Tree Physiology

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.

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.

Studies of wood growth in common tree species across the United States generally indicate that springwood (earlier growth) and summerwood (later growth) layers have different thicknesses, influenced by both species and environmental conditions. Springwood forms during periods of active growth and has larger cells with thinner walls, making it typically wider than the denser, thicker-walled summerwood, which forms as growth slows later in the season.
For instance, in Douglas-fir, average ring widths for combined springwood and summerwood range from 0.094 to 0.471 inches, with an average of about 0.261 inches across multiple annual rings . In southern pines like loblolly and slash pine, juvenile growth tends to produce wider rings with a high proportion of springwood, leading to greater yearly trunk diameter increases in younger trees. As trees age, the diameter increase per year decreases, with more defined summerwood layers and less annual ring width overall.

Growth rates are also highly variable based on environmental factors like soil moisture, which can impact cell wall thickness and the transition from springwood to summerwood. For example, high soil moisture in the growing season can expand springwood, while reduced moisture supports the formation of denser summerwood cells .

These growth dynamics are essential to wood density and strength properties, especially in timber species widely used in construction and forestry. For a detailed analysis by species and age, many forestry and environmental journals provide precise data, often specific to growth studies under controlled conditions.

Michael WeiNovember 10, 2024