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A tree is a large plant with a woody stem. There is no clear definition of what constitutes a tree, and they do not form a distinctive biological group or clade, as angiospermWikipedia trees like maples are more closely related to grass than gynmospermWikipedia trees like pines,[1] and there are other trees like tree fernsWikipedia that aren't closely related to either group. They are generally considered a good thing: they produce oxygen, they can combat global warming by absorbing atmospheric carbon dioxide and storing carbon, they produce lots of useful foodstuffs, they provide shelter and home to many animals and other plants, and they look nice.

Types of trees[edit]

Trees are a great example of convergent evolution, as many different plant lineages have evolved into trees as it's a great way of being able to grow taller than other plants to get more sunlight. Some of the major types of trees include:


A fossil of Lepidophloios,Wikipedia a scale tree.

Lepidodendrales,Wikipedia also known as "scale trees", are a group of lycophytes,Wikipedia a group of plants that was once very diverse but now are largely limited to club mosses.Wikipedia Being some of the earlier vascular plants, having diverged from the lineage in the Silurian, these trees reproduced using spores instead of seeds. While the scale trees are long extinct, they left a lasting impact, being the main plants that are responsible for the massive coal swamps.[2]


While today are mainly associated with the smaller plants of their clade, ferns have actually evolved into trees no less than three times, with the most recent lineage (appearing in the late Jurassic), tree ferns, still being alive today. Originally native to Australia and New Zealand, tree ferns were introduced to other places as a result of human activity, and like their smaller cousins tree ferns have fronds and reproduce using spores.[3] The extinct lineages include TempskyaWikipedia as well as some extinct members of the still extant Osmundaceae.Wikipedia

Seed ferns[edit]

Despite their name, these were not closely related to modern ferns. Although they looked superficially similar, these plants produced seeds instead of spores, and they were likely the first lineage to do so. While they were also contributors to the coal swamps, they sadly went extinct in the K-Pg extinction event.[4]



Ginkgo bilobaWikipedia is a living fossil, being the last remaining descendant of an order that first showed up in the Permian. In fact, they barely avoided extinction during the ice age, with cultivation by Chinese monks being the main reason they survive today, and since then they've been cultivated across the planet.[5] The ginkgo is famous for its smelly seeds that are used in Traditional Chinese Medicine, although Western woo-meisters seem to prefer the leaves (probably due to the aforementioned smell). As it thrives quite well in urban environments, it's one of the plant species that humans have actually significantly aided the survival of.[6]


Conifers are so named because they bear cones that contain their seeds. Most conifers have needles or scale-like leaves, and most of them don't shed their leaves during the winter (larchesWikipedia being the exception). Conifers are most known for dominating the vast taigaWikipedia of North America, Europe, and Asia,[note 1] although they can really survive in many different environments, including arid and tropical ones. The tallest (coast redwood),Wikipedia oldest (bristlecone pine),Wikipedia and largest by volume (giant sequoia)Wikipedia of all extant trees are all conifers.[7]


Angiosperms are currently the dominant group of plants, having split from the gymnosperms 300 million years ago but only truly diversifying during the Cretaceous. They are distinct from other plants because they bear flowers, and by extension, fruit.[note 2] Angiosperm trees are often referred to as deciduous, meaning they lose their leaves in winter, but keep in mind larches are technically deciduous despite being conifers and most tropical angiosperm trees don't lose their leaves over winter.[note 3][8]

How many[edit]

It isn't clear how many trees there are on Earth, but a 2015 estimate reported in Nature was 3 trillion of the things, which is a lot more than there are people.[9]

Tree-planting and global warming[edit]

Tree-planting is often seen as a good way of combatting global warming by absorbing large amounts of carbon.[10][11] One problem is that if, as mentioned, there are 3 trillion trees already, then making a significant difference will require planting a lot. Research led by Tom Crowther at ETH Zurich published in Science in 2019 recommended planting one trillion trees on 11% of the Earth's land to eventually absorb 200 gigatonnes of carbon from the atmosphere, around two-thirds of the total anthropogenic carbon in the atmosphere.[10]

There are also risks in that you can't just stick trees anywhere. They require water and other resources, and may have effects on local ecosystems, particularly with low-quality schemes that just plant lots of one or two types of tree rather than seeking to preserve or restore existing types of forest.[11] One traditional method of reforestation is known as "pines in lines", in which pine trees are planted in rows, but this requires expensive maintenance by foresters meaning it may be prohibitively expensive.[12] It's also important to preserve farmland for growing crops if we want to stop people dying of hunger.

