Trunk (botany)

Clockwise from top left: Ficus subpisocarpa, Adansonia grandidieri (giant baobab), Eucalyptus deglupta, and cross-section of a one year old Quercus

Trunks are the stems of woody plants and the main structural element of trees. The woody part of the trunk consists of dead but structurally significant heartwood and living sapwood, which is used for nutrient storage and transport. Separating the wood from the bark is the cambium, from which trunks grow in diameter. Bark is divided between the living inner bark (the phloem), which transports sugars, and the outer bark, which is a dead protective layer.

The precise cellular makeup of these components differs between non-flowering plants (gymnosperms) and flowering plants (angiosperms). A variety of specialised cells facilitate the storage of carbohydrates, water, minerals, and transport of water, minerals, and hormones around the plant. Growth is achieved by division of these cells. Vertical growth is generated from the apical meristems (stem tips), and horizontal (radial) growth, from the cambium. Growth is controlled by hormones, which send chemical signals for how and when to grow.

Plants have evolved to both manage and prevent damage from occurring to trunks. Trunks are structured to resist wind forces, through characteristics such as high strength and stiffness, as well as oscillation damping, which involves taking energy, and therefore damage (by extension), out of the trunk and into the branches and leaves. If damaged, trunks employ a complex and slow defence mechanism, which starts by creating a barrier to the incoming disease. Eventually, diseased cells are replaced by new, healthy cells, once the threat is contained.

Ecologically, trunks not only support the extensive ecological function of living trees, but also play a large ecological role when the trees eventually die. Dead trunk matter, termed coarse woody debris, serves many roles including: plant and animal habitat, nutrient cycling, and the transport and control of soil and sediment. Most trees grown outside the tropics can be dated (have their age estimated) by counting their annual rings. Variations in these rings can provide insights into climate, a field of study called dendroclimatology. Trunks have been in continuous use by humans for thousands of years including in construction, medicine, and a myriad of wood-related products. Culturally, trunks are the subject of symbolism, folk belief, ritual, and feature in art of many mediums.