[Ko Gyu-hong's Trees and People] Fierce Struggle for Life... That Is Why Danpung Is Beautiful

<7> The Principle of Autumn Leaves and Trees' Mistletoe Control Measures

The autumn foliage is at its peak. Every tree on this land has shed its green and begun preparing for winter. Poet Lee Je-ha sang of the autumn leaves as "lives falling in the void, blood vessels vomiting on every leaf" (from the poem 'Autumn Leaves'). It is a brilliant metaphor for the life and survival of trees.

To understand the reason for autumn leaves, we must first examine how trees live. Leaves are the link between the sky and the earth. They use the light from the sky to produce nourishment on the ground for all living things. Among all living beings on earth, only trees produce nutrients. Every living thing depends on the nutrients created through photosynthesis by the green chlorophyll in leaves.

In fact, leaves contain other pigments besides chlorophyll, each with its own color. However, these other pigments remain hidden because photosynthesis must be prioritized. For photosynthesis to occur, leaves must absorb sunlight well. Therefore, nothing should interfere with the sunlight that chlorophyll needs to absorb. Other pigments must lie beneath the chlorophyll. This is why leaves appear green.

As the season for photosynthesis passes, the green color disappears from the leaves, revealing a colorful palette. This signals the approach of winter. Autumn foliage is a survival strategy for trees preparing for winter. The arrival of winter means temperatures drop below 0 degrees Celsius, causing water to freeze.

Trees draw water needed for photosynthesis up from their roots. The tricky part is that even for trees with thick trunks, the water-conducting vessels are located on the outer part of the trunk, which is greatly affected by the outside temperature. The inner part of the trunk, called heartwood (심재, shimjae), is dead tissue, while the sapwood (변재, byeonjae) is the living tissue. The water vessels exist only in the sapwood.

There is no way to keep the sapwood vessels warm. At temperatures below freezing, the water inside these vessels inevitably freezes. When this happens, the water expands and the vessels burst. This is the same principle as water pipes bursting in winter.

Trees form a tissue called the 'abscission layer' (떨켜, tteol-kyeo) at the junction between branches and leaves. This is to drain water. It is a pathway extending from the roots to the leaf veins that spread out delicately above the leaves. It is also the life channel that transports nutrients produced by sunlight to the tree trunk. Before the temperature drops further, the tree must thicken the abscission layer to block this pathway. Once blocked, water cannot rise. The moisture remaining in the vessels loses its destination and escapes into the air.

Hidden autumn foliage spot, the autumn scenery at Seungbu Station, a small station on the Yeongdong Line. Maple leaves dyed red due to their high anthocyanin content.

Green chlorophyll in leaves produces nutrients through photosynthesis
In winter, water vessels risk freezing and expanding
To drain stored water, the pathway is blocked
Chlorophyll naturally breaks down, revealing various true colors
A minimal survival strategy to endure winter

Gradually, the leaves dry out. Photosynthesis, which requires sunlight, carbon dioxide, and water, can no longer occur. Until the previous season, chlorophyll actively performed photosynthesis, gathering sunlight and carbon dioxide. But without water, photosynthesis cannot be completed. Chlorophyll loses its strength and is slowly destroyed. Other elements in the leaves, which were initially present but hidden beneath the chlorophyll for photosynthesis, now have their turn to show their colors.

Each tree has different pigments; some contain a lot of anthocyanin, others have abundant carotene or tannin. Even trees of the same species can have slightly different pigment compositions. These pigments each have their own colors: anthocyanin is red, carotene is yellow, and tannin is brown.

The true colors of leaves, revealed only after chlorophyll breaks down, are diverse. Ginkgo and locust trees, rich in carotene, turn yellow; maple and sumac trees, abundant in anthocyanin, turn red; oak species such as the sawtooth oak, Mongolian oak, and Quercus serrata, which contain much tannin, turn brown and become splendid.

