Light Pollution's Impact: Plants Under Threat

Light pollution is a growing problem, with about a quarter of the planet already affected by it. Caused by outdoor lighting, it has a detrimental impact on plants, disrupting their seasonal rhythms and their relationship with pollinators.

Plants rely on light as a source of energy and information. They use light to regulate their circadian rhythms, seasonal phenology and the expression of phenotypic variation, including growth form and resource allocation.

Artificial light can disrupt these natural cycles, affecting the timing of plant reproduction, growth, flowering and fruiting. It can also reduce pollination by deterring nocturnal pollinators from their nightly routine. This can lead to a reduction in plant reproduction and fruit production.

Light pollution can also indirectly affect plants by impacting other species that they rely on, such as pollinators and seed transporters. It can also reduce nutrient cycling by soil-digging nocturnal mammals.

The effects of light pollution on plants are still being studied, but it is clear that it has the potential to cause ecological chaos and put our food supply in jeopardy.

Light pollution disrupts the natural sleep-wake cycle of plants

Light pollution can disrupt the natural sleep-wake cycle of plants, also known as their photoperiods, by altering their perception of day and night. Plants rely on the duration of uninterrupted darkness to govern developmental processes such as dormancy, shoot growth, and flowering.

Artificial light at night, or ALAN, interferes with these processes by tricking plants into perceiving longer days. For example, trees near streetlights tend to retain their leaves longer in the fall, making them more susceptible to ice damage. This disruption can also impact leaf expansion, increasing the plant's exposure to air pollution and water stress.

The type of lighting commonly used in street lamps, emitting high-intensity red to far-red wavelengths, is particularly effective at disrupting plant photoperiods. This light can induce flowering patterns and promote continued growth beyond what is safe for the plant, leaving it vulnerable to winter weather.

The impact of light pollution on plant sleep-wake cycles can have ecosystem-wide consequences. For instance, changes in the timing of flowering or bud break can disrupt insects and birds that rely on these events for food and shelter.

Additionally, plants that bloom only at night, such as cacti, depend on nocturnal pollinators for reproduction. Increased lighting conditions can prohibit these plants from flowering, disrupting their reproductive cycle and affecting their pollinator species.

The effects of light pollution on plant sleep-wake cycles are complex and vary depending on factors such as plant species, light intensity, and duration of exposure. While higher levels of light pollution may increase photosynthesis and growth, lower levels can disrupt other light-dependent processes, leading to negative consequences for plants.

It affects the seasonal rhythms of plants

Light pollution can affect the seasonal rhythms of plants by disrupting their photoperiods. Photoperiodism is the process by which plants use the length of day and night to control vegetative growth and reproductive activities. It is governed by the duration of uninterrupted darkness during a 24-hour cycle.

Plants are classified as long-day, short-day, or day-neutral plants according to their response to day length. Long-day plants flower in early summer and continue vegetative growth until days shorten in the fall, while short-day plants flower and enter dormancy when day length shortens in late summer.

Artificial lighting at night, particularly in the red to far-red or red to infrared range of the spectrum, can extend the day length and disrupt flowering patterns and growth cycles. For example, trees near streetlights tend to retain their leaves longer in the fall, making them more susceptible to ice damage. They may also break buds and flower earlier in the spring, increasing their vulnerability to frost damage and disrupting their relationship with pollinators.

Additionally, light pollution can promote leaf expansion, increasing the plant's exposure to air pollution and water stress. It can also alter the makeup of plant communities, with some species producing more biomass and vegetative offshoots, pushing out other species.

The disruption of seasonal rhythms in plants can have far-reaching consequences, impacting the food web and ecosystems that depend on these plants.

It can reduce plant pollination

Light pollution can reduce plant pollination in several ways. Firstly, it can disrupt the natural relationship between plants and their pollinators. Nocturnal pollinators like moths, which are vital for the reproduction of many plant species, are attracted to artificial lights and fly towards them, thus neglecting their pollination duties. A 2014 study found that 70% of moths flew towards streetlamps and away from flowering plants, resulting in reduced plant pollination.

Some nocturnal pollinators are deterred by well-lit areas, while others are attracted to and disoriented by the lights, wasting their energy and distracting them from pollinating plants. Artificial lights also increase the risk of nocturnal pollinators being predated as they become easier for predators to see.

Light pollution can also affect the timing of plant flowering and dormancy. As plants use the relative length of uninterrupted darkness to time events like flowering and dormancy, artificial light at night can confuse them, causing them to flower earlier in the spring and retain their leaves for longer in the fall. This can disrupt their relationship with pollinators, as pollinators may not be active at the time the plants flower.

Additionally, light pollution can alter the composition of plant communities. Some plant species respond well to artificial lights and produce more offshoots, outcompeting other species. This can further impact pollination as certain pollinators may be attracted to specific plant species.

It can cause plants to retain their leaves for longer

Light pollution can cause plants to retain their leaves for longer. This is because plants depend on cycles of light to determine their behaviour, including leaf shedding. Light pollution disrupts these cycles, which can have a significant impact on plants.

Plants depend on light for energy. They gather sunlight in microscopic, chemical solar panels called chloroplasts. The light energy moves through a series of chemical reactions that turn carbon dioxide into usable energy, like glucose, in a process called photosynthesis.

However, the impact of light pollution on plants is not always positive. While increased light might be expected to lead to more photosynthesis, providing plants with more chemical energy for growth and other functions, this is not always the case. In fact, light pollution can disrupt the natural cycles of plants, including their daily rhythm, which can be disruptive to plants as it deprives them of the rest and repair that occurs at night.

The effects of light pollution on plants can vary depending on the intensity of the light and the type of plant. Low-intensity light may not provide enough energy to stimulate growth but can still disrupt other processes that depend on light as a signal, leading to negative consequences. For example, in a study of grassland plants, low-intensity LED lighting at night reduced the amount of one species, colonial bentgrass.

Trees living near street lamps provide one of the earliest and most obvious examples of the effects of light pollution on plants. These trees often retain their leaves longer than normal, as artificial lighting tricks the red to far-red receptors in plants, convincing them that the days are longer than they actually are. This disruption to their natural cycles can have serious consequences, as retaining leaves for longer makes trees more susceptible to ice damage.

The impact of light pollution on plants can also vary depending on the season and the age of the plant. For example, in a study of periphyton, a microscopic plant community, exposure to LED lighting reduced growth by more than half compared to controls that received no artificial nighttime lighting. However, this effect was only observed in newly developing periphyton communities, not in older, more established communities.

It can change the flowering patterns of plants

Light pollution can change the flowering patterns of plants. Plants depend on cycles of light to determine when to flower. These cycles are usually synchronized by light, so light pollution can disrupt their natural rhythms.

Plants that flower depend on cues from their environment to determine when to produce flowers. The duration of light and darkness affects the growth, development, and flowering of plants. This is called photoperiodism, a process in which a plant responds to the duration of light or dark. It is a kind of biological clock.

Artificial lighting can extend the length of the day and trigger a plant's photoreceptors, changing its flowering pattern. For example, cacti species that bloom only at night may be prohibited from ever flowering if there is too much light. This also affects their pollinator species, such as nocturnal insects or small animals, principally moths and bats.

Trees living near street lamps retain their leaves longer than normal, which can make them more susceptible to ice damage. They may also break buds and flower earlier in the spring, making them vulnerable to frost damage.

Light pollution can also reduce fruit production in plants. For example, tomatoes, eggplants, ladyfingers, cucumbers, and bell peppers may be affected.

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