A number of ambitious schemes have been announced. Ethiopia planned to plant 4 billion trees by October 2019, including 350 million on just one day, although it is unclear how many they actually planted.[13] In 2017 Pakistan completed a goal of planting 1 billion trees (despite allegations of corruption).[14] Pakistan's scheme was inspired by the International Union for Conservation of Nature's (IUCN) Bonn Challenge launched in 2011, which aimed to restore 150 million hectares of forest globally by 2020 and 350 million hectares by 2030.[15] A Turkish scheme for National Forestation Day on 11 November 2019 is alleged to have ended in failure with 90% of the 11 million trees dead within a few months.[16]

Before the UK's 2019 general election, the British Labour Party announced plans to plant 2 billion trees; this is a bit more than the recommendations of the UK government Committee on Climate Change which recommended 1.5 billion trees be planted by 2050 and appears very ambitious but not impossible.[17][18] Labour lost the election, and we'll have to wait a while to see if 1.5 billion trees are planted.

Growing trees isn't as simple as dropping a seed in the ground. Most schemes, such as those in Pakistan and Ethiopia, rely on tree nurseries to produce seedlings which must then be planted in the correct location. Growing trees require protection from grazing animals, as well as the risk of disease and insect pests, and can suffer from a lack of water and nutrients.[19] So they may require water, fertilizer, pesticide, and the infrastructure (roads/tracks, accommodation for foresters, etc) to plant and maintain them. All this adds to the costs and can provide a negative environmental impact, but it's the sort of thing farmers and foresters have been doing for millennia.

Frequently, when governments get behind the idea of tree planting on a massive scale, it winds up being tree plantations, which have a relatively relatively minor impact compared to restoring land to natural forest (3 vs. 42 gigatons of sequestered carbon).[18] Restoring natural forests is also a much more difficult task than making plantations because it usually requires specific plans tailored to each location so that that there is buy-in from the communities who will be responsible for protecting the forests.[18] There is likewise an important need to protect natural forests that are still intact because there is constant economic pressure for deforestation, and this too requires tailoring solutions to local communities.[20] Adding trees within croplands can both increase carbon sequestration and reduce pressure on deforestation with often the added benefit of increased food yield.[21]

Woo and legend[edit]

Trees cause pollution[edit]

Quick, someone stop this dastardly producer of pollution!

The idea that trees cause pollution is a gloriously stupid concept foisted upon the public in 1981 by U.S. President Ronald Reagan. The full quote of his statement was:

Trees cause more pollution than automobiles do.[22]

Reagan was falsely referencing the photochemical reaction that causes ground-level ozone pollution (GLOP), also known as photochemical smog.[23] Smog is created when automobile and power plant emissions are broken down into ozone and other chemicals by strong UV radiation. These reactions are then further amplified by the presence of various volatile organic compounds (VOCs).[24] During hot and sunny weather, many tree species release volatile organic compounds into the atmosphere, notably terpenesWikipedia and isoprenes (pine pitch is a terpene). Other VOCs that contribute to GLOP include gasoline, kerosene, paint, paint thinner and other industrial solvents.

The truth[edit]

Reagan's statement was disingenuous because volatile organic compounds produced by trees do not cause pollution any more than the Sun itself causes pollution. Reagan might as well have said, "The Sun causes more pollution than automobiles do." Photochemical ozone pollution is created when automobile and power plant pollution is broken down by strong sunlight in the presence of any number of volatile organic compounds.