Sometimes, even trees of the same species show different autumn colors. For example, zelkova and cherry trees may display red, yellow, or brown hues alternately depending on the annual growth and climate.

There are also conditions that make autumn foliage more beautiful. Considering the principles of autumn leaves mentioned earlier, this can be easily guessed. For the colors to be vivid, chlorophyll must break down quickly so that anthocyanin, carotene, and tannin in the leaves can show their true colors. Above all, the tree must quickly drain water from its trunk within a short time.

It is basic that it should not rain much during the autumn foliage season. A lot of sunlight is needed to quickly drain moisture. However, excessive drought is also undesirable. A moderate amount of moisture is necessary during the time it takes for the leaves to draw water. If the leaves dry suddenly, they fall before the red color can develop, preventing us from seeing beautiful autumn foliage.

A large daily temperature range is also a condition for beautiful autumn leaves. The weather must have a big temperature difference between a cold morning and a warm afternoon so that the water inside the tree dries out thoroughly. This allows the leaves to reveal colors that contrast sharply with their previous green.

If nighttime temperatures drop significantly, the tree recognizes the signal of winter. The abscission layer that blocks water supply grows faster. Transpiration, which expels remaining moisture, becomes more active. However, if it suddenly gets too cold, leaves may suffer partial frost damage and fall before turning color. Adequate moisture is needed until the very last moment.

If one more condition is added, it would be a clear blue autumn sky. Among the typical autumn colors of red, yellow, and brown, blue provides the most extreme contrast. Therefore, a blue autumn sky is essential to enjoy the autumn foliage. The fact that Korea’s autumn skies are clear and high is a reason why our autumn leaves are so beautiful. Moreover, this year, since last spring, the sky has become clearer and higher due to the COVID-19 pandemic. All the conditions for beautiful autumn foliage are met.

Ginkgo trees, which are rich in carotene, display a fluorescent yellow color in their autumn foliage.

The autumn foliage of Cheongnyangsan in Bonghwa, Gyeongbuk, illuminating the season in a myriad of colors

When trees turn yellow or red with autumn colors, we say they are beautiful. But trees do not produce such beautiful colors to please human emotions. Each tree has its own circumstances.

The maturity of the abscission layer between branches and leaves signals that the tree is ready to shed its leaves. That is why leaves with autumn colors soon fall. Now the tree can lay down all its efforts and enter a long winter sleep. However, the tree still has worries to resolve for a peaceful winter sleep.

It worries about having to endure the winter in one place, exposed to snowstorms without moving to a better spot. In winter, trees survive with minimal energy. But they can be attacked by pests and diseases active during winter. Without nutrient supply and in deep winter sleep, trees have no way to resist these attacks. Winter survival is inevitably a harsh ordeal for trees. That is why trees produce more anthocyanin during the previous season.

Anthocyanin has a 'pest and disease control' effect. It turns out that red autumn leaves are the tree’s minimal defense strategy to ward off pests that might come during deep winter sleep. The tree now drops red leaves full of anthocyanin near its roots. The anthocyanin in the red leaves gradually seeps into the soil around the roots. It is the only weapon that protects the tree during its deep winter sleep.

Looking back, autumn foliage is not meant to please humans but is the tree’s minimal survival strategy to live on this land. Enduring the cold wind alone in a snowstorm on an open field is the fate given to trees. Though it looks calm, it must be fierce.

All living things in the world have their own charm and beauty. That beauty always contains a desperate will to survive. The brilliant festival of colors that trees display in this season is ultimately a trace of a desperate survival strategy acquired over many years. The reason autumn leaves are so beautiful is because they are the result of a fierce struggle for life.

Lee Je-ha’s poem 'Autumn Leaves,' which sings "lives falling in the void... blood vessels vomiting on every leaf," is exquisite. The season to love the tree’s desperate struggle more deeply, autumn, has deepened. Winter is coming soon.

Tree Columnist