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Why can the same woody plant species grow as a tree in one habitat and a shrub in another?

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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.

The classification of a woody plant as a tree or a shrub is not always fixed, as the same species can exhibit different growth forms depending on its environment. This variation occurs primarily due to phenotypic plasticity, which is the capacity of a single genotype to produce different morphological expressions under varying environmental conditions.12

The shape and size of a woody plant are ecological constructs dependent on habitat stresses, such as temperature, light availability, and soil quality. In favorable conditions, a plant may develop a dominant trunk and reach the height of a tree. Conversely, in extreme or limiting environments—such as high elevations, subarctic regions, or areas with poor resources—the same species may assume a stunted, shrub-like, or arborescent form to survive.21

Plants adapt to these constraints through physiological and morphological responses, including the reallocation of nutrients and water to the organs most in need. Consequently, factors like climate and resource competition dictate whether a species reaches its full potential as a tree or remains a smaller, multi-stemmed shrub.21