As we commute home after a long day of work or school, perhaps this will be the view that most of us see. In a developed city like Singapore, we rely heavily on street lamps to light our roads and ensure the safety of pedestrian and drivers. However, what we might not see is the figure below, the fact that the light emitted from street lamps, especially if poorly illuminated, can lead to harmful effects on trees. 

 

Trees and the Electromagnetic spectrum 

Before we look at the impacts of light pollution on trees, let us first understand the potential types of light spectrums and wavelengths that trees can be exposed to. 

With respect to night lighting, we will be focusing on the visible light segment that is between 380nm to 760nm on the electromagnetic spectrum as seen above. This segment is important not only because our eyes detect this band of light for us to see, it is also vital for photosynthesis and other processes that determine the growth and development of trees. Generally, the growth and development of trees depend on three main aspects of this electromagnetic spectrum: quality (wavelength or colour), intensity (brightness) and duration in 24 hours (photoperiod) (Chaney, 2002). For example, photosynthesis would require visible blue (400-450 nm) and red (625-700 nm) wavelengths. 

 

What night light does to trees

However, other than the process of photosynthesis that most are familiar with, photoperiodism, which controls the vegetative growth and reproductive aspects of trees should also be looked at. Photoperiodism depends on the length of day, also known as photoperiod, which can be significantly altered because of the intensity and duration of night lighting. 

(a) too much growth 

According to Chaney (2002), artificial lighting, particularly from sources that emit light from the red to infrared segment of the spectrum, lengthens day length. This upsets the flowering patterns of trees and most importantly induces continued growth of the trees even nearing the winter season. Under natural conditions, the trees will shed their leaves in time to prevent branches from breaking due to the snow that accumulates on leaves and branches of the trees. However, over exposure to artificial lighting can cause the trees to not shed its leaves in time. Snow that is built up on these trees can cause branches to break, damaging the trees.  

(b)increased stomatal pore size

Additionally, prolonged lighting and the increase in the size of foliage can cause trees to be more susceptible to other forms of pollution like air and water pollution because the stomatal pores of the leaves are left open for a longer period of time. 

 

What can we do?

As seen from the table above, different types of light sources result in different impacts on trees. Where and when artificial lighting is necessary, fluorescent, mercury vapour or metal halide lamps should be prioritised to minimise the impact on trees. 

 Apart from the type of light source, we can also ensure that the lighting design is shielded, meaning that all light is directed downwards onto pedestrians and vehicles, away from trees. Uplighting and light that shines over large horizontal distances should also be avoided. 

In Singapore’s context, most of the street lamps that we see on roads uses high pressure sodium vapour lamps with white light which results in high photobiological impacts on trees as seen from the table above. However by 2022, the Land Transport Authority (LTA) aims to replace all street lamps with Light Emitting Diodes (LED), which are “directional” light sources which emit light in a specific direction unlike incandescent bulbs which emit light in all directions. While this is good news, the reasons for this still hinges largely on being more energy efficient rather than to reduce the impact of light pollution on species like trees. 

 

Stay tuned for more!

Jean

 

References:

• Chaney, W. R. (2002). “Does Night Lighting Harm Trees?” Purdue University Extension Forestry and Natural Resources. 
• The Straits Times. (2017). LTA installing smarter, energy-saving street lights. retrieved from https://www.straitstimes.com/singapore/lta-installing-smarter-energy-saving-street-lights#:~:text=%22This%20helps%20conserve%20energy%20and,roads%2C%20which%20require%20greater%20illumination.
• Image 1 retrieved from https://www.lumica-asia.com/portfolio-item/streetlight-series/