How do growth rings form on trees? It’s a multi-layered process that requires an understanding, not just of how growth rings form on trees, but what a tree ring does and the significance of what its results mean. And since the forming of growth rings on trees is a multi-layered process, a multi-layered answer from a reputable tree company like Mr. Tree is necessary.

Primary vs. Secondary Growth

Growth rates among different trees vary immensely, but even as it pertains to trees of the same species, growth rates are incredibly diverse based on factors like temperature and rainfall. When a tree grows, it generally grows in one of two ways: primary or secondary growth. If primary growth occurs, it results in the tree growing in height at the tips of stems and roots. This could mean the tree either gets longer or taller, depending on the type of tree and the way it’s planted.

By contrast, if secondary growth occurs, the result is a bigger diameter in the trunk or stem of the tree in the cork cambium or vascular cambium, which is located between the bark and old wood of a tree. This means the tree’s girth becomes substantially wider than it had been previously.

Phloem and Xylem

Xylem and phloem are produced in the vascular cambium, which is a thin layer of cells that creates conducting cells. The water and minerals of a tree are conducted through the xylem, which is vascular tissue found near the inside of the cambium layer. Xylem often replaces water lost during photosynthesis and transpiration, and while its sap consists largely of water, it does contain some organic chemicals too. Transport is passive and it becomes exceedingly difficult to transport upward as the tree gets taller.

By contrast, the phloem is essentially the outer bark of the tree and the tissue that conducts food. Unlike xylem, whose cells are mostly dead, phloem is made up of cells that are still alive and transport sap that’s loaded with sugars produced by photosynthesis. These sugars are brought to areas of the tree such as the storage structures and roots. When growth periods occur, storage units like the roots become sources for sugar. Whereas transport in the xylem is passive and travels upward, despite the struggle, in the phloem, transport is multidirectional.

Secondary xylem, which is what wood is essentially comprised of, is added near the cambium layer’s inside. Secondary phloem, which is primarily the bark of the tree, is the outside layer that’s made up of a thin layer of cells that comprise the cork cambium. The bark plays an important role in increasing the girth of the tree.

Growing Season

All of this is crucial for understanding the development of a tree ring. Tree rings are created from new cells that are made in the cambium just below the tree bark. When the growing season begins, growing conditions are often almost perfect and cells are both less dense and fast to grow. It’s during this period that most of the tree’s growth happens each year. So, when looking at the cross-section of a tree, this is indicated by the lighter, wider portion of a tree ring.

As the growing season continues, drought and heat become more common, and cells become denser and slower to grow, as indicated by the darker, thinner portion of a tree ring. This is when the tree’s growth typically stagnates, so if the earlier period didn’t result in strong growth, then that year’s growth ring may ultimately end up being disappointing. At this stage of the process, you can almost always find the youngest wood of the tree underneath the bark and the oldest wood of the tree in the center.

Growth rings on trees often correlate to the passing of a single growing season. Sometimes, it’s even possible for a tree to have two growth rings in one growing season if conditions occur that are both extremely dry early in the season and extremely wet later in the season. Extreme weather conditions can produce other strange results as well, as a volcano eruption could cause nearby trees to have no annual growth rings altogether. In fact, this has happened several times, including notably in Indonesia in the year 1816 when oak trees failed to grow at all as a result of the eruption of Mt. Tambora.

Tree Stories

Annual growth rings function to tell the story of a tree’s life and what occurred during each year. When you look at a tree’s stump, you’ll generally see the growth rings that represent that tree’s first three years of life close together and hard to tell apart. This is usually because during the first few years of a tree’s life, it grows somewhat slowly.

After that, though, the tree’s roots are firmly anchored into the soil, and the tree itself starts to grow very fast. These years in a tree’s life could increase its girth by an inch or so annually. This can continue for around 20 years, depending on the level of moisture during each growing season, as the tree evolves from its youth phase to adulthood, in which it can produce fruit and flowers.

But growing conditions are what ultimately tell the tale of how large or small an annual growth ring ends up being. To that end, obviously, something like a long drought would have an incredibly negative impact on growth rings, whereas consistent moisture and rainfall would produce the opposite result.

Just as growth rings can help tell a tree’s story in life, so too can they help tell a tree’s story in death. You can tell a tree is on its last legs, for example, if its growth rings begin to get narrower by the year. If this process repeats itself multiple years in a row, that means the tree’s growth rings are done getting wider permanently. What this tells you about the tree’s future survival prospects is that its health is in major decline, and the downturn is going to continue indefinitely, which will ultimately result in the tree’s death.