What is a Tree? How Does it Work? — #Colorado State Forest Service

Limber pine. Photo credit: Colorado State Forest Service

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Every year, trees grow two annual rings. In the spring, the usually wider and thinner-walled layer, called springwood, grows. In the summer, a thicker-walled layer, called summerwood, develops. Annual rings are typical in temperate forest trees.

Tree Physiology

  • A tree is a tall plant with woody tissue. Trees gather light for photosynthesis through their leaves; this process creates “food” for the tree.
  • Most of a tree trunk is dead tissue and serves only to support the weight of the tree crown. The outside layers of the tree trunk are the only living portion. The cambium produces new wood and new bark.
  • The band of tissue outside of the cambium is the phloem. Phloem transports new materials (the sugars created from photosynthesis) from the crown to the roots. Dead phloem tissue becomes the bark of a tree.
  • The band of tissue just inside of the cambium is the xylem, which transports water from the roots to the crown. Dead xylem tissue forms the heartwood, or the wood we use for many different purposes.
  • Every year, trees grow two annual rings. In the spring, usually a wider and thinner-walled layer called springwood forms. In the summer, a thicker-walled layer, called summerwood, develops. Annual rings are typical in temperate forest trees.

Parts of a Tree

  • Leaves – broadleaf or needles; primary location for photosynthesis and production of hormones and other chemicals.
  • Twigs and Branches – support structures for leaves, flowers and fruits.
  • Crown – the upper part of the tree composed of leaves, twigs, branches, flowers and fruit.
  • Flowers – the site of reproduction. Trees can be male, female or both. Conifers, however, do not have petals and typical flower structures.
  • Fruits and Seeds – all trees have seeds, most are inside of the fruit.
  • Trunk – generally a single “stem,” but can be multiple-stemmed. Main functions are materials transport and support.
  • Bark – main function is to protect the living tissue called cambium from damage.
  • Roots – two main functions: (1) collect nutrients and water and (2) anchor the tree.

Trees Grow:

  • At the twig tips (apical meristem)
  • At the root tips (root apical meristem)
  • At the cambium (old xylem cells become heartwood, old phloem cells become bark)

Why do Leaves Change Color in the Fall?

Chlorophyll production goes down as night length increases (fall and winter). The green colors are no longer reflected and other chemicals in the leaf become dominant, revealing red and yellow pigments. Weather during the period of declining chlorophyll production influences intensity of colors.

  • Warm fall weather generally reduces color quality.
  • Moist soils following a good growing season contribute to greater color intensity.
  • A few warm, sunny days and cool nights increase brilliance.
  • Drought usually results in poorer displays.

Leaves fall in autumn as part of a tree’s preparation for winter dormancy. Because it is too cold for water to remain in the plant tissues (freezing water would rupture cells in the tree), and because the water in the soil is frozen and cannot be absorbed, trees shut down major processes in the cold months. Deciduous trees drop their leaves; conifers have strategies to maintain their needles during the winter.

Concentric rings of various widths mark the annual growth of trees. Photo by Peter Brown, Rocky Mountain Tree-Ring Research. Photo credit: NOAA

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