The 5 layers of the tree trunk

The 5 layers of the 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 even more value! There are thousands of tree species in the world, all with different shapes, sizes, colors, yield and much more. But they all have one thing in common: a trunk. All trees, both deciduous and coniferous, have a 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.


As mentioned, tree trunks have 5 distinct layers. They are the outer crust, the inner crust (phloem), the cambium cell layer, the sapwood and the pith. Each layer has its own purpose, but overall the main task of the trunk is to protect and support the tree. Look below to review each layer and what they do.

Outer bark:

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

Phloem (inner bark):

The phloem, or inner layer of the bark, is where food and nutrients travel through the tree. This layer has a very important task, but with a very short life. Eventually it dies, turns into cork and becomes part of the outer layer of bark!

Cambial cell layer:

The cambium cell layer is interesting because it is the part of the stem that grows. Each year, this layer produces more bark and wood in response to hormones transmitted by 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 plumbing, delivering water throughout the tree. And as new sapwood is created, the inner cells lose their strength and turn into pith.


The heartwood is the innermost part of the trunk. It plays an important role in the balance, stability and security of a tree. Technically, the pith is dead, but it does not atrophy or decay (unless the outer layers are threatened). It consists of hollow, needle-like cellulose fibers that are bound together with a glue-like chemical called lignin.

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