The trunk, often called the bole, is the main woody axis of a tree that connects the roots to the crown and supports the branches. In everyday and commercial contexts a severed trunk is called a log. In scientific usage — for example in botany — the trunk is the principal structural member that bears mechanical loads and conducts water, nutrients and photosynthates between roots and leaves. The outer surface of the trunk is covered by bark, which varies widely in texture, thickness and color among species.

Basic structure and layers

  • Outer bark: the protective, nonliving layer that shields the inner tissues from physical damage, pathogens and desiccation (bark).
  • Inner bark (phloem): living tissue that transports sugars from leaves toward roots and storage organs.
  • Vascular cambium: a thin, actively dividing layer of cells that produces new xylem (wood) inward and phloem outward (vascular cambium).
  • Sapwood (outer xylem): functional xylem that conducts water and dissolved minerals upward from the roots.
  • Heartwood (inner xylem): older xylem that is often darker and provides structural support but no longer transports water.

Growth, form and physiology

Trunk diameter increases primarily through the activity of the vascular cambium; seasonal rates produce growth rings visible in cross section. The trunk also develops vertical and radial patterns — including taper, buttresses or fluting — that reflect genetics, site conditions and mechanical stresses. Specialized reaction wood (tension or compression wood) forms to correct leaning stems. Trunks contain living tissues that participate in wound response, storage and internal transport, and they play a central role in a tree's carbon balance.

Uses, measurement and ecological value

Trunks are the principal source of timber and wood products because of their straightness, uniformity and volume. Foresters commonly measure trunk diameter at a standard height to estimate biomass and timber yield. Beyond human uses, trunks provide habitat (cavities, bark surface, epiphytes), structural support for ecosystems, and long-term carbon storage in forests.

Variation, threats and management

Trunk form differs by species, age and environment: some tropical trees develop wide buttresses; others invest in thick, protective bark for fire resistance. Trunks are vulnerable to pests, fungal decay, mechanical injury and girdling; management practices such as proper pruning, wound treatment and monitoring reduce damage. When harvested, a trunk becomes a log graded for different uses, from construction lumber to pulp.

Understanding trunk anatomy and growth is fundamental to disciplines from arboriculture and forestry to ecology and wood science. For more technical details on tissues, measurement methods and species-specific variations see related resources and field guides (main stem, vascular cambium, species differences, root connections).