Five things you should know about trees and forests


In honour of the International Day of Forests, Tim Wallace presents some fascinating lesser-known facts about our arboreal companions.


1. There are about 3 trillion trees (and falling) in the world

A global map of tree density.
A global map of tree density.
Crowther et al.

Until 2015 no one really had any idea how many trees there are. The best global estimate was about 400 billion. When researchers from Yale University were asked to come up with a more accurate figure by the organisers of a UN-launched tree-planting initiative called the Billion Tree Campaign, they discovered this number was dramatically on the low side.

The conclusion of the research (led by Thomas Crowther, now with the Swiss Federal Institute of Technology in Zurich) was that there were more than 3 trillion trees. This result was found by collating data including satellite tree inventories verified by on-the-ground counting, to compute tree numbers down to the square kilometre.

The study also delivered a recalibrated estimate of the number of trees being destroyed by humans – more than 15 billion a year. The total tree population had been halved as a consequence of human activities over the past 5,000 years or so.

These figures made the 14 billion trees planted by the Billion Tree Campaign in its first decade seem hopelessly inadequate. In 2017 it became the Trillion Tree Campaign.

2. The planet’s lungs have emphysema

Degradation and disturbance account for 69% of total carbon losses from the world’s tropical forests.
Degradation and disturbance account for 69% of total carbon losses from the world’s tropical forests.
Eco / UIG

The Yale research also gave us some reasonably precise figures on where trees are. Twenty-two per cent are in the temperate zones, between the tropics and the polar regions; the most dense forests are in the sub-arctic boreal regions of Russia, Scandinavia and North America, which account for another 24 per cent. The biggest forests, however, are in the tropics. Home to about 43 per cent of all trees, they are rightly known as the lungs of the planet.

But those lungs are now emphysemic. In September 2017 there came the depressing diagnosis that tropical forests are now emitting more carbon than they capture, due to the rate of forest destruction, degradation and disturbance.

This finding was made by researchers from the Woods Hole Research Center and Boston University, both in Massachusetts, who refined satellite monitoring tools to identify subtle tree losses as well as complete clearing. The study showed that deforestation – long recognised as damaging – is now less a problem than more subtle incursions, with degradation and disturbance accounting for 69 per cent of total carbon losses from the world’s tropical forests.

“It can be a challenge to map the forests that have been completely lost,” said one of the paper’s authors, Wayne Walker. “It’s even more difficult to measure small and more subtle losses of forest. In many cases throughout the tropics you have selective logging, or smallholder farmers removing individual trees for fuel wood. These losses can be relatively small in any one place, but added up across large areas they become considerable.”

3. Trees are connected

As above, so below: The root systems of tropical trees may represent 30 percent of the total tree biomass.
As above, so below: The root systems of tropical trees may represent 30 per cent of the total tree biomass.
Jefferson Hall / STRI

One aspect of tree life that satellites can’t shed much light on is what is going on underground. Root systems can account for almost 30% of the total biomass of young tropical trees, according to a study conducted in Panama and published in October 2017.

The implications for carbon accounting and storage programs are only part of the significance of the research, which involved carefully excavating the root systems of six different tree species.

Not only did scientists find root systems extending more than 20 meters from tree trunks but about 5 per cent of the time those roots were connected with trees of neighbouring species via grafts.

“Are these trees sharing resources?” pondered study co-author Jefferson Hall, of the Smithsonian Tropical Research Institute. “Would we have found a higher percentage of root grafts if we had the ability to look at fine roots? Clearly there is more work to be done.”

4. Trees share friends, and resources

Though competition for resources has historically been considered the dominant relationship between trees, as with all other species, the evidence continues to mount that trees communicate and interact in far more complex and cooperative ways. Experiments in a Swiss forest, for example, suggest trees effectively share food via an underground carbon-trading system.

Trees, of course, use photosynthesis to convert sunlight, water and carbon dioxide into sugars that are then transported from the leaves to feed branch, stem and root construction. The tree’s roots also exchange the carbon-rich photosynthate for water and nutrients from symbiotic fungal partners.

By flooding the crowns of individual trees with carbon dioxide carrying a specific atomic signature, scientists were able to track the path of carbon in that sugar, using atomic mass spectrometry. They found the carbon also ended up in the roots of neighbouring trees. Their conclusion: it was transported by fungal intermediaries. “Evidently,” says study co-author Christian Körner, “the forest is more than the sum of its trees.”

5. Trees sleep too

Detailed tracking of tree limb movement shows that they exhibit a slight, sleepy droop overnight.
Detailed tracking of tree limb movements shows that they exhibit a slight sleepy droop overnight.
Eetu Puttonen

Like almost every living organism, trees are attuned to the planet’s day-night cycle. During the day they photosynthesise and breathe out oxygen; at night they rest in what researchers have likened to sleep – a state indicated by a quantifiable physical droop.

While the nocturnal hibernation of small flowering plants has long been known, proving the same in fully grown trees only became possible with advanced technology – using laser scanning to track millions of points on a tree. Laser scanning means each data point needs only to be illuminated with infrared light for a split second, so the nocturnal cycle is not disrupted.

A team of researchers from Austria, Finland and Hungary used this technique in 2015 to scan trees in different countries from sunset to sunrise. Though the changes were not large – with leaves and branches dropping about 10 cm in five-metre-tall trees – they were “systematic and well within the accuracy of our instruments”, said study lead author Eetu Puttonen.

The drooping, which occurred over a few hours and was reversed over roughly the same time come daylight, was credited to the water balance within individual tree cells. During the day, when the leaves are busy photosynthesising, they open their stomata to take in carbon dioxide. The open stomata mean leaf cells can also lose the water they need to for photosynthesis. So the tree pumps water from its roots and trunks out through its branches to the leaves. It’s a bit like pumping water through a fire hose, effectively inflating the entire structure through maintaining cellular water pressure.


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  1. https://www.nature.com/articles/nature14967
  2. https://cosmosmagazine.com/climate/tropical-forests-no-longer-a-carbon-sink
  3. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185934
  4. http://science.sciencemag.org/content/352/6283/342
  5. https://www.frontiersin.org/articles/10.3389/fpls.2016.00222/full
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