https://books.google.com/books?id=4UsHOAGc5x0C&pg=SA1-PA4 page 1-4, section 1.3 Of the many variations in form of the Eucalyptus (shrubs, mallees, trees, etc.), virtually all species being tested in the above category are those which exhibit good tree form (e.g., height, straightness of trunk, etc.). […] https://books.google.com/books?id=4UsHOAGc5x0C&pg=SA2-PA1 page 2-1, section 2.2 Shrubby eucalypts are usually referred to as "mallees" although the term strictly applies to those species which have many stems growing from an enlarged rootstock (lignotuber).