GLOP or photochemical smog is only created in areas that receive large amounts of pollution from automobiles and power plants.[25] Trees are not required to create it. Without the input of large amounts of synthetic nitrogen oxides by cars and power plants, the VOCs released by forests on a summer day have nothing to react with, and thus do not create any pollution. On the contrary, photochemical smog is actually very harmful to trees— reducing their growth, making their shed leaves decay slower and affecting symbiotic insects and other small organisms that live on them.[26]

Carnivorous trees[edit]

This apparently goes back to a disreputable journalist named Edmund Spencer who in 1874 wrote a report about natives of Madagascar sacrificing somebody to a carnivorous tree. Untrue.[27]

Tree of the knowledge of good and evil and Tree of life[edit]

These appear in Genesis 2:3. Adam and Eve ate the fruit of the tree of knowledge, and as a result were thrown out of the Garden of Eden. There is a lot of speculation about what kind of trees they were. While commonly believed to be an apple, there is no evidence in the Bible for this, and it's likely out of place anyway as apples originated in Kazakhstan and not the Middle East.[28]

See also[edit]

Tree topics:

Specific types of tree and their products:

Famous people whose lives involved trees in popular mythology:


  1. Which is actually a new environment, geologically speaking: the taiga's existence is a direct result of the most recent glacial maximum of the ice age
  2. Note that the botanical definition of a "fruit" (which includes things like acorns and star anise, not to mention tomatoes) is quite a bit wider than the culinary one.
  3. The opposite of "deciduous" isn't "coniferous" as many believe, but rather "evergreen".


  1. Forest Stewardship: Gymnosperms vs Angiosperms, Mount Grace Land Conservation Trust 5 December 2022
  2. Fossil Lycophytes, Sam Noble Museum
  3. Tree Fern, San Diego Zoo
  4. Seed Fern Fossil, National Park Service
  5. Ginkgoales: Fossil Record, University of California Museum of Paleontology
  6. The Life Story of The Oldest Tree on Earth, Roger Cohn, Yale School of the Environment 1 May 2013
  7. Oh Christmas Tree: The Science of Conifer Trees, US Geological Survey
  8. What Are The Differences Between Evergreen And Deciduous Trees?, World Atlas
  9. Earth's trees number 'three trillion', BBC, 3 September 2015
  10. 10.0 10.1 Tree planting 'has mind-blowing potential' to tackle climate crisis, The Guardian, 4 July 2019
  11. 11.0 11.1 We Can’t Just Plant Billions of Trees to Stop Climate Change, Discover Magazine, 10 July 2019
  12. Tree-planting programs can do more harm than good, National Geographic, 26 April 2019
  13. Deforestation: Did Ethiopia plant 350 million trees in a day?, BBC Reality Check, 11 Aug 2019
  14. Pakistan has planted over a billion trees, World Economic Forum, 02 Jul 2018
  15. About the Bonn Challenge Goal, Bonn Challenge website
  16. Most of 11m trees planted in Turkish project 'may be dead', The Guardian, Jan 30, 2020
  17. Is Labour's plan to plant 2bn trees realistic?, The Guardian, 28 Nov 2019
  18. 18.0 18.1 18.2 Why planting tons of trees isn’t enough to solve climate change: Massive projects need much more planning and follow-through to succeed – and other tree protections need to happen too by Carolyn Gramling (July 9, 2021 at 6:00 am) Science News.
  19. What Are the Most Common Threats To Trees in Your Landscape?, Big Blog of Gardening, Sep 2018
  20. The first step in using trees to slow climate change: Protect the trees we have: By holding onto the big, old trees, more carbon will stay sequestered by Susan Milius (July 13, 2021 at 6:00 am) Science News.
  21. Mixing trees and crops can help both farmers and the climate: Agriculture that includes trees boosts food production, stores carbon and saves species. by Jonathan Lambert (July 14, 2021 at 6:00 am) Science News.
  24. See the Wikipedia article on Photochemical smog.
  26. Study shows how pollution affects tree growth. University of Wisconsin.
  27. See the Wikipedia article on Man-eating tree.
  28. Was the 'forbidden fruit' in the Garden of Eden really an apple?, Ashley P. Taylor, Live Science 27 March 2021
  29. Cherry Tree Myth Mount Vernon Ladies’ Association.