The Amazing Process of Evapotranspiration

Mother Nature’s Moment • July/August 2019
by: Lesley Bruce Smith, ISA Certified Arborist

Bi-colored Oak tree leaves after a rain in the process of photosynthesis and evapotranspiration.  photo by Lesley Bruce Smith

Bi-colored Oak tree leaves after a rain in the process of photosynthesis and evapotranspiration. photo by Lesley Bruce Smith

It is difficult for us to remember that less than four weeks ago the temperatures were consistently in the 60’s and rainy, and we were wearing coats and rain gear almost everyday. We are all happy for the more summer like weather, however, the trees view it a bit differently.

When we were complaining bitterly about day after day of rain, the trees were hard at work soaking up all the daylight of the summer solstice and all that readily available water. In the process they do what trees do best, photosynthesize like crazy. Our lives all depend on that elegant process of the trees that use the sun’s energy to create oxygen, swallow up carbon dioxide and make stems, buds, roots and leaves, lots and lots and lots of leaves! Because of the excessively wet weather, the wettest in Chicago history, they also were sending their roots up very close to the surface of the soil. That is because, roots need O2 to survive, and they can’t do that like fish, they need to come up for air when the soil in super saturated with water, and if that is not possible they will suffer and often die. Every leaf on a tree or shrub is a tiny “evapotranspirator”, which means it shoves water through capillary action up from its roots, out to its leaves and into the atmosphere. A mature tree can, in fact, transport over 100 gallons of water on a hot sunny day.

The amazing thing about that process is that it is the osmotic action, remember your lower school chemistry classes, that creates the great water transpiration express which allows the tree to move minerals and nutrients up where they are needed for the tree to produce its own food from the sun’s energy. It is estimated that 97% of the water absorbed by the roots will be lost to the atmosphere, but not before it does its job of bringing life and verdant growth to the tree.

Problems arise when a tree has to switch gears from a huge water pumping machine into a slower water pump and a more drought tolerant life form, as the temperatures soar from the average daily in the 60’s to the 90’s. But those amazingly wise trees know what to do. They compensate for lost roots baked by the sun and drowned by the water by dropping some of those magnificent evapotranspirators (leaves) to prevent desiccation. It is a very clever tactic, because they are shedding what they no longer need. It is a bit alarming to us because it can look like autumn in the yard in July and August. Remember the leaves have done their job and all that lovely food is now packaged up in the tree’s long term storage.

Our job in this water train is to notice when the ground gets hard and dry and give our friends, the trees, a good 4 hour drink every few weeks, if we don’t get rain. The lesson here is to not look at the dead leaves all over the lawn, but the healthy full crown that is still in the tree. It is true in all of life, that we are served best by looking up!

Backyard Wisdom • Tree Gutter Garden