Recent research revealed that artificial light and urban heat are extending the growing seasons of plants in cities across the Northern Hemisphere. Lin Meng from Vanderbilt University and her colleagues analyzed satellite observations taken between 2014 and 2020 from 428 urban centers, including major cities such as New York, Paris, Toronto, and Beijing. Their findings indicate that urban plants are experiencing prolonged periods of growth compared to their rural counterparts.
In the cities analyzed, the start of the growing season occurs an average of 12.6 days earlier than in rural environments due to artificial light. Additionally, the end of the growing season is delayed by 11.2 days in urban areas, also influenced by artificial light. This results in city plants growing up to three weeks longer than plants in less populated regions. The extended growing season is influenced more by increased artificial light than by temperature differences, which shows the impact of light pollution on urban ecosystems.
Urban areas experience higher atmospheric temperatures during both day and night due to the phenomenon known as urban heat islands. Concrete buildings and asphalted roads absorb and radiate heat more than areas with less development, contributing to warmer conditions that can affect plant growth. However, the research indicates that light pollution has a greater impact on the extended growing seasons than temperature.
The amount of artificial night light in cities increased by 10% over the past decade, making urban streets brighter than rural ones. Artificial light acts like an extension of natural daylight, disrupting the photoperiodic signals that regulate seasonal transitions in plants. This disruption leads to earlier budding and blooming in the spring and delayed leaf coloration and fall in the autumn.
The study found that these patterns are consistent across all cities in the Northern Hemisphere. The effect of artificial light on the end of the growing season was consistently observed across Northern Hemisphere cities. The researchers also identified differences between continents regarding the timing of the growing season and the influence of artificial light on plant growth. The start of the growing season is earliest in Europe, later in Asia, and latest in North America, although North American cities are the brightest.
Artificial night light is more intense at the start of the growing season in some climatic zones, including temperate climates with dry summers and cold climates without a dry season. The influence of artificial lights is evident in Europe, while it is less pronounced in Asia and North America.
The research team analyzed data on artificial night light, near-surface air temperature, and plant growing seasons. They compared satellite data from urban centers with data from the suburbs and surrounding rural areas to understand the impact of urban environments on plant growth. For the first time, satellite data allowed the measurement of alterations in urban environments, including the impact of artificial light on plant growing seasons.
The findings have implications for urban planning and environmental management. Meng and her colleagues suggest that future urban infrastructure plans should include lighting that minimizes the effect on plants while satisfying functional requirements. More research is needed to understand the effects of different types of artificial lighting on plant growing seasons.
Moreover, the effect of artificial light on the growing season may be further complicated by the relatively recent general switch from high-pressure sodium lamps to LED lighting. Plants may be more responsive to LED lighting than to high-pressure sodium lamps, although more research is needed to fully understand this impact.
Written with the help of a news-analysis system.