Why are trees important?

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Trees are not only beautiful to look at they also provide protection, security and shade but, most importantly, they are fundamental to maintaining life on Earth as we know it.

Trees absorb CO2, cool and moisten our air, create oxygen, act as filtering systems to clean our air and soil and stabilize the ground against erosion. And if this wasn’t enough, they also provide food, fuel, cork, rubber, medicines and building materials for human activity.

Not surprisingly, this is why good forest management and tree replanting is and always has been essential for life on Earth.

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How did trees evolve?

South African forests © Michael Lambert

Plants evolved on Earth around 475 million years ago – during the Ordovician period, which is part of the Palaeozoic era.

On an herbaceous plant’s stem you can see buds growing at the base of most leaves in the upper angle (axil) between the leaf stalk or branch and the stem or trunk from which it is growing (called “axillary buds”).

To prevent the plant from becoming top-heavy and maintain a low centre of gravity, axillary buds on flowering plants normally grow into branches near the base of the shoot system. Because herbaceous plants have soft-structure stems the number of branches, leaves and flowers that they can support is limited.

It was only when plants evolved to have woody stems (around 425 million years ago – the Early Devonian period of the Palaeozoic era) that the ability to support branches at higher levels (known as arborescence) became possible.

The first woody-stemmed plants to achieve any major height were ferns and by the Middle Devonian period one species called Wattieza had already reached the giddy height of 8 metres!

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What is classified as a tree?

Dicksonia antarctica Labill © Mmparedes

Trees and shrubs are woody perennials that bear large numbers of branches. Generally speaking, trees have one or a small number of main trunks that support their leaf crowns whereas shrubs are smaller plants with many woody stems branching close to the ground.

Just to make things a little more complicated (as is often the case in nature), these definitions cannot be totally adhered to since tree-like shrubs and small shrub-like trees also exist!

Trees are part of an ancient group of flowering plants belonging to the family called Gymnosperms. This group includes conifers, cycads and allies although the very first “trees” were ferns. These belonged to the family now called Pteridophytes. More details.

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Why are trees essential to the planet?

What we should really ask is, “Why was tree evolution so essential to the development of our planet and why do trees remain important to life as we know it on Earth today?” Once you understand this it is difficult to see why humans continue to practise and/or allow deforestation. See Threats to trees – deforestation.

By the Late Devonian period (around 370 million years ago) trees had developed into an extremely successful species; they were so profuse and their roots bound the soil together so firmly that they even caused existing river systems to take a more meandering course around these immoveable obstacles.

This vast quantity of trees also impacted on Earth’s atmosphere because they absorbed carbon dioxide from it to such an extent that they greatly reduced the greenhouse effect (but may have been so effective at that time that they could have caused an ice age which occurred around 350 million years ago).

As new life forms evolved over the millennia, each took their place in local ecosystems to help create a natural balance. These new life forms then established a status quo by limiting and controlling the extremes of other species.

Today, forests offer food, water, shelter and protection to an incredible vadriety of wildlife and that wildlife in turn interacts with our forests and ecosystems to keep things naturally in balance. See Deadwood.

Every element in our ecosystem is essential to the whole and to maintaining the biodiversity of life on Earth as we know it.

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How do trees help cool Earth’s temperature?

Date palm Phoenix dactylifera © Jennifer Hope-Morley

Besides simply being beautiful to look at, trees also work very hard for their living. Trees are Earth’s natural air conditioners and water pumps. They cool the Earth by providing shade and recycling water. By keeping the air and ground around them cool and by helping to moderate rainfall, trees help maintain temperatures at levels that keep Earth fit for human habitation.

Trees clean the air and release oxygen

Trees release oxygen (O2), which means that while they are cleaning the air of harmful CO2 they are also creating oxygen, which they put into the atmosphere – making it easier for humans and other living things to breathe.

Trees absorb water from the soil. Heat then causes this water to be released and absorbed back into the atmosphere from the tree’s leaves (this process is called transpiration).

Clouds form when the atmosphere becomes saturated with water vapour (in the form of extremely tiny droplets), which is later discharged on the Earth as much-needed rain to help plants grow and provide us with drinking water. A cubic metre of air can contain as many as 100,000,000 of these droplets!

Forests act as a giant filter to clean the air we breathe – a mature leafy tree produces as much oxygen in one season as ten people inhale in a year – so imagine how much oxygen a whole forest produces! It is interesting to note what percentage of land was covered by trees centuries ago compared to today.

While the world’s population has increased the number of trees supporting it has decreased. If it were possible to calculate the total number of trees in the world and relate that to the number of people on the planet – you will probably find that currently there are not enough trees to provide sufficient clean air for all of us.

When trees respire they emit chemicals called Volatile Organic Compounds (VOCs), which interact with chemicals in the air to form particles that clouds develop around. Climate models suggest that these clouds slow global warming by around 1% by reflecting sunlight back into space. Locally, it is thought that these VOCs could cut warming by up to 30%!

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How do trees absorb carbon dioxide?

Copper beech Fagus silvatica atro-purpurea © Ita McCobb

Trees are important for the continuation of all life on Earth. Excesses of carbon dioxide are a major threat to life on Earth. Trees absorb carbon dioxide – CO2 (a broad-leafed tree will absorb on average as much as 10 kg of CO2 per year). To produce its food a tree absorbs and locks away carbon dioxide in its wood, roots and leaves. A forest can lock up and store in its wood as much carbon as it produces – meaning that this CO2 is not released into the atmosphere as a “greenhouse” gas.

Trees bind and clean the soil

Trees continue to bind the soil and act as breakers against erosion caused by wind and rain. Trees also operate an incredible filtering system (phytoremediation) through their deep spreading roots. (Phytoremediation is the direct use of living green plants for the removal, breakdown or containment of contaminants in soil, sludge, sediment and groundwater.)

The highly sophisticated root system of trees filters sewage and farm chemicals, reduces the effects of animal waste and cleans water runoff. Flash flooding can be dramatically reduced by a forest or simply by tree planting. More than 1,000 gallons of water annually have been known to have been absorbed by just one tree!

So when we hear of trees being destroyed, we should be aware that someone is actually depriving us of the clean air we breathe and ultimately taking away the ability of future generations to live on Earth.

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References: NASA Earth Observatory; United Nations Environment Programme (UNEP), Division of Technology, Industry and Economics; Colorado University; Brian Capon, Botany for gardeners, Third edition, Timber Press; Royal Botanical Gardens, Kew, London; Missouri Botanical Garden; Latin for gardeners, Lorraine Harrison, Royal Horticultural Society; International Botanical Congress (IBC); RHS Gardeners’ encyclopaedia, Christopher Brickell ed., Dorling Kindersley; The Royal Horticultural Society; Frances Rose, The Wild Flower Key, Warne.