Artificial Light At Night (ALAN) profoundly affects the natural environment, and trees are particularly vulnerable. Trees have evolved to rely on the predictable cycle of day and night, but chronic exposure to artificial illumination acts as a misleading signal. This continuous light confuses the tree’s internal clock, which governs its fundamental life processes. The consequences range from altered physical growth to a failure to prepare for changing seasons. The light intensity required to cause this disruption is far less than what is needed for photosynthesis, meaning even distant streetlights or landscape uplighting can have a significant biological impact.
How Artificial Light Disrupts Seasonal Dormancy
The most severe harm caused by light pollution is the disruption of a tree’s natural seasonal cycle, specifically preparation for winter. Trees measure the changing seasons using photoperiodism, their biological response to the relative lengths of daylight and darkness. As days shorten in autumn, the lengthening period of uninterrupted darkness triggers a hormonal cascade that signals the tree to stop growing and develop cold hardiness.
Artificial Light At Night interferes with this sensing mechanism, making the tree believe the summer day length is continuing. This false signal prevents or significantly delays leaf senescence, the process where leaves change color and drop off. The tree maintains its foliage and continues a reduced form of growth instead of moving nutrients into its roots and trunk.
The failure to enter true dormancy means the tree does not accumulate the necessary compounds to protect its cells from freezing, leaving it “soft” and unprepared for the cold. When the first hard frost inevitably arrives, the water inside the unprotected cells of the leaves, buds, and twigs expands, rupturing the tissues. This damage, known as frost desiccation, can lead to widespread branch dieback, severe dehydration, and total mortality in younger or highly exposed trees. Studies show that the phytochromes—the light-sensing pigments trees use—can be activated by light levels as low as 0.06 to 3 microeinsteins per square meter per second.
Altered Growth Patterns and Reproductive Cycles
Night lighting also alters the physical structure and reproductive success of trees throughout the growing season. Phototropism causes plants to grow toward a light source, leading to uneven or asymmetrical canopy development in illuminated trees. This uneven growth results in structural weaknesses, as the crown leans heavily toward the consistent artificial light, making it more vulnerable to wind and ice damage.
The spectral quality of the light also influences physical growth. Blue wavelengths, commonly found in modern white LED lights, can suppress stem elongation. This effect can cause the tree to develop a shorter, bushier form that is not typical for the species, potentially affecting its ability to compete for sunlight.
Reproduction is frequently suppressed because many tree species require a specific period of uninterrupted darkness to initiate flowering and fruiting. If this dark period is not met, the reproductive cycle is delayed or entirely inhibited, reducing seed production. Indirectly, ALAN also impacts reproduction by disrupting nocturnal insect and bat pollinators, which are disoriented or deterred by the light. A stressed tree that has delayed dormancy or an asymmetrical canopy is already weakened, making it an easier target for pests and pathogens.
Practical Steps to Reduce Light Pollution Harm
The effects of light pollution are often reversible, and several practical steps can mitigate the harm to nearby trees. The most straightforward solution involves minimizing light trespass by ensuring fixtures are fully shielded and directed exclusively toward the ground. Shielded, downward-facing lights prevent illumination from spilling upward into the tree canopy, eliminating the disruptive light signal.
The spectrum of the light is another significant factor. Blue and white lights are the most problematic because they closely mimic the wavelengths of natural daylight. To reduce disruption, homeowners and municipalities can choose warmer-colored light sources, such as amber or red LEDs, which have a lower color temperature. These warmer spectrums are less biologically active in triggering the tree’s light-sensing pigments.
Controlling the duration of light exposure is also an effective strategy, as trees only need a brief period of darkness to reset their internal clocks. Using timers or motion sensors ensures that lights are completely off during the critical midnight hours, allowing the tree to perceive the necessary dark period. Careful placement of new lighting fixtures, keeping them as far away from the tree canopy as possible, is a simple preventative measure.