The 5 layers of a tree trunk

The anatomy of a tree would really interest even the most apathetic person. That’s because trees are complex parts of nature that provide an endless list of benefits and an even greater amount of value! There are thousands of tree species in the world, all in different shapes, sizes, colors, yields, and more. But one thing they all have in common: a trunk. All trees, both deciduous and coniferous, have a tree trunk in one form or another. And one of the most interesting facts about tree trunks is that they have 5 different layers! Read on to learn more about each layer of a tree trunk and the purpose they serve.

tree trunks

As mentioned, tree trunks have 5 separate layers. They are the outer crust, the inner crust (bast), the cambium cell layer, the sapwood, and the heartwood. Each layer has its own purpose, but in general, the main job of the trunk is to protect and support the tree. Watch below to review each layer and what they do.

outer bark:

Like a shield, the outer bark of a trunk is there to protect the tree from its outside environment, including inclement weather, wildlife, pests, and more. It also controls humidity, preventing excess moisture from rain and snow and retaining sufficient levels of moisture during dry seasons. It also provides insulation in cold weather and protects against sunburn in the summer.

Phloem (inner bark):

The phloem, or inner layer of the bark, is where food and nutrients pass through the tree. This layer has a very important job, but a very short useful life. It eventually dies, turns into cork, and becomes part of the outer layer of the bark!

Cambium Cell Layer:

The cambium cell layer is interesting because it is the part of the trunk that grows. Each year, this layer produces more bark and wood in reaction to hormones that are transmitted from the leaves along the alimentary canal. These hormones are called auxinsAnd they are very important because they stimulate the growth of new cells!


Sapwood is new wood and plays an important role as the tree’s water pipe, supplying water to the entire tree. And as new sapwood is created, the inner cells lose their vigor and turn into heartwood.


The heartwood is the innermost part of the trunk. It plays an important role in a tree’s balance, stability, and safety. Technically, the heartwood is dead, but it does not atrophy or rot (unless the outer layers are compromised). It is made of hollow, needle-shaped cellulose fibers that are held together by a glue-like chemical called lignin.

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