Winter photosynthesis of trees was well studied for boreal,Mediterranean,and some temperate forests,while little is known about the forests from tropical-subtropical transition zones and subtropical areas.Forests in these areas are subject to seasonal low temperatures and probably seasonal water deficits.Evergreen broadleaf trees dominate the forests from elevation 1000 to 2600m in the subtropical area of SW China,while forests in eastern parts of Chinese subtropical areas with similar elevations are dominated by deciduous broadleaf trees.In order to understand the eco-physiological mechanisms in explaining this unique pattern and the importance of winter carbon assimilation for evergreen broadleaf trees to establish dominance in subtropical montane forests in SW China,seasonal dynamics in photosynthesis of 10 evergreen broadleaf tree species from a montane forest in SW China (Mount Ailao) was studied.Environmental factors were monitored and low temperature effects on photosynthetic system I and II (PSI and PSII) were also studied to reveal the underlying environmental and eco-physiological mechanisms.Leaves of all the 10 species studied experienced freezing in winter but their PSI and PSII could tolerate extracellular freezing.All of the 10 species down-regulated maximum photosynthetic rate by 13 to 53% in winter,depending on species.Maximum quantity of PSI decreased by 25 to 53% while maximum quantum efficiency of PSII deacreased by 6 to 20% respectively,depending on species.However,all the tree species could maintain fairly high winter carbon assimilation,and the monthly carbon accumulation in winter was 60 to 70% of that in summer.During the winter,low temperature effects on photosynthesis were compensated by relatively high levels of incoming solar radiation.Low precipitation in winter was compensated by fog persistence and lowered evapotranspiration,and trees did not experience water deficits in winter (predawn water potentials > -0.2MPa).The considerably high winter carbon assimilation of evergreen broadleaf trees in this area may convey advantages for them to establish dominance and allow these forests to sequestrate more carbon.