Tree

The Royal Oak of England

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Divide and multiply

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Great Apes

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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 angiosperm trees like maples are more closely related to grass than gynmosperm trees like pines, and there are other trees like tree ferns 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

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:

Lepidodendrales

A fossil of Lepidophloios, a scale tree.

Lepidodendrales, also known as "scale trees", are a group of lycophytes, a group of plants that was once very diverse but now are largely limited to club mosses. 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.

Ferns

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. The extinct lineages include Tempskya as well as some extinct members of the still extant Osmundaceae.

Seed ferns

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.

Gymnosperms

Ginkgos

Ginkgo biloba 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. 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.

Conifers

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 (larches being the exception). Conifers are most known for dominating the vast taiga of North America, Europe, and Asia, although they can really survive in many different environments, including arid and tropical ones. The tallest (coast redwood), oldest (bristlecone pine), and largest by volume (giant sequoia) of all extant trees are all conifers.

Angiosperms

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. 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.

How many

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.

Tree-planting and global warming

Tree-planting is often seen as a good way of combatting global warming by absorbing large amounts of carbon. 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.

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. 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. 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. In 2017 Pakistan completed a goal of planting 1 billion trees (despite allegations of corruption). 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. 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.

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. 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. 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). 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. 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. Adding trees within croplands can both increase carbon sequestration and reduce pressure on deforestation with often the added benefit of increased food yield.

Woo and legend

Trees cause pollution

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.

Reagan was falsely referencing the photochemical reaction that causes ground-level ozone pollution (GLOP), also known as photochemical smog. 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). During hot and sunny weather, many tree species release volatile organic compounds into the atmosphere, notably terpenes 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

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. 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.

Carnivorous trees

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.

Tree of the knowledge of good and evil and Tree of life

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.

See also

Tree topics:

• Dendrochronology — calculating dates using tree rings
• Petrified forest — stone trees
• Tree of life — various meanings
• Superfoods — including apple, cacao, walnut, and possibly other trees
• American tall tales — including Johnny Appleseed who planted trees and Paul Bunyan who chopped them down again
• Ancient giant silicon-based tree hypothesis — your guess is as good as mine

Specific types of tree and their products:

• Açaí berry
• Apple cider vinegar
• Christmas tree — usually pines
• Coffee is a small tree — some varieties grow well in shade as an understory crop
• Graviola
• Palm oil
• Tar water — an aromatic pine tree product
• Yew

Famous people whose lives involved trees in popular mythology:

• Adam and Eve
• Isaac Newton
• George Washington