Month: February 2012

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Roots and All

Roots are poorly appreciated. They are not observed when we admire a tree’s beauty, and when they are, it is often due to encroaching roots that may be too close to a property’s foundations, or when a ferocious storm has passed. To be honest, they can be aesthetically less pleasing; but there are some beautiful exceptions (see pic).

The magnificent flowing roots of the Ficus

However, roots are more important than leaves; providing not only water and minerals through absorption and nutrients in winter, but also strong anchorage. Roots naturally seek minerals and water so they tend to be found in the surface layer of soil and can extend about 3 times the width of the branches. As oxygen is vital for growth (for the conversion of sugars created by photosynthesis to energy), compacted soil underneath a tree can seriously hinder oxygen transfer and a tree’s health. Sometimes this may take 2 or 3 years to become noticeable. Trees in more arid regions have deeper roots, some such as the Eucalyptus may have roots extending more than 50m into the earth. Access to oxygen in water-logged regions is a challenge and some trees have developed clever adaptations e.g. mangroves extend roots like snorkels above the waterline to gain access to air (see pic), as well as to provide oxygen for their unique desalination process.

Mangrove roots act as 'snorkels' to get air

Along with roots come all the bacteria and fungi, which are even less appreciated. For some families of trees there are rhizobia, soil bacteria found in root nodules (e.g. legumes) which fix nitrogen from air for the tree’s use. There are also mycorrhiza (see pic), which are root fungi which provide increased absorption of nutrients (including phosphorous and nitrogen) to the tree, while receiving sugars in return. These fungi may have been vital in helping plants colonise land millions of years ago.

Mycorrhizal fungi have a symbiotic relationship with trees

Not only are roots integral to trees, but they have direct benefits to humans who share the landscape. This is because roots hold valuable soil in place and prevent erosion. Soil erosion is a natural process but has been exacerbated by deforestation and poor farming techniques.  For example, Madagascar has lost over 90 % of its original forests and the effects can be seen in the below picture. And let us remember that it takes 500 -1000 years to form one inch of soil.

Vast erosion in a deforested Madagascar

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The Natural Powerhouse

While humans have proudly developed their own power sources using mostly fossil fuels, uranium and rivers, photosynthesis in nature captures approximately 6 times as much energy consumed by modern civilisation. This capture rate is about 100 terawatts, gained from our closest star, the Sun.

The Sun

Trees love the Sun (Soho Extreme Ultraviolet Imaging Telescope, EIT Consortium)

How is this done? The leaves of trees and plants have tiny organelles containing chlorophyll which absorb mostly blue and red light, so we see them as green. An idea of this global activity can be observed in the world map showing concentration of chlorophyll in the sea (from phytoplankton), and vegetation concentration on land.

Chlorophyll Map

Chlorophyll Map of Earth. Sea shows chlorophyll concentration and land shows relative vegetation index.

This energy captured promotes, through the production of electrons, the reaction between carbon dioxide from the atmosphere and water to produce sugar and oxygen. In trees, the water comes from the roots transported via the xylem (see previous blog) and reacts with carbon dioxide which enters through tiny holes in the leaf, or stomata (see pic).

Tomato leaf Stoma

Leaf Stoma

The sugars produced are then transported via the phloem to the roots. The basic reaction is 6CO2 + 6H2O –> C6H12O6 (sugar) + 6O2, meaning for each part of carbon dioxide reacting, an equivalent amount of oxygen is produced.

In this way a mature tree can provide enough oxygen for 2 people to live per year, while in total, photosynthetic organisms convert about 100-115 petagrams (15 zero’s!) of carbon into biomass per year.

The competition for light in the forest is intense and sometimes trees are growing too close to each other, thereby undermining each other’s health. Shade tolerance is therefore a key competitive advantage. However, with human activity, competition for light is the least of a tree’s worries. Trees have evolved at different latitudes in different ways to capture this light. At the equator where the sun is overhead all year round, trees have broad canopies. While at higher latitudes trees generally have narrow and extended crowns to capture light at lower angles, e.g. conifers.

There is thus perhaps a simple way of being more conscious of trees and having them in our awareness, if on a sunny day we can occasionally be mindful when we enjoy a deep, luxurious breath of air, and think about that oxygen that sustains us.

