A tree is a perennialwoody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to 6 m; some authors set a minimum of 10 cm trunk diameter (30 cm girth). Woody plants that do not meet these definitions by having multiple stems and/or small size are called shrubs. Compared with most other plants, trees are long-lived, some reaching several thousand years old and growing to up to 115 m (379 ft) high.
Krummholz or Krumholtz formation (German: krumm, "crooked, bent, twisted" and Holz, "wood") — also called Knieholz ("knee timber") — is a particular feature of subarctic and subalpinetree line landscapes. Continual exposure to fierce, freezing winds causes vegetation to become stunted and deformed. Under these conditions, trees can only survive where they are sheltered by rock formations or snow cover. As the lower portion of these trees continue to grow, the coverage becomes extremely dense near the ground.
Axel Erlandson (December 15, 1884 – April 28, 1964) was a Swedish American farmer who shaped trees as a hobby, and opened a horticultural attraction in 1947 advertised as "See the World's Strangest Trees Here," and named "The Tree Circus."