Tree leaves often change color with the seasons, marking the passage of time in the natural world.

In spring, newly sprouted leaves are a tender green; in summer, they turn dark green; in autumn, they become golden yellow; in winter, they return to the soil, waiting for the next spring.

Why are most leaves green?

The vitality of life on Earth comes from the sun's blessings, as sunlight drives the process of photosynthesis in plants. Through photosynthesis, plants use the sun's radiation and carbon dioxide from the air to synthesize sugars.

Sunlight consists of 7 visible light spectra (red, orange, yellow, green, blue, indigo, violet), similar to the colors of a rainbow, also known as the "seven-color rainbow spectrum." Green is the most common color in plants, related to the sunlight spectrum absorbed by plant leaves during photosynthesis.

When a substance absorbs one or more colors of the seven-color rainbow spectrum and reflects or transmits the rest of the light, it shows the color of the remaining light. The color of the sun, its temperature, and its distance from Earth determine that plants on Earth can absorb most wavelengths of light but do not absorb green light.

Why do leaves turn yellow in autumn?

When autumn arrives, days shorten and nights lengthen, leading trees to shed their leaves. During the cold season, plants still need a lot of energy and water to maintain leaf health. We know that sunlight provides energy to plants, but winter is cold and dry, often without sufficient sunlight.

To conserve nutrients, most plants are forced to shed their leaves. Before shedding, trees no longer produce large amounts of chlorophyll as in spring and summer; chlorophyll begins to break down. As the chlorophyll content decreases, the colors of other pigments gradually appear on the leaf surface, resulting in yellow, red, and other colors.

As the weather cools, approximately 10% of tree species in temperate regions exhibit leaf color change over a few weeks. Many types of leaves turn yellow, such as poplar and ginkgo trees, because the chlorophyll involved in photosynthesis breaks down and is transported back to the trunk and roots for storage. The colors of accessory pigments like carotenoids, which usually assist in photosynthesis, become visible.

Some tree species, like maple trees whose leaves turn red, produce a chemical substance called anthocyanin when temperatures reach 0-7°C. As temperatures drop in autumn, sugar content increases in these plants, coupled with dry weather and daytime sunlight, which favor anthocyanin production.

Why do some plants remain evergreen?

Most tree leaves turn yellow and wither in autumn, yet pine and cypress trees wear green even in harsh winter, standing lively.

Pine and cypress trees originally grew in cold zones and high mountains, developing unique cold-resistant structures due to their long-term adaptation to cold environments. The leaves of pine and cypress trees are generally needle-shaped, linear, or scale-like, with small leaf areas that reduce water evaporation. Additionally, these leaves have low moisture content and contain resin.

When temperatures drop, they can quickly increase the concentration of cell sap to prevent freezing by increasing sugar and fat content. Thus, even in winter, pine and cypress trees do not wither due to lack of water, ensuring their vitality persists.

Every leaf has a certain lifespan, and when each leaf reaches a certain age, it will shed. Pine and cypress trees are no exception, except their leaves have a long lifespan, lasting 3-5 years. Typically, old leaves wither and fall after new leaves grow, giving the appearance of evergreenness throughout winter and summer.

Although the leaves of pine and cypress trees remain green in winter, the color is much duller compared to spring, summer, and autumn. This is because the low winter temperatures limit the production of chlorophyll in the leaves. This color change reduces photosynthesis within the leaves, slowing the tree's physiological activities, which is beneficial for pine and cypress trees to safely endure winter.

In conclusion, the colors of leaves changing with the seasons and the evergreen nature of some trees are fascinating adaptations of nature to the changing conditions, ensuring the survival and resilience of plant life.