at noon when temperature was high. There might be fundamental defects if we ignore the contribution of branch photosynthesis when predicting carbon process of desert ecosystem under the background of climate change.