A Tree Story - Reading Between the Rings

You likely already knew that counting the dark rings of a cross-section of a tree will determine how old that tree is, but did you know that they show much more than just its age? Read between the rings and you might just find this tree’s whole life story.

I became intrigued with the life of this particular Oak tree while counting its growth rings. It was evident that it had really hard conditions to survive in. Life isn’t always a breeze, for a tree. Many environmental conditions impact a tree leaving their mark on the tree’s growth rings - wind events, excessive rain, drought, thinning, injuries, insect infestation, disease and more.

Figure 1: area within the circle represents 30-35 years of growth

Oak trees, in particular, don’t grow well under the shade of other trees. Being starved of sunlight, they struggle to grow. Slow growth (narrow growth rings) is an indication of a lack of sunshine. You can see, that for the first 30 to 35 years of this tree’s life, it only grew 2-1/2-3 inches in diameter. (refer to Figure 1)

Then, suddenly, its conditions changed. This is evident in the shift in the pattern of the growth rings at this stage of it’s life. My best guess is that one of two things happened, either logging or a strong wind event downed some of the surrounding trees. This opened it up to more sunlight but in the process the tree got bent over at a severe angle. Now it had to struggle to upright itself to reach for the sun. As the years continued, the increased branching and leaves of the canopy caused more and more stress on the bent over trunk. Each year, with added growth, it would have more weight to support.

When a tree is faced with these conditions it must respond properly to keep it’s trunk from cracking and breaking. Picture this Oak bent over at a severe angle, the trunk begins to form an arch as it reaches for the sunlight. This gradual bending of the trunk is commonly referred to as a sweep, or “banana” tree, due to it’s shape. This bending causes opposing tensions on the upside and underside of the sweep. The upside of the sweep is known as tension wood while the underside is known as compression wood. Hardwood trees such as this Oak respond by ramping up growth on the tension side of the sweep. While, interestingly, softwood trees such as conifers, ramp up growth on the compression side of the sweep.

Figure 2: Tension wood growth (upside) - 20” growth/65 years; Compression wood growth (underside) - 2-1/2” to 3” growth/65 years

In this cross-section it is evident that the tree was leaning, as it formed tension wood on the upside of the sweep to enable it to support the weight of the canopy. The wider rings on the tension side of the trunk indicate that growth was faster here. The resultant new growth had to respond to these demands for the tree to survive.

So for the next, approximately, 65 years this Oak added about 20 inches of growth on the upside of the sweep and only about 2-1/2-3 inches on the compression side. (refer to Figure 2)

This incredible tree was able to adapt and survive under those extreme conditions for about 100 years before falling victim to the elements, or possibly someone’s chainsaw.

If trees could talk, what a tale it might have to tell.