The four levels of accreditation recognize arboreta at various degrees of development, capacity, and professionalism. Accredited arboreta are encouraged to seek and achieve higher professional standards and move up through the levels. Categorization also allows an assessment of which arboreta are capable of participating or collaborating in certain scientific, collections, or conservation activities.
Level I arboreta are generally smaller publicly accessible sites with at least 25 species of woody plants, one or a few employees or volunteers, a governing body, and an arboretum plan. Examples of arboreta at this level may include golf courses, college arboreta, cemeteries, zoos, private estates, or towns with an organized tree collection.
Level II arboreta have at least 100 species of woody plants, employ paid staff, and have enhanced public education programs and a documented collections policy.
Level III arboreta have at least 500 species of woody plants, employ a collections curator, have substantial educational programming, collaborate with other arboreta, publicize their collections, and actively participate in tree science and conservation.
At the highest level of accreditation, Level IV arboreta employ well-qualified tree scientists engaged in publishing sophisticated research, manage living tree collections for the purpose of conservation, and take an active role in supporting tree conservation through the Global Trees Campaign. Level IV arboreta are world-renowned tree-focused institutions.