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Known as "living fossils", large old trees play an important role in ecology, landscape, and culture and are an important part of ecosystems and human settlements. The aim of this paper is to provide suggestions for the protection of urban large old tree resources and the selection of large old tree backup resources. First, we conducted a statistical analysis on the composition, distribution, and important values of large old tree species in Beijing and used Downtrend correspondence analysis (DCA) to analyze the composition of different types of habitat tree species. Second, we created a 3 km × 3 km grid within the administrative scope of Beijing, extracted the number of large old trees and tree species richness in the grid, and used geographic detectors to determine the driving factors of the spatial distribution of large old trees, as well as tree species richness differences and their interactions. A total of 40,590 large old trees in Beijing were found, belonging to 72 species, 52 genera, and 29 families. Platycladus orientalis (L.) Franco was the dominant tree species, with an importance value of 0.51. Among the different habitats, the large old trees were found in parks and temples, and the greatest tree species number of old trees were found in communities and the countryside; meanwhile, microgreen spaces had the lowest number of trees and tree species. The distribution of large old trees and tree species was mainly concentrated in the center of the city and the northwest. The distribution pattern of large old tree resources in Beijing is affected by the interaction of various factors. Social factors were the dominant in the distribution of large old tree resources in Beijing. The spatial distribution of large old trees was mainly affected by the scenic resort and historic site (SRHS), and the SRHS and gross domestic product (GDP) level were the most important factors influencing the richness of large old tree species. In addition, the functional value and characteristics of tree species determined the distribution of large old tree habitats. Therefore, the protection of large old tree resources requires developing scientific management and planning by managers, increasing investment in management and protection, and strengthening ecological culture publicity. [ABSTRACT FROM AUTHOR]

• The cavity index was introduced to quantitatively describe tree trunk decay. • Tree cavity characteristics were determined by defense mechanisms and external interference of tree species. • The outside parts of urban trees were more prone to damage, frequently leading to cavity formation. Cavity trees play a crucial role in enhancing urban biodiversity, despite limited habitat space. Understanding these aspects is of paramount importance for promoting biodiversity in urban environments. Nevertheless, there is a paucity of extensive research on the characteristics and formation mechanisms of cavity trees across diverse tree species in urban landscapes. In this study, Arbotom sonic tree tomography was used to identify cavity cores within four representative large old tree species in Beijing, encompassing two broadleaved species [ Styphnolobium japonicum (L.) Schott, n = 301; and Ginkgo biloba L., n = 100] and two conifer species [ Platycladus orientalis (L.) Franco, n = 515; and Pinus tabuliformis Carr., n = 404]. We introduce a cavity index to quantify tree decay and explore cavity characteristics and formation mechanisms across different tree species. Our results reveal that the cavity index is primarily linked to diameter at breast height (DBH), tree age, and trunk health in the four large old tree species studied. The formation of cavities in trees is primarily attributed to external disturbances that lead to tree damage. Subsequent fungal infections occur when a tree's defense mechanisms fail to prevent them, resulting in trunk decay and the formation of cavities. It is worth noting that different tree species exhibit varying defense mechanisms, with coniferous species generally displaying stronger defenses than broad-leaved species. In summary, the formation of cavity trees in urban landscapes is influenced by a combination of internal and external factors, with the tree species' defense mechanisms and the extent of external disturbances determining cavity characteristics. The findings indicate a higher likelihood of cavities in the outer parts of large old trees in southern regions compared with those in northern regions. Coniferous species also exhibited fewer cavities than broad-leaved species. Further, urban trees are more susceptible to external damage that can lead to cavity development, thereby necessitating attention to their growth environment and morphological conditions. However, understanding these factors can aid managers in crafting conservation policies that enhance urban biodiversity and provide habitats for urban wildlife, thereby maximizing the societal benefits. [ABSTRACT FROM AUTHOR]