Shining a Light on Urban Plant Growth: Why Cities Cause Plants to Blossom Longer
Urban Vegetation Blossoms Prematurely and Thrives for a Prolonged Period.
Hustle and bustle, neon signs, and an endless stream of people – cities are anything but sleepy. This incessant activity has surprising consequences for Mother Nature, as a group of researchers found out. Artificial city lights, combined with higher urban temperatures, can extend the growth period of plants by as much as 12.5 days, starting the blooming season up to 10 days earlier compared to rural areas.
Cities experiencing the most prominent shift from country to city in the beginning of the growing season are mostly located on the northwest coast of Europe, Southeast Asia, and North America. On the other hand, cities where the urban-rural difference is most noticeable at the end of the growing period are primarily found in South Asia, Europe, and the west of North America, according to a study published in the journal "Nature Cities."
The Earliest Season Start in Europe: A Dance with Light and Heat
The research team, led by Lin Meng from Vanderbilt University in Nashville, USA, aimed to further explore the impact of artificial light and the higher heat radiation from urban buildings on plant flowering, growth, and the seasonal timing of shedding leaves. They found that in the spring, both temperature and light play a roughly equal role in causing urban plants to sprout earlier than rural ones. However, it is primarily artificial light that delays leaf coloring and shedding in autumn, extending the growth period in the city.
There are exceptions: In cities with higher temperatures, researchers found shorter growing periods in some cases. They attribute these cases to water scarcity, as increased temperatures during summer and early autumn can lead to reduced water availability, accelerating autumnal developments.
Interestingly, the type of lighting seems to be a factor as well. Traditional lamps, such as sodium vapor lamps, are gradually being replaced by energy-efficient LED (light-emitting diodes). The study notes that the shift to LED lighting, with its pronounced blue light emissions, could have a fundamental impact on the influence of artificial light on plant development, a topic that remains largely unexplored.
Man-made Light: A Game Changer for Plants' Growth Cycle
The Magic Ingredient: LEDs
Street lamps, neon signs, brightly lit shops – cities never truly darken. This artificial glow influences the natural growth period of plants in autumn by an average of about 11 days, making it last roughly a week longer. In addition, the combination of artificial light and higher city temperatures causes the growth period to start about 12.5 days earlier than in rural areas. The research, published in "Nature Cities," analyzed data from 428 cities in the Northern Hemisphere between 2014 and 2020.
Calculations by Meng and his colleagues revealed that while both temperature and lighting contribute roughly equally to the earlier flowering of city plants in spring, in autumn it is almost exclusively artificial light that extends the growth time by delaying leaf coloring and shedding.
From Warmth to Illumination: The Impact of Temperature and Light
In contrast to what was previously believed, temperature and artificial light play a more significant role in plant growth and development cycles than previously thought. While the urban heat island effect contributes to earlier flowering, it is the increase in artificial light that predominantly advances the onset of urban growing seasons and delays leaf senescence across all climate zones.
It is worth mentioning that the shift to LED lighting is a key factor in this effect. The implications of LED lights on plant life and the ecosystem aren't fully understood yet, but they might have significant consequences for soil health and microbial activity and impact local ecosystems, pollinators, and biodiversity.
So, the next time you wander through a well-lit city street late at night, spare a thought for the plants, gazing up at the artificial lights, their circadian rhythms confused, dutifully growing that much longer than their rural counterparts.
Sources: ntv.de, Stefan Parsch, dpa
Connections: Temperature, Ecosystem, Lighting, Urban Development, Plant Growth, Urban Heat Island, Circadian Clocks
Additional Insights:
- LED Lighting: The switch to LED lighting in urban environments significantly influences plant growth periods, primarily by prolonging the growth season and altering plant phenology—the timing of biological events such as budding and leaf senescence.
- Extension of the Growing Season: Urban environments with LED and other artificial lights experience a growing season extended by 2–3 weeks compared to surrounding rural areas, based on analysis of satellite data from 428 cities in the Northern Hemisphere. Some studies report urban centers turning green about 12–13 days earlier and delayed leaf senescence by a similar period.
- Mechanism: LEDs and other artificial lights mimic sunlight, confusing plants’ biological clocks (circadian rhythms), which sense day length and regulate seasonal changes, leading to altered flowering and leaf production schedules.
- Implications: The ecological consequences of extended growing seasons and altered phenology in urban plants are still under investigation but could affect local ecosystems, pollinators, and biodiversity. Additional research is needed to fully understand the impact of LED lighting on the health of soil and microbial activity.
- The study published in "Nature Cities" reveals that the expansion of LED lighting in urban environments could have a fundamental impact on plant development, as it’s believed to alter the influences of artificial light on plant growth cycles, a topic that remains largely unexplored.
- The research indicates that temperature and artificial light play a more significant role in plant growth and development cycles than previously thought, with the urban heat island effect contributing to earlier flowering and the increase in artificial light predominantly advancing the onset of urban growing seasons and delaying leaf senescence across all climate zones.
- As cities shift towards energy-efficient LED lighting, it may have significant consequences for soil health and microbial activity, impact local ecosystems, pollinators, and biodiversity, due to the extended growing seasons and altered plant phenology caused by LED lights mimicking sunlight and confusing plants’ biological clocks.
