Contributed by Amanda Wood, The Martha S. Miller and Rusty Miller Urban Forestry Intern at Morris Arboretum

Human impacts can lead to a wide range of environmental changes, from droughts to flooding to affecting plant phenology, e.g., when they bloom. Artificial lights can be a significant factor in changing plant and animal behaviors. For birds, it can lead to an increase in vocal activity in nocturnally migrating birds, causing more energy to be used on their already demanding route (Watson, Wilson, and Mennill, 2016).

Photo: This flowering dogwood photo was taken December 19, 2018.
Note that the tree is dormant except for the section under the light, which is flowering.

Photo credit: Jason Lubar, Associate Director of Urban Forestry at Morris Arboretum

Artificial lights can also affect plants’ biological clocks, potentially disrupting the day-night cycle to which trees have evolved to respond. This can result in changes in flowering timing, the promotion of continuous growth, and the prevention of dormancy, a state which allows trees to survive the winter. As seen in the photograph, this flowering dogwood, Cornus florida, on Penn’s campus has become dormant, except for the one section under the light which is not only still in leaf but also blooming. Flowering dogwoods are particularly sensitive to artificial lighting, while other tree species are more tolerant (Peterson, 2017).

Given the large amount of artificial light in cities, it is best to choose tree species (or varieties) which can innately tolerate light stress. This is particularly important for plantings which will be directly under artificial lights, such as street lights. People who plant trees do not usually consider artificial light as a stressor, but it can impact a tree’s health and may eventually lead to an overall decline. Future plantings should not only consider the tree’s soil and water requirements, but also how the nighttime light needs of a specific species. Like humans, trees need to “sleep” as well.


Peterson, N. (2017). Hit the lights! Light pollution's negative impact on urban trees. Retrieved from

Watson, M., Wilson, D., & Mennil, D. (2016). Anthropogenic light is associated with increased vocal activity by nocturnally migrating birds. The Condor Ornithological Applications, 118, 338-344.