Michelle Sutton


Tree Canopy and Evaporation: A River to the Sky  

By George Profous, NYSDEC Region 3 Senior Forester [email protected]

References listed at end of essay

A tree is a disguised water fountain that converts high energy solar radiation into low temperature latent heat by moving water from the soil to the sky—a river to the sky, if you will. By redistributing water and energy, trees help balance local and regional weather extremes.

Planting trees has become a ubiquitous social brainstorm with cities and countries outdoing each other to grow a hundred trees, a thousand trees, or even a million trees. Yet it isn’t just how many trees are planted, but where and how they are planted, that makes a difference.

The U.S. Forest Service estimates that New York State’s increasing forest cover, mostly the result of abandoning farmland, has slowed and probably stopped. Urban lands are now making a difference. But at the same time, urban development has caused urban tree cover to decrease more than 1% from 2009 to 2014, and the trend is continuing.

In Westchester County, immediately north of New York City, developed land increased by over ten square miles since 2001, a full 5% of the county’s land. Every year in the United States an area the size of the state of Delaware becomes urban. By 2030, worldwide urban land will nearly triple what it was in 2000. In other words, it’s a local and a global trend, with land cover change contributing to as much as 18% of current global warming.

Trees only provide cooling affects when they are healthy, in well designed, adequately sized growing spaces to allow root systems access to sufficient water. Photo Michelle Sutton

In most of our cities and towns, even those actively striving to plant trees, removals exceed planting.  Increasingly, tree canopy cover in neighborhoods toward the center of large cities, such as those in the lower Hudson Valley, hovers around 20%. Experts say that is not enough to make a difference; the current consensus is that increasing tree canopy cover up to 40% in urban areas is necessary to effectively mitigate local, regional, and even global climate impacts.

Right now it’s impossible to get an exact measurement of how much trees, forests, and green spaces cool, but there is ample evidence that cooling is happening. Researchers are closing in on the exact values, taking into account canopy cover, humidity, prevailing winds, altitude, solar radiation at different latitudes, ambient temperatures, available water, tree species, and more.

The day may be coming when you see a line on your electric bill that says something like, “Tree Cooling Degree Days in the Growing Season,” reflecting your utility’s support of tree planting programs. Already, measurements indicate that increasing canopy cover from 5 to 15% up to 40% would decrease cooling degree days (CDDs) and energy use in summer by 29 to 45 percent!

We know that locations with lawns, shrubs, or bare ground can have air temperatures 15 degrees F higher than those with trees. With their deeper roots and multiple canopy layers, trees achieve substantially greater cooling and stormwater reduction than grasses and shrubs. Although sustainable long-term tree survival requiring specially designed tree pits and landscapes may initially cost more, replacing high maintenance shrubs and grasses, avoiding hardscape damage, and more effectively lowering temperatures actually saves money in the long run.

Tree benefits and adequate planting spaces are often an afterthought. Above left, inadequate site preparation and insufficient usable rooting volume. Above right, a tree planting in a parking lot underlain with CU Structural Soil. The design consideration shown in the latter photo is what every urban tree needs to thrive. Photos by George Profous

Of course, tree species must be carefully chosen and planted, so we need to hire professionally trained urban foresters and arborists to work alongside landscape architects, engineers, public works departments, and other local organizations. Advice from these professionals, based on the latest research, will make municipal decisions much easier.

Trees lock up solar energy by recycling the surplus in their canopies, evaporating water during the day and condensing it at night. In the cooler morning air, the moisture they release returns as dew, swapping the heat we feel (sensible heat) for gentle release later (latent heat). That’s why on a cold autumn morning, it’s slightly warmer in the woods than in an open field. A tree’s water-circulating “air-conditioning system” evens out the highs and lows of daily temperature swings, absorbing five times more heat than an urban surface—but to do this, trees need access to water in the soil.

Though forests are substantial carbon sinks, their ET Impact (EvapoTranspiration Impact) is even more important. A tree’s rate of photosynthesis can be slowed down by excessive daytime heat stress, but by cooling their immediate environment through ET during the night, trees decrease their own rate of respiration. This slowed respiration is helpful because although trees overall store more carbon than they release), at night they release more of it than they absorb, via respiration.

Swamp coolers are used in many hot and dry areas of the United States. Trees are gigantic outdoor swamp coolers: they lower temperatures and increase comfort by taking in drier air and adding moisture. A swamp cooler uses 4-10 gallons per hour and can lower temperatures by 20 degrees F by increasing humidity. If well hydrated, a small tree with a 30 foot canopy can transpire 5 gallons per hour for the ten hours of a hot sunny day. Illustration from Fort Collins (fc.gov) publication “Application of Evaporative Cooling”

Many deserts have no less water than the forested areas where three quarters of the U.S. population lives. It’s the distribution and timing of the precipitation that matters. Most of the water passing through trees continues up through the leaves and is carried by winds to eventually settle elsewhere.  With less tree cover, there is less water and therefore more extreme temperatures and weather patterns in that “somewhere else.” Your neighborhood may be that somewhere else!

More evidence that deforestation decreases rain and accelerates drought is reflected in the historic landscape changes of the northern Mediterranean, the Middle East, and the Sahel in Africa. In the past, moisture held by soil and vegetation helped keep the Sahara Desert lush by helping to create clouds and boost rainfall. Satellite scans have revealed the hidden river valleys and lakes buried under the sands.

The average time evaporated water spends in the atmosphere is only nine days, but in that time it can make a difference locally and move thousands of miles. There are regions within continents that rely heavily on this water from elsewhere. More than 40% of rainfall over land comes from evapotranspiration– the forest “river to the sky.” We catch visible glimpses of this invisible water movement in the remnants of Sahara and Gobi Desert dust storms, and from smoke plumes from our large western fires passing overhead.

The shade under a tree is cooler than under an awning due to the tree’s evapotranspiration (Hesslerova et al, 2021).

In dry places with low humidity, heat depletes water supplies causing drought and more heat. High humidity can also increase heat stress on people by reducing our sweat’s ability to evaporate. When nights don’t cool down, the heat index rises, affecting people by decreasing productivity and increasing stress, illness, violence, and deaths. Luckily for us here in the Northeast U.S., trees decrease temperatures and our average specific humidity is only 10-12%, so increasing water to cool the air is well within our comfort zone.

Although the many tree benefits are frequently discussed, their climate change role has often stressed carbon sequestration and energy savings, with not enough emphasis on the story behind the invisible movement of water. Planting a tree is terraforming—restoring the balance altered by a wide range of human-caused changes to the atmosphere.

George Profous

[email protected]., New York State Department of Environmental Conservation, Region 3, 21 South Putt Corners Road, New Paltz, NY (845-256-3082)


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