Why do leaves fall off trees?

 

To survive in the environment as it becomes hostile, trees initiate abscission, the process of separation of leaves, naturally.

The ethereal beauty of Kashmir has inspired scores of poets and writers, artisans, and even Bollywood to churn out memorable works of art. Besides the breathtaking landscapes, the stimulation is also derived from the beauty of the Chinar leaf, the colour of which changes from green in summers to blood-red, amber, and yellow in autumn before they wither and fall. The tree that this leaf belongs to, the Chinar, is a majestic one known for its antiquity and magnificence. Another widely recognised leaf that is beautiful and unique is that of the Sugar Maple, an indigenous tree of Canada. The maple leaf in its autumn, fiery red avatar occupies a place of pride in the Canadian national flag.

Both Chinar and Maple are examples of deciduous trees that have broad and flat leaves which they shed once a year, leaving behind bare branches and a skeletal trunk. The plummeting of these leaves is actually an act of self-preservation against harsh weather conditions. They do not simply fall off, but are separated intentionally through a meticulous process. This loss of leaves in deciduous trees like oak, elm, beech, and maple happens primarily in winters which protects them from foliar frostbite. On the other hand, the tropical deciduous trees such as Peepal, Palash, Neem, and Ashok shed leaves between February and May which help them deal with seasonal droughts.

When we take a look at fossil records, they show that about 250 million years ago there was not a single tree on Earth designed to survive in temperatures below freezing point. They were all built for life in the tropics where the weather was always warm, water was liquid and leaves were safe year-round. If such trees grew in extreme cold regions, their water would freeze into sharp-edged ice crystals, fatally puncturing the living cells. Additionally, the Xylem tissue’s water would also freeze and bubbles would form from the gases that were previously dissolved in liquid water. When the ice would thaw, these bubbles would impede the upward flow of water, destroying the tree. And thus we see that for survival, the trees needed to prevent damage to the living cells due to ice crystal bubble formation.

Apart from extremely cold weather, it is also difficult for trees to survive in regions that are exceedingly dry. In this weather, the plants have to work harder to get water from the soil and in the process, they might suck in tiny pockets of air from the surrounding tissues which would lead to their destruction. Therefore, it is to cope with such severe weather, both extremely dry and/or cold, that the deciduous trees shed their leaves.

To survive in the environment as it becomes hostile, trees initiate abscission, the process of separation of leaves, naturally. Hormones are produced in the terminal buds, the tip of the stem that connects to the leaf. This halts the production of chlorophyll and enables the underlying yellow-orange pigments to shine through, making the trees look like a beautiful painting! The trees also pull out the moisture and other nutrients from leaves, while the xylem and the phloem which carry water and food, respectively are closed off. A layer of cells, termed an abscission layer then grows, cutting the connection between the leaf and the tree. The tree as a result then becomes dormant, similar to hibernation in animals, and the colourful leaves then become redundant and are dismantled.

Thus, by going leafless, trees conserve energy and moisture while also ensuring that all the valuable nutrients which were laboriously extracted from the soil to build the leaves earlier are absorbed and stored in twigs and branches until it is time for renewal. The exquisite gold and russet displays help deciduous trees recover as much as 50 percent of nitrogen and phosphorus from the old leaves. It would be right to say that such trees are perhaps the world’s prettiest recycling plants.

How beautifully nature has provided for creation, sustenance, self-preservation and recycling of leaves growing from and going back to Mother Earth. Nature indeed is truly marvellous!

Source: Indian Express, 1/07/22