2024-06-12 by Shakunthala Natarajan
Why Do Trees Not Stop Growing? | Science Blog
It is a scorching summer day and you are desperate for shelter. Just then you spot a large bunch of trees offering you shade with their emerald canopies. As you enjoy the cool natural shed, you are just awe-struck by the gigantic tall trees. You wonder, do these green friends grow forever? Do they have some super power of immortality that keeps them on the growth track? Let us join Utsuka and Jigyasa as they embark to explore the puzzle of tree growth in this episode of IndiaAsksWhy.
First. Does “growth” mean only increase in tree height? No! Irreversible increase in tree size is growth. So processes like, increase in height, girth, branching, and developing new leaves, together cause tree growth. Now let us look at the magic cells that cause trees to grow to great heights. These cells present in the stem, root tips and axial regions are called ‘meristematic’ cells. Their only task is to divide and make more cells whose function is yet to be determined. Based on communication from other plant regions, they move out of the meristematic region to be made into specialized cells with specific functions. Now as they try to fit into their new job roles, they need to adapt accordingly right? So the cells grow bigger in size by taking up water, nutrients and making cell organelles. These activities of cell specialization and growth occur simultaneously.
Going back to magical meristematic cells once more. These clever cells don’t go on dividing unmindfully. They get instructions from the genes or a cocktail of chemical messengers from other plant regions to start and stop dividing. Growth needs a lot of investment in terms of nutrients, water and energy, which is expensive for the organism. Let’s talk about height. In case, trees grow too tall, there will be difficulty in transporting water, and minimizing water loss by transpiration. So the tree gives a red signal to its meristematic cell army to stop division. This theory is popularly called the ‘hydraulic limit theory’. In some situations where conserving energy becomes more important than growth, like cold winters, meristematic cells are again restricted from dividing.
If growing tall is so difficult, why do trees do so? Although water is a main factor determining tree height, there is another interesting phenomenon taking place. Trees have an invisible sensor system that spies their neighbours who shade them. Based on the feedback from this sensing system, they instruct the meristematic cells to divide. This increase in tree height helps them absorb more sunlight. But sometimes the trees are so smart to realize that they can never surpass their gigantic neighbours. At this point, they further stop growing in height. However, as the trees get older, they resort to other modes of growth like increasing its girth and width rather than height. Tree growth is hence a highly coordinated event whose results come out of the strategic decisions and risk analysis undertaken by our efficient tree friends! What a splendid display of natural intellect :)
Tree growth is a never-ending phenomenon due to the immortal meristematic army. But there is a particular limit to the height that a tree species can grow, as we saw before. In reality, we see that trees are so diverse in their heights and not uniform. But have you thought what would happen if all the trees grow to the same height? Utsuka and Jigyasa too are puzzled by this. They call upon Dr Vinita Gowda, a plant biologist, to understand tree growth to a greater extent. Dr Gowda explains that tree height is a combined output of competition for resources, features of the neighbouring tree community and their structures. For trees, to grow tall, means more sunlight, exposure to strong winds and a more dynamic surrounding. Trees that have strong sturdy branches are tolerant to these situations. But weaker ones break. For example shrubs, herbs have different bark textures and requirements for sunlight and are of varying heights. Thus, based on the inherent qualities of the tree, its need for water and sunlight, each tree has its own sweet spot in terms of height. It is much like different individuals having different preferences. This means that everyone can get what they need and survive together. ‘Unity in diversity’ is the secret behind tree heights and growth, and that’s precisely why not all trees grow up to the same height.
We are finally at the end of the journey into the woods. We looked at the beautiful mechanism operating behind tree growth and determining tree heights. I bet you must be more curious to directly hear the tree adventures of Utsuka and Jigyasa. And it is time to gear up your headphones and get started with your Science quest!
References
- Apr 28, 2016 — by Natural Selections (2016) Why don’t trees just keep growing?, NCPR. Available at: https://www.northcountrypublicradio.org/news/story/7893/20160428/why-don-t-trees-just-keep-growing (Accessed: 15 July 2023).
- Do trees ever stop growing? (2022) EarthSky. Available at: https://earthsky.org/earth/what-makes-a-tree-stop-growing/ (Accessed: 15 July 2023).
- Do trees stop growing? (no date) Do trees stop growing? | Science Illustrated. Available at: https://scienceillustrated.com.au/blog/nature/do-trees-stop-growing/ (Accessed: 15 July 2023).
- Dorn, L. (2019) Why trees stop growing after a certain height, Laughing Squid. Available at: https://laughingsquid.com/why-trees-stop-growing/ (Accessed: 15 July 2023).
- Harris, R. (2014) An old tree doesn’t get taller, but bulks up like a bodybuilder, NPR. Available at: https://www.npr.org/2014/01/16/262479807/old-trees-grow-faster-with-every-year (Accessed: 15 July 2023).
- Trees accelerate growth as they get older and bigger, study finds (2014) The Guardian. Available at:https://www.theguardian.com/environment/2014/jan/15/trees-grow-more-older-carbon (Accessed: 15 July 2023).
