Trees or Lawns — which will provide a better future?
For 12 years I’ve studied the proposed Lake Powell Pipeline (LPP). There has been a constant refrain from LPP proponents regarding lawn in our area: grass is needed to help cool and lower energy needs. Additionally, this area prides itself on lush landscaping and abundant golf courses. But is grass the only way to achieve a verdant look? Is grass the best way to ensure a water-wise, sustainable future? Are there other ways to achieve lower temperatures and save energy?
A 2014 water district Water Line revealed that 61 percent of our residential water (mostly culinary/potable) went outside, generally on grass. Information currently on the district’s website, which includes residential and institutional, commercial and industrial water use, reveals that 58 percent of water is used outside, again most of that culinary.
High outside water use is driving our high overall usage — around 303 gallons per capita per day — encouraging a perceived need for the LPP. What options are there? How do trees compare with grass? A UCLA study revealed that trees studied in Los Angeles use less water than unshaded turfgrass, and lawns accounted for 70 percent of the total water used on landscapes that consumed nearly 100 billion gallons of water per year. As we grow, this is important to keep in mind.
Many studies regarding the “urban heat effect” and associated need for building cooling have been conducted. Growing cities create blacktop, concrete and other materials that add heat by absorbing solar radiation during the day, much released at night into the air, compounding the problem. Comparing the effects of trees and grass on temperature and energy needed to cool buildings is very complex, involving issues such as evapotranspiration — the transfer of water evaporating from the soil and transpiration from plants to the air — but also solar radiation, wind speed and more.
Studies reveal that in most cases, tree canopies are the optimal solution for shading pedestrian space and areas between buildings, and also provide evapotranspirative cooling. One study determined that unshaded grass was found to cause only a small air temperature depression and had the highest water requirement. Trees can significantly promote building energy efficiency – reducing energy use – due to shading effects. Comparisons indicate that shading is more effective than evapotranspiration. Grass provides no shading effect, only evapotranspiration.
This is not to say that lawns do not provide some measure of cooling, but given the amount of water needed and the overwatering we currently see in our area, lawns are not the best choice.
As climate change progresses temperature mitigation should not be the only objective of urban greening but is an important one. There is sufficient information available for local governing bodies to take positive action to help mitigate rising temperatures.
However, there are challenges with using trees. Consider our area’s ozone issue. More cars and use of lawn equipment create emissions that add to the problem. Volatile organic compounds (VOC), also known as BVOC (biogenic volatile organic compounds), combine with heat and emissions to increase ozone. Studies have identified trees that either help or hurt. Desert willow, mimosa, thornless honey locust and chitalpa are examples of trees with low BVOC levels, while poplars and willows have higher levels. Trees with low levels should be the focus.
Placement of trees is important to provide proper shading and obtain maximum benefit. Both lawns and trees act as carbon sinks — absorbing carbon from the atmosphere — to help counter global warming. Studies are being done on how well each succeeds.
These issues require further study, but given current studies, lawns should be out and trees along with drought-tolerant landscaping used to provide the green people want. For a list of the benefits of trees go to https://projects.ncsu.edu/project/treesofstrength/benefits.htm
Lisa Rutherford lives in Ivins and is a member of the board with Conserve Southwest Utah. Learn more about CSU at http://conserveswu.org.