is there a mathematical formula or explanation to the growth of trees?-applied mathematics
There is no definite formula for tree growth, as the amount trees grow depends on a great number of things, such as light, temperature, water, minerals, whether or not it has parasites, etc. You can explain how much trees usually grow, and how much these factors limit or aid their growth, but you can't make a universally working formula for tree growth (If this is something you need to do theoretically as an example problem, however, then you could pretend it was a very controlled environment and base your formula off one or two factors).
