Studying the photosynthetic characteristics,carbon fixation,and oxygen release capacity of tree species,along with screening the dominant species for carbon fixation and oxygen release,provides important guidance for the development of forestry carbon sinks and ecological environment construction.This study focused on 12 tree species that are predominant in Baotou City,and their photosynthetic physiological parameters were measured using a CI-340 portable photosynthesis system.Pearson correlation and systematic cluster analysis were used to explore the diurnal variation in the net photosynthetic rate of tree species and its relationship with environmental factors.Additionally,and the carbon fixation and oxygen release capacity of tree species was evaluated.1)Among the 12 tree species,Ulmus pumila,Pinus sylvestris var. mongholica Litv.and Picea asperata showed unimodal diurnal variation of the net photosynthetic rate.The bimodal tree species are Pinus tabuliformis,Populus simoniiand and Reaumuria songarica.2)Tree species with a single peak of water use efficiency were Picea asperata,Juniperus rigida and Pinus tabuliformis.The tree species with a bimodal type are Sorbaria sorbifolia;the tree species that decreased first and then increased were Ulmus pumila,Pinus sylvestris var.mongholica Litv.and Platycladus orientalis.3)The tree with the highest net photosynthetic rate was Ulmus pumila.while the shrub with the highest net photosynthetic rate was Tamarix chinensis.The tree with the strongest water use efficiency is Pinus sylvestris var.mongholica Litv.,and the shrub with the strongest water use efficiency is Caragana tibetica.4)Net photosynthetic rate was significantly positively correlated with both transpiration rate and stomatal conductance.5)The carbon fixation and oxygen release capacity of tree species in Baotou City can be classified into three grades according based on strength:Ulmus pumila and Populus simonii are grade I;Pinus sylvestris var.mongholica Litv.is grade II;and Picea asperata.,Platycladus orientalis,Pinus tabuliformis and Juniperus rigida are grade III.The carbon fixation and oxygen release capacities of shrub species were classified into three grades:Tamarix chinensis was grade I;Juniperus sabina and Sorbaria sorbifolia are grade II;and Reaumuria songarica and Caragana tibetica Kom.are grade III.It is recommended to select Ulmus pumila,Populus simonii and Tamarix chinensis as the preferred tree species for carbon fixation and oxygen release.In areas with poor water conditions,Pinus sylvestris var.mongholica Litv.,Picea asperata and Caragana tibetica are recommended.