Objective With the development of urbanization and industrialization, greenhouse gas emissions continue to increase, and the issue of global warming has attracted more and more attention. Actively responding to the issue of climate change, China has put forward the carbon peaking and carbon neutrality goals for carbon emission reduction. Landscaping trees play an important role in increasing urban natural carbon sinks and neutralizing urban anthropogenic carbon emissions. To maximize the carbon sink of the limited urban green spaces, it is necessary to choose landscaping tree species with high carbon sequestration capacity. Beijing is facing various environmental problems, such as severe urban island heat effect caused by vast greenhouse gas emission, and air pollution. The low-carbon landscaping construction practice in Beijing can not only alleviate urban problems, but also improve the human settlement environment, making people live healthier and happier. Furthermore, such practice in Beijing, the capital city, can set a significant example for other cities. Therefore, the research on carbon sequestration capacity of common landscaping tree species in Beijing is of great importance, especially considering that relevant research is still not sufficient and complete at present.

Methods According to the principles of ecological adaptability and representativeness, this research mainly considers tree species seldom involved in the current research (Vitex negundo var. heterophylla, etc.), while highly recommended in Beijing in recent years (Chionanthus retusus, etc.), and then takes 30 common landscaping tree species in Beijing urban green space as the research object. The research selects 5 standard plants for each specie. All the selected plants are located in the open spaces (without shade) of urban parks in Beijing, which are far away from water bodies and buildings and have similar habitats. The photosynthetic physiological parameters were monitored with Li-cor 6400 portable photosynthetic analyzer in four seasons in 2022 and 2023. Based on the monitoring results, the average daily photosynthetic rate in different seasons is calculated. According to the assimilation amount method, the annual carbon sequestration per unit leaf area, per unit land area and per plant are calculated respectively. Moreover, the carbon sequestration capacity of tree species is graded by cluster analysis, and the differences in carbon sequestration capacity between tree species with different life forms are tested by independent-sample t.

Results Among the tested tree species, the daily average photosynthetic rates of evergreen and deciduous tree species in different seasons follow the same order: Summer > spring > autumn > winter. According to the indicators of annual carbon sequestration per leaf area, annual carbon sequestration per land area and annual carbon sequestration per plant, the tested tree species can be divided into levels I, Ⅱ and Ⅲ (from high to low), respectively. The carbon sequestration capacity of Ulmus pumila is outstanding, which belongs to level Ⅱ and above in all the three indicators. The comprehensive carbon sequestration capacity of the tested tree species is evaluated based on annual carbon sequestration per unit land area and per plant, and the performance of Salix matsudana (among all the tested arbors) and Prunus tomentosa (among all the shrubs) is the best. Among all the tested tree species, the annual carbon sequestration per plant of arbors is significantly higher than that of shrubs, but there is no significant difference between the two in terms of both the annual carbon sequestration per unit land area and the annual carbon sequestration per unit leaf area. The reason why there is no significant difference in the annual carbon sequestration per unit land area between arbors and shrubs may be related to the application forms.

Conclusion In the practice of low-carbon landscaping in Beijing, the application of landscaping tree species should be comprehensively considered from multiple perspectives, including ecological adaptability, ecological benefits, aesthetics, etc. On the basis of fulfilling other functional needs, from the perspective of carbon sink, landscaping tree species with high carbon sequestration capacity should be fully applied. The arbors such as Salix matsudana, Ulmus pumila, Paulownia tomentosa, Eucommia ulmoides, Aesculus chinensis, and shrubs such as Prunus tomentosa, Vitex negundo var. heterophylla, Viburnum opulus subsp. calvescens and Rosa xanthina are highly recommended. Future research is supposed to further discuss the differences in carbon sequestration capacity between the colored-leaf plant species and its original plant species, and carbon budget in the whole life cycle of landscaping tree species, which can further provide a scientific basis for the construction of low-carbon landscape in Beijing, thus helping achieve the carbon peaking and carbon neutrality goals.