Increasing artificial light
In recent decades, the urban population has grown exponentially worldwide, with the expectation that the urban population will continue to increase the next decades. As cities expand, so does the reliance on artificial light sources to illuminate the night. This increases the visibility for humans when it is dark, but it can pollute the nighttime environment for many organisms by differing light timing regimes when compared to natural circumstances. This can be done by creating skyglow, which is created by artificial lights raising night sky luminance when rays of light scatter back to the ground from the atmosphere. Artificial light can also alter natural patterns of light and darkness in ecosystems by direct glare. The direct glare of artificial light at night (ALAN) can induce strong responses in terrestrial ecosystems by altering activity patterns, life history traits and community structure, thus causing biological impacts on organisms living within the terrestrial ecosystem. Globally, nearly a quarter of the land area lies under artificially light-polluted night-time skies.
ALAN and the terrestrial ecosystem
When investigating the effects of ALAN on the terrestrial ecosystem, much of the research conducted has been towards the effect on animals. A meta-analysis of Sanders et al. shows that circadian rhythms of organisms are disrupted, especially for nocturnal animals. This happens through alterations of hormone levels and gene expression. This analysis contained more than a thousand researches regarding the effects of artificial light on animals, but only 143 researches regarding the effects on plants. This shows a clear bias of interest towards animals. As plants are the basis of the food web, it is important to understand the effects of light at night in order to predict how life of urban communities will change when there are changes in plant phenology and physiology. From these two parameters, there are yet no clear general patterns when it comes to its effects. Individual studies find evidence of phenological shifts in plants under ALAN, but when these are compared, these studies suggest both positive and negative effects. Even a smaller number of studies apply the knowledge of physiological effect of light on plants, as their natural photoperiod is changed by the light at night.
ALAN and plant phenology & physiology
One of the few plant researches investigated the effects of ALAN on phenology and physiology on urban herbaceous plants by Czaja & Kołton. Bud development of trees and shrubs was measured as an indicator for possible changes in plant phenology and sugar contents measured as an indicator for changes in physiology. Different urban species were exposed to low and medium light intensities at night, while the control had darkness at night. The results showed that all species had an altered phenology under low and medium light intensities. All species developed buds faster than the control group. However, the changes were visible at different stages of bud development for the different species. This result can show that there are changes but that there is no clear pattern for plants. This is especially shown in this research by the results for the sugar contents. Two species had increased sugar contents, four species had decreased sugar contents and two showed no change in sugar content. All plants showed phenological and some physiological changes even at low light intensities, which shows the possible sensitivity of plants to artificial light at night.
Conclusion
To conclude, there is still a lot we do not know when it comes to the effects of artificial light at night on plant phenology and physiology and is thus understudied. The results from a handful of researches on these two parameters show no clear pattern as the underlying mechanisms can be different for species. It would thus be great to know if there possibly is a general underlying mechanism for most species, as they might be different from woody species and non-woody species. Next to this, different plant species should be evaluated on their sensitivity to artificial light to see which could be more resilient to life in the city. Researches are conducted on seasonal basis, which leaves a lack of knowledge on how urban areas alter plant physiology on long term. As plants in the city are exposed to a heterogeneity of light intensities and colours, research can be quite difficult towards possible effects, but plants seem to respond to even low intensity light pollution. All this knowledge can contribute to a better understanding of the basis of the food web in urban areas. So, in my opinion: Yes, urban plant phenology and physiology is understudied when it comes to artificial light at night!
Sources:
Barentine, J. (2023). Artificial Light at Night: State of the Science 2023. Zenodo. https://doi.org/10.5281/zenodo.8071915
Czaja, M., & Kołton, A. (2022). How light pollution can affect spring development of urban trees and shrubs. Urban Forestry & Urban Greening, 77, 127753. https://doi.org/10.1016/j.ufug.2022.127753
Sanders, D., Frago, E., Kehoe, R., Patterson, C., & Gaston, K. J. (2021). A meta-analysis of biological impacts of artificial light at night. Nature Ecology & Evolution, 5(1), 74–81. https://doi.org/10.1038/s41559-020-01322-x
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