Zelkova trees belong to the species Ulmus, which is a genus within the family Ulmaceae. This genus includes both deciduous and evergreen species, with Zelkova serrata being the most well-known deciduous species. The classification of Zelkova within the Ulmus genus is based on several botanical characteristics, including leaf shape, flower structure, and wood properties. According to the Angiosperm Phylogeny Group (APG) classification system, which is widely accepted in the botanical community, Zelkova is grouped with other Ulmus species due to these shared characteristics.

The APG classification system, which was last updated in 2016, is based on extensive molecular phylogenetic studies. These studies analyze the genetic relationships between different plant species by comparing their DNA sequences. The results of these studies have led to a more accurate classification of plants, including trees like Zelkova. The data from these studies indicate that Zelkova is closely related to other Ulmus species, such as the elm trees, which further supports its placement within the Ulmus genus.

Additionally, the morphological similarities between Zelkova and Ulmus species, such as the leaf shape and flower structure, provide further evidence for their classification within the same genus. For example, Zelkova leaves are typically ovate with a pointed tip and a toothed margin, which is consistent with the leaf shape of other Ulmus species. Similarly, the flowers of Zelkova are small and greenish, with a four-lobed calyx and four sepals, which is a characteristic shared by many Ulmus species.

In conclusion, Zelkova trees belong to the species Ulmus within the family Ulmaceae. This classification is supported by molecular phylogenetic studies and the morphological similarities between Zelkova and other Ulmus species. The APG classification system, which is based on these studies, provides a comprehensive framework for understanding the evolutionary relationships between different plant species, including Zelkova.