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A Multitude of Trees

September 21, 2015

Scientific articles in the popular press are rare, so you can be sure that it's a "slow news day" when you read an article about how insects greatly outnumber humans on Earth. A good estimate, based on several published papers, is that there are 1019 insects (10 billion billion) representing about 1-2 million species.[1]

The number of insect species can only be estimated, and it may run up to 30 million. With slightly less than 10 billion humans on our planet (1010), the insects outnumber us by at least a billion to one. I'm starting to itch.

Illustration of a 'glimwormen' by Jan Brandes (Rijksmuseum Collection)

Illustration of a Dutch East Indies "glimwormen" by Jan Brandes (1743-1808)

(Rijksmuseum Collection, via Wikimedia Commons.)

While 10 billion billion insects is a large number, the number of bacteria on Earth is much larger. A recent estimate has 2.9 x 1029 single-celled organisms (prokaryotes) living in ocean sediments, which is about 10 billion billion bacteria per human.[2-3] The number of bacteria in soil could be ten times this number.

The relative numbers of macro-sized humans, millimeter-sized insects, and micro-sized bacteria suggest the existence of some biological principle. It's found that that small organisms have a greater number of species than large organisms, and this may influence the total number of individuals.

Figure caption

Biosafety level 4, 17th century style.

This costume, as described by Jean Jacques Manget (1652-1742) in his Traité de la peste (Treatise on the plague), was worn by physicians who attended plague patients in the 17th century.

The bird beak acted as a filter.

(Via Wikimedia Commons.)

One problem involved in an estimate of the number of a particular species is sampling bias. When I look outside my window, I see a hillside with hundreds of trees. A desert-dweller, however, would see few, if any, trees. So, how many trees are there in the world? This is an important question, since trees act as a significant carbon sink in our environment.

This biosequestration of carbon reduces atmospheric greenhouse gas, but the effect is not as complete as what we would desire. The US Environmental Protection Agency found that in 2013 US forests sequestered only 10% of the 6,673 million metric tons of carbon dioxide released by fossil fuel combustion in the United States.[4]

Although it's still unclear how many scientists it takes to change a light bulb, we now know how many it takes to count trees: 38. That's the number of authors of a recent paper in Nature describing the most accurate estimate of tree number to date.[5-8] The number found, 3.04 trillion, is an order of magnitude higher than previous estimates.[5] The breakdown of trees by habitat is about 1.39 trillion in tropical and subtropical forests, 0.74 trillion in boreal regions, and 0.61 trillion in temperate regions.[5-6]

There's a joke that the definition of a virgin forest is a forest where the "hand of man has never set foot." This study shows that the hand of man has been very active since the start of civilization. It estimates that the number of trees today is only about 54% of the number at the start of civilization, and we are presently felling more than 15 billion trees each year, although about 5 billion are replanted.[5-8]

Entrance to the Wood at Ville-d'Avray by Jean-Baptiste-Camille Corot (1796–1875)

Once a familiar scene, now typically a vacation photo opportunity.

Entrée du bois à Ville-d'Avray (Entrance to the Wood at Ville-d'Avray), oil on canvas painting, c. 1825, by Jean-Baptiste-Camille Corot (1796–1875).

(Scottish National Gallery accession number NG 1681, purchased with the aid of A E Anderson in memory of his brother Frank 1927, via Wikimedia Commons.)

This project was launched two years ago at the request of the group in charge of the United Nations "Billion Tree Campaign." The group wanted an estimate of tree number so it could set goals and evaluate its progress in reforestration. Says lead author, Thomas Crowther, a postdoctoral fellow at the Yale School of Forestry & Environmental Studies,
"Trees are among the most prominent and critical organisms on Earth... Yet you ask people to estimate, within an order of magnitude, how many trees there are and they don't know where to begin. I don't know what I would have guessed, but I was certainly surprised to find that we were talking about trillions."[6]

The best previous estimate of tree number, 400 billion trees, or about 61 trees per person, was far less than what was found.[6] That estimate was made solely through satellite imagery without ground observations.[6] The new estimate of 3.04 trillion corresponds to 420 trees per person.[7-8]

The study used a combination of satellite imagery and tree density information from more than 400,000 forest plots to obtain data for supercomputer modeling.[6] Satellite imagery, while good for showing forest extent, can't count the number of individual trees beneath the forest canopy, thus the need for ground observation.[8] The result was a global map of tree density with square-kilometer resolution.[6]

Global tree density

This global map of forest density, resolved at the square-kilometer level, was made from data derived from a combination of satellite imagery, forest inventories, and supercomputer modeling. (Yale University image by Crowther, et al.)[6)]

This more accurate estimate of tree number will have application in many environmental areas, such as assessment of animal and plant habitats, and for climate models relating to the removal of carbon dioxide from the atmosphere.[6,8] Although the number of trees has "increased," Crowther cautions that nothing has really changed. These trees have always been there. It's just our estimate of their numbers that's changed.[8]


  1. Numbers of Insects (Species and Individuals), Encyclopedia Smithsonian.
  2. Jens Kallmeyera, Robert Pockalnyc, Rishi Ram Adhikari, David C. Smith, and Steven D'Hondt, "Global distribution of microbial abundance and biomass in subseafloor sediment," Proc. Natl Acad. Sci., vol. 109, no. 40 (October 2, 2012) http://dx.doi.org/10.1073/pnas.1203849109.
  3. Kathryn Lougheed, "There are fewer microbes out there than you think," Nature News, August 27, 2012.
  4. U.S. Greenhouse Gas Inventory Report: 1990-2013, US Environmental Protection Agency.
  5. T. W. Crowther, H. B. Glick, K. R. Covey, C. Bettigole, D. S. Maynard, S. M. Thomas, J. R. Smith, G. Hintler, M. C. Duguid, G. Amatulli, M.-N. Tuanmu, W. Jetz, C. Salas, C. Stam, D. Piotto, R. Tavani, S. Green, G. Bruce, S. J. Williams, S. K. Wiser, M. O. Huber, G. M. Hengeveld, G.-J. Nabuurs, E. Tikhonova, P. Borchardt , et al., "Mapping tree density at a global scale," Nature, Published online September 2, 2015, doi:10.1038/nature14967.
  6. Kevin Dennehy, "F&ES Study Reveals There are Many More Trees Than Previously Believed," Yale University Press Release, September 2, 2015.
  7. 'Three trillion' trees on Earth, BBC, September 3, 2015.
  8. Jonathan Amos, "Earth's trees number 'three trillion'," BBC News, September 3, 2015.

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