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Why are trees considered an artificial category in scientific classification?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

Trees are considered an artificial category in scientific classification because the term is a human concept based on visual criteria rather than a formal phylogenetic or taxonomic group.1

From a scientific perspective, the classification of a plant as a "tree" is not an immutable biological category for the following reasons:

Lack of Phylogenetic Basis: Trees are found across many diverse plant families, including club mosses, ferns, gymnosperms, and angiosperms. Because these forms appear in both primitive and highly specialized lineages, the "tree" form does not represent a single evolutionary descent from a common ancestor.12
Environmental Plasticity: A plant's growth habit can be shaped by its environment. A species that grows as a tree in one habitat may assume a shrub-like form in another due to stresses such as altitude, temperature, or oxygen tension.1
Adaptive Strategy: While not a taxonomic group, the tree is recognized as a natural construct on an ecological basis. It represents a successful adaptive strategy used by many unrelated taxa to dominate the habitat above the ground by developing specialized conducting and supporting tissues.1