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How Do Trees Grow?

We may imagine the whole tree grows upwards but this is an illusion. If we tied a yellow ribbon round an old oak tree and came back a year later it would, luckily for romantics, be in the same place and not higher off the ground. Only the apical or terminal buds of trees (see pic) will grow upwards and maintain dominance through the production of a hormone called auxin which keeps lateral buds dormant.

Apical or terminal bud

The Apical (terminal) and Lateral Buds (http://landsat.gsfc.nasa.gov/)

This is how trees maintain their traditional shape, although this has been influenced by humans through the practice of coppicing, pollarding and pruning where dominant stems are cut to enhance lateral growth and the production of new shoots.

The yellow ribbon, however, would have been stretched tighter. This is due to the thin part of the trunk that is actively growing called the cambium. The cambium produces cells which divide and specialise into the xylem and the phloem. The xylem tissue transports water and minerals from the tree’s roots upwards. The xylem forms the sapwood and as these cells age and die they turn into harder heartwood, which is found towards the centre of the tree. The phloem is the thin green layer under the bark which transports the tree’s lifeblood, the sap. The bark is made up of several layers, primarily the cork cambium which is an extremely thin layer of cells that divide to form the cork, or more commonly called bark (see pic). In this way, as trees age their diameter increases.

Tree cross-section

The Cross-Section of a Tree (Mirriam Webster)

It is thus not necessary to cut down a tree to kill it; all that is needed is to remove a ring of bark and the thin underlying tissue to prevent the transport of the sap to the roots via the phloem. This is called girdling, and sometimes practiced to remove a particular tree from an ecologically sensitive area without causing greater damage. This also shows for all their mightiness, trees are certainly vulnerable.

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The First Tree

The first tree did not grow overnight. In fact it took 3.4 billion years for life to develop from single-celled organisms to the first plant with a woody stem. 385 million years ago in the mid-Devonian period it was relatively warm, with tropical seas at 30 °C (86 °F) and cloudless skies. In this balmy weather Wattieza (see pic) grew to a height of 8m with frond-like leaves,  and reproduced by means of spores.

Wattieza

Wattieza grew to a height of 8m

This was a momentous occasion for the planet Earth. Now plants could compete for light vertically and horizontally and convert CO2 at higher rates. Thanks to the rise of forests in the later part of this period, CO2 concentration was reduced which resulted in a cooling of the planet. The rise of Wattieza and also Archaeopteris (of the upper-Devonian, see pic),

archaeopteris

Archaeopteris grew up to 30m and lived for 40-50 years

had major effects on soil chemistry and their litter would have fed streams. It is no surprise that the numbers and varieties of freshwater fish exploded at that time.

What we can conclude from these early beginnings is that the appearance and growth of trees in the history of this planet had significant effects on other life and on the climate. The tree is thus an integral part of the global ecosystem, and significantly decreasing their numbers one would expect disruption of the climate and life. What we can also take from this early period, is that planting more trees will decrease CO2levels. While scientists and engineers scratch their heads thinking of economical methods of carbon sequestration, there is already a simple and beautiful solution.

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Eyeing the Landscape Ahead

Teratrees. A trillion trees. Currently we have around 400 billion trees on Earth and we are losing 50 000 – 100 000 km2 a year of primary forest, this is roughly 3 – 6 billion trees per year. Rainforests used to cover 15 % of the Earth’s surface in 1950…in 50 years we lost more than half of that, and at current rates they have 40 – 50 years left.

As more “thinking” creatures we pride ourselves on our rationality. As Thomas Berry said “To remain viable a species must establish a niche for itself that is beneficial both for itself and for the surrounding community”. This means our current behaviour should either be described as suicide or insanity.

But current is not necessarily future. We have it within our means to alter our behaviour, and a trillion trees is where Earth needs to be. This is the first blog of many exploring trees on our planet, what they give us, how they feature in our collective minds, and how, as a human, trees are synonymous with life.

This story has begun with words but in stories to come it will end with action. While the tragedy of deforestation continues there are people and organisations planting trees, but much help is needed, and the work is indeed great. I will be working on a project in this regard, but in the meanwhile I will hope to weave a memory of trees.