Spring is transpiring

How do you know it’s spring in Colorado? The days have become longer, flower buds are opening and snow is on the ground. After we experience those first few days over 70 degrees, a tense expectation can linger in the air. “OK, when is it going to snow?” It turns out the “bomb cyclone” wasn’t too bad this time. Maybe we can all breathe a sigh of relief?

For many of us the slow greening of the land and the awakening of dormant plants in the spring beckons us to get outside. It also happens to be a great time to check in on your trees to see what they are up to.

If you look closely at trees in our area you might notice small linear scars and swollen bumps on the upper sides of the twigs and branches. These are the marks left by last year’s hail storms. As trees grow this year they will continue to develop wound wood over these old injuries. In fact trees don’t actually heal from wounds but instead compartmentalize or close off damaged cells. This is referred to as CODIT or compartmentalization of decay in trees. The final step of this process is for the tree to grow over the wounds with living tissue.

Since water is the greatest limiting factor for tree growth, it helps to give the roots of trees a good soaking in the spring. This helps them get enough water to the newly forming leaves. The upward movement of water in trees is in many ways the engine that allows the other functions like photosynthesis to take place. As the leaves and buds expand in the spring, their cells are being filled with water.

Transpiration is the process of water moving from the soil up through the roots and stem and evaporating, mainly through stomata in the leaves. Stomata, the tiny openings on the surfaces of leaves, are opened and closed by guard cells. This opening allows for water to evaporate and the absorption of CO2 which is required for photosynthesis. It’s a seemingly miraculous process to contemplate that a mature cottonwood tree can move more than 100 gallons of water up to the top of its crown in one day. The bulk of this water evaporates from the leaves, while about one percent of the water transpired by a tree is actually used for photosynthesis.

There is an easy way to prove transpiration and see how much water is evaporated from leaves. A plastic bag can be tied around a group of leaves on the branch of a tree. Leave the bag on for a day and when you return you will see that water vapor from the leaves has condensed inside the bag. All of this water has been pulled from the soil up through the stem and out of the leaves. Perhaps the next time you find yourself taking a breath under the shade of a tree you’ll be aware of all the water being released into the air.