Gravity will be the short answer…
The Long Answer
Biology of Trees
Trees are biological organisms, which means they will go through stages of life in establishing themselves, attaining maturity and after multiple years of reproduction, they die. This process can last for any number of years but usually within hundred in the case of trees in the city (and even less).
As years goes by, trees will grow taller and wider in size. The gain of height and weight works against gravity and the tree has to deal with it every single day as long as they live.
The gain of height and weight works against gravity and the tree has to deal with it every single day as long as they live.
Older trees that already passed their maturity, say over 60 years or so, are much more difficult to handle gravity, it is not only because they are taller and heavier, but it is also because they are less responsive to the environment as their metabolism starts to subside. Any stress, be that too much/less rain (environment), infection/infestation of fungus, bacteria or insects (biological), pruning (human intervention) or other reasons (e.g. pollution, soil pH), older trees won’t respond well to these things.
As for younger trees, as they are in their rapid stage of growing, they usually capable to overcome those stresses mentioned above. However, this also subjected to the intensity and duration which the consequences of these disturbances might take years before taking the toll on trees.
Challenges in Urban Environment
In the case of trees planted in the city, it is a very challenging environment for them to survive.
Trees are planted in tree pit which the size already dictated how big a tree would grow.
Trees are planted in tree pit which the size already dictated how big a tree would grow. When the tree finally outgrown the tree pit, the tree needs to destroy the structure on the floor while maintaining growth on top. The tree base not only wounded from struggling with bricks, tree grilles or concrete but also weaken the supporting capabilities of the tree trunk as it needs to be deformed to suit the shape and size of space. Even more deadly so, is the wounds that would invite bacteria, fungus and insects that they infect and infest the tree to kill.
So these are my explanation on Why Trees Fall, hopefully it would bring answers to people to understand the biology of trees on how they keep themselves right up until they can’t anymore.
…we are stretching the biological limit of trees, question is…
Keeping old trees is equally meaning that we are stretching the biological limit of trees, question is, we need to prevent tragic to happen when trees fall. It seems to me is a management challenge on balancing conservation of old trees and public safety.
Meanwhile, understanding public expectations are equally important. After all, the reasons for all greening and tree conservation projects are for the public right?
Let’s hope no more tragic happen in the future.
Trees related incidents in 2015
Chan S. (2015, April 20). Lucky escape for Hong Kong driver and wife after falling tree crushes car on Mid-Levels road. South China Morning Post. Retrieved from: http://www.scmp.com (self note: OVT DSD CW/1)
Lo, C., & Kao, E. (2015, July 20) Massive Chinese banyan tree falls on woman on busy street in Hong Kong’s Central. South China Morning Post. Retrieved from: http://www.scmp.com
Lo, C. & Mok, D. (2015, July 22).Highways Department called for fallen banyan tree to be removed a year ago – but local residents objected. South China Morning Post. Retrieved from: http://www.scmp.com
Lee, D. & Ben, W. (2015, August 26). Another massive banyan tree crashes down onto busy Hong Kong street as city braces for week of heavy rain. South China Morning Post. Retrieved from: http://www.scmp.com
The post is also available in Chinese http://royceto.com/chinese-why-trees-fall/