What is a tree?

The Great Otway National Park. Photo by author

I love trees. I am a third generation Arborist. When I tell people I am an arborist, most people give me a confused look for which I explain that essentially, I look after trees.

What is a tree?

It’s hard to define what makes a tree as they come in all shapes and sizes.

This tallest tree in the world is a species of Redwood (Sequoia sempervirens), measuring a whopping 115.7 metres tall.For some time, it was thought that the largest organism in the world was a tree. A Quacking Aspen (Populus tremuloides), named Pando, has 47,000 individual stems/trees arising from the vast root system, which has an area of 433, 014 square metres – about 21 MCG’s. This has been surpassed by a species of fungi which spreads 8.8 square kilometres.

Most simply, a tree is ‘a woody perennial shedding organism with a mature height greater than three to five metres, and usually with a single stem’.

A 400+ year old Eucalyptus camaldulensis (River Red Gum) with an arborist in the canopy. Photo by author

How do trees grow?

I am lucky enough to plant a lot of trees. Once I have finished a long day of planting, I will have a snack and some water. Trees are like us.They need food and water to grow.

Water is taken in by the roots and pulled up the tree. Only 5% of the water absorbed by the roots is used by the tree. The remaining 95% exits the tree via microscopic openings on the leaves. It is this evapotranspiration that is so important for cooling cities and reducing the urban heat

Unlike us humans, trees can’t go down to their local deli and order a toasted cheese sandwich. Trees are autotrophs, meaning they rely on other means for creating food/energy for their growth.

Trees absorb certain wave lengths of light, absorbed by green pigments which give leaves their green colour. Carbon dioxide is absorbed by the tree in the same openings which the water exits the tree. The process by which trees make their ‘food’ is photosynthesis. Light energy from the sun is mixed with the CO2 taken in by the leaves, and the water taken in by the roots. What is produced are carbohydrates for the plant oxygen for us. Pretty amazing process!!

Trees can’t do it all by themselves

 Much like us, trees can’t do it all by themselves and need a little help from their friends in what is two truly symbiotic relationships from the unlikeliest of allies – Fungi and Bacteria.

A type of fungi, called mycorrhizae, form symbiotic relationships with trees where the fungi help the tree by extending further than what a normal root system can and by making nutrients available that would otherwise be locked up in the soil. The tree provides the fungi sugars in return.

Around 78% of the air we breathe is nitrogen. And nitrogen is the most important element in tree growth and function. The problem though, is that it is not in a form that trees can absorb. In comes bacteria. Some species of trees and plants have developed symbiotic relationships with nitrogen fixing bacteria. These bacteria grow on the roots of trees and plants and absorb atmospheric nitrogen and turn it into a form that is available to the plants. The bacteria in turn is provided sugars.

Nitrogen fixing nodule on a root. Photo by author

Trees are great. Go touch them.

4 Responses to “What is a tree?”

  1. NortheShore says:

    I wanted to thank you for your time for this wonderful read!! You described the life cycle of the tree in a very easy and more interesting way. Well done!

  2. Eliza Kelly says:

    I am a budding tree enthusiast, have just started my first garden and it is very exciting! I loved this blog and love your passion!

  3. Matt James says:

    Thanks, Gabriela. The symbiotic relationships trees have with other organisms really is amazing.

  4. Gabriela Eder says:

    I love trees too! Thank you for this nice overview about the life of a tree. It is really interesting how trees and bacteria benefit from each other.