Article Tools

Font size
Share This

Photo: N/A, License: N/A

With summer’s heat just beginning to take hold, the cooling provided by trees is worthy of note. Have you ever gotten out of your car on a summer day and thought to yourself that there is no escaping the heat? The blacktop and haze seem endless and the contrasting air conditioning provided in malls, offices and cars seems to be an illegitimate, artificial and unhealthy escape from the sun’s unrelenting heat.

Now change that image. In your mind’s eye, drive west on Pennsylvania Rte. 118 and enter Rickett’s Glen State Park. As you descend into the hemlock-canopied glen, feel the temperature drop. It’s as if you have entered a refuge from summer’s oppressive heat.

Did you know it is not just the shade that is cooling? Studies show that the shade from a tree feels as much as 10 degrees cooler than the shade from a structure. The reason for this is transpiration. Trees not only block the sun’s heating rays, they constantly give off moisture and that moisture cools the air. Have you ever noticed that on hot NFL sidelines they have big fans blowing mist on the players benches? This is the same air conditioning that trees provide.

In the heat, don’t retreat away from the summer that you prayed would come soon. Don’t hide yourself from nature in your air-conditioned fallout shelter of a house. Instead, look for the shade and cooling of a tree.

Incidentally, municipal arborists have economic formulas for the reduction in cooling costs provided by trees over their lifespan. Long ago, before the advent of electric air conditioning, builders of homes in the American South were aware of the cooling provided by a canopy of trees. So they surrounded at least the southern and western sides of their structures with large canopy trees.

How do trees do this? The simple answer is through osmotic pressure. Transpiration of moisture through the leaves is ultimately responsible for triggering the negative pressure that pulls water and nutrients out of the soil and up through the tree. The tree is a complex straw for the sky to drink out of. See transpiration vapor for yourself by putting a clear bag over a plant.

The downside is that as long as trees have leaves, they are basically transpiring moisture. This is only a problem when not enough moisture is available. In winter, like this past one, many evergreen hollies dried out because they transpired moisture that could not be replaced due to the frozen ground. On the other hand, in a summer like this one, especially in the case of wet and stressed trees, there may not be enough moisture to sustain the tree. If you are watering your outdoor geraniums, soak your young trees as well.

Joshua Arp is an ISA-certified Municipal Specialist, a Clarks Summit Tree Commissioner and an operator of a landscape maintenance business. He can be reached at