Mechanisms of trunk cavity formation of large old trees in urban landscapes: A case study on four tree species in Beijing

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.