July 31, 2023 at 1:49 pm | Updated August 2, 2023 at 1:02 pm | 4 min read
- Chlorophylls, carotenoids, and anthocyanins are the pigments coloring leaves green, orange, and red, respectively.
- Chlorophyll degradation, a crucial phenomenon in autumn, is getting delayed due to climate change cutting the fall color change time short with repercussions on nitrogen cycling and carbon storage.
- Warm temperature makes tropical forest leaves darker and reduces light reflection, which could increase global warming.
Leaf pigments not only color leaves but also have functional roles for plants. Changes in pigment concentrations can affect plants’ physiology and their surrounding environments. Research has just begun on how climate change impacts leaf pigments. Some changes are apparent, but others are not; find out what they are in this article.
Important Leaf Pigments
Pigments are vital for plant growth, health, and development. Leaves have three groups of pigments crucial for plant functioning- chlorophylls, anthocyanins, and carotenoids.
Chlorophylls: The most abundant pigment in plants that makes leaves green are chlorophylls a and b, which capture light in photosynthesis to synthesize food for plants. Chlorophylls absorb blue and red colors. The pigments are produced throughout the leaves’ life and broken down during senescence so that the plant can absorb nitrogen in them.
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Anthocyanins: A type of flavonoid, anthocyanins’ color leaves red, as it absorbs blue-green light. Anthocyanins are antioxidants and protect the leaves and plants against biotic and abiotic stress. The pigment helps in nitrogen reabsorption during chlorophyll degradation Anthocyanins also slow the leaf aging process.
Carotenoids: This pigment is responsible for yellow-orange colors in plants and absorbs blue light. Carotenoids are necessary for plant health.
The rise in temperature and carbon dioxide associated with climate change has a different impact on leaf pigment composition and concentrations in the tropics and temperate regions.
Climate Change Impacts leaf Pigments in the Tropics, Causing Darker Leaves
Leaves do not absorb all the radiation from the sun. A portion of the unused light is reflected. The light captured by leaves also includes heat waves, so more light absorption leads to more heat being trapped by plants. When more light is reflected, the heat is also reflected away from leaves, forest canopy, and the earth’s surface.
Leaf reflectance or leaf albedo occurring in the visible spectrum (400–700 nm) is driven by leaf traits like chlorophyll content and near-infrared (NIR) reflectance (700–2,500 nm) by leaf mass per area (LMA). The LMA, in turn, is influenced by leaf thickness and density.
In lowland tropical forests, leaves had lower LMA and reflected less NIR light. Leaves reflect less NIR because they have become darker due to warm temperatures. Darker leaves absorb more sunlight and reflect less light, increasing global temperatures.
Moreover, simulations indicate that reducing leaf albedo will be more significant in high carbon dioxide conditions, again leading to global temperatures rise.
Changes to LMA have already been observed in many tropical forests. Subsequent changes to leaf reflectance from tropical forests significantly affect the global climate since they are the largest terrestrial biomes. However, further research is necessary to check if leaf albedo in other biomes is also changing since it could produce darker leaves there.
Shorter Autumn Colors
Figure 1.: “The pigments involved in leaf color changes,” Ashford et al. (Image credits: https://www.nottingham.ac.uk/naturalsciences/study-with-us/teaching/synoptic-projects/the-effect-of-climate-change-on-autumn-leaf-colour.aspx)
As temperatures drop and light changes in autumn in temperate regions, leaves stop producing chlorophyll and start making carotenoids or anthocyanins, see Figure 1. Earlier, it was assumed that chlorophyll degradation exposed preexisting carotenoids or anthocyanins. However, it is now established that the two pigments are produced when chlorophyll production stops because they have a functional role during leaves senescence.
Climate change extends the growing season as warm days and carbon dioxide levels increase. For example, 2021 had more number of warmer days than 2020, extending the growing season. So, leaf color change is delayed. However, the sunlight received has remained over the years. With reduced lighting, leaves can’t photosynthesize, and the trees shed their leaves at the same time as they did earlier, shortening the time of fall colors.
The color quality is also changing. The trees are stressed during drought and produce fewer photosynthates, and the leaves color is not as bright as in wet years. Moreover, trees shed their leaves faster during dry periods, shortening or missing the colorful phase.
Autumn plant senescence is a critical time for perennial plants, as the plants reabsorb leaf nutrients like carbon and nitrogen. Anthocyanins act as a sunscreen and protect chloroplasts vulnerable to low temperatures. The presence of anthocyanins increases the reabsorption of nitrogen and other photosynthesis enzymes, for storage in branches and reuse in the next season. Redder leaves have been found to have less leaf nitrogen, so anthocyanin production is good for plants.
Since nitrogen is limited in most ecosystems, a change in its cycling will also be affected if chlorophyll degradation is delayed.
Carbon storage is also affected. Delayed chlorophyll degradation can increase carbon sequestration. However, there is more autumn respiration than photosynthesis by trees, leading to carbon dioxide release and reducing carbon sinks.
Many animal species depend on leaf color change cues for migration and food sourcing, and they will be affected by the delay in color change. Increased plant water use will also impact soil water availability.
Quantification of Pigments
Many effects of climate change associated with temperature rise and carbon dioxide levels on pigment concentrations are yet unknown. Studying changes in leaf albedo outside the tropics and understanding nitrogen dynamics in the plant and ecosystems during leaf senescence are only some of the questions that need to be explored. Tools like the NIR spectroscopy-based CI-710 Miniature Leaf Spectrometer, developed by CID Bio-Science, can measure light transmission, absorption, and reflection. The device is also efficient in the quantification of pigments and color analysis. Researchers can use these tools on-site to get non-destructive readings in real time to expand the research questions they address, increasing our understanding of climate change effects.
Ashford, T., Day, C., Fernandes, R., et al. (n. d.). The effect of climate change on autumn leaf colour. Retrieved from https://www.nottingham.ac.uk/naturalsciences/study-with-us/teaching/synoptic-projects/the-effect-of-climate-change-on-autumn-leaf-colour.aspx
Doughty, C. E., Santos-Andrade, P. E., Shenkin, A., et al. (2018). Tropical forest leaves may darken in response to climate change. Nature Ecology & Evolution, 2(12), 1918-1924.
Fecht, S. (2022, Sept 26). How Climate Change Impacts Fall Foliage. Retrieved from https://news.climate.columbia.edu/2022/09/26/how-does-climate-change-impact-fall-foliage/
Shi C, Sun G, Zhang H, Xiao B, Ze B, Zhang N, et al. (2014). Effects of Warming on Chlorophyll Degradation and Carbohydrate Accumulation of Alpine Herbaceous Species during Plant Senescence on the Tibetan Plateau. PLoS ONE 9(9): e107874. https://doi.org/10.1371/journal.pone.0107874
The Biological Significance of Leaf Color Change | Harvard Forest. (n.d.). https://harvardforest.fas.harvard.edu/leaves/biological
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