Light pollution has become a frustration for many a star-gazer. Those who seek the heavens must drive far outside of urban centers and away from light-lined interstates in order to truly see the night sky, and even then, the effects of light pollution cannot be completely escaped. Light from our streetlights and buildings travels farther than many realize. The sky glow of Los Angeles, for example, is visible from an airplane over 200 miles away. We are affected in many ways by this wayward light. Light pollution disrupts our sleep, wastes energy, and adversely affects wildlife. It also affects the health of trees.
Urban trees live with all kinds of stresses that their forest cousins do not – road salt, soil compaction, and lawn mower related-injuries to name a few. They are also subjected to a different light environment, one where it is never completely dark. Trees evolved with a regular, predictable transition from day to night, following the natural patterns of the sun and moon. They measure light with a kind of molecular clock, so that they know the length of the day, the season, and can also measure shadows to determine their distance to other trees. This information tells them when to carry out certain important processes, such as photosynthesis, the timing of spring budburst, leaf coloring, and the shedding of dead leaves. Artificial light confuses trees by extending the day length, which can change flowering patterns, and promote continued growth, preventing trees from developing dormancy that allows them to survive the rigors of winter weather.
While all artificial lighting is harmful to trees in some way, there are two kinds that do the most damage: continuous lighting and light rich in red and infrared wavelengths. Continuous lighting is very common, and eliminates a natural light cycle for the tree. The foliage of trees grown in continuous lighting may be larger in size and therefore more susceptible to air pollution and water stress during the growing season due to the stomatal pores in leaves remaining open for longer periods.
There are ways to lessen the impact of artificial light on urban trees.
When artificial lighting is necessary, mercury vapor, metal halide, or fluorescent lamps give off less harmful wavelengths of light. Fixtures should be shielded so the light is directed on the ground, and not up into the sky, and uplighting trees should be avoided. Where possible, lights should be turned off or dimmed during off-peak hours to give the trees a period of darkness. There is also some variation in the susceptibility of trees to lighting, as seen in Table 1. Using tree species that are less sensitive to artificial light can help with overall tree health and longevity. If you are planting trees where night lighting already exists, select those with low sensitivity to light to ensure the best possible outcome.
We can improve tree health, sleep better, and save resources by thinking critically about lighting choices and where lighting is really needed. Trees contribute so much to our towns and cities – isn’t it the least we can do to dim the lights? After all, so much artificial light isn’t good for people either, and there is a lot for us to enjoy in the night sky.
Chaney, William R. “Does Night Lighting Harm Trees?” Purdue University Extension Forestry and Natural Resources, 24 June 2002.
“Street Lights and Tree Growth.” Venerable Trees, 3 May 2016, www.venerabletrees.org/street-lights-tree-growth/.
“The Natural Night Sky Is Our Universal Heritage.” International Dark-Sky Association, www.darksky.org/.