Every fact in the three tree lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: 'one ring per year' is the usual rule, but in harsh conditions a tree may not form a ring every year: the bristlecone pine 'Prometheus' had 4,862 rings but its age was estimated at about 4,900 years [8]. The water figure for a large oak (about 151,000 litres a year) is a USGS illustration for one large tree, not a value for every oak [5]. The carbon method from Forestry England [10] gives a rough estimate from trunk girth alone; 'about half of the dry weight is carbon' is approximate. In Forestry England's worked example the final figure (about 10,590 kg) is the carbon dioxide taken up over the tree's life, although the sheet labels it as carbon; we use it as CO₂. Record heights and ages change as new trees are measured; we give the values stated by the US National Park Service at the time of writing [7][8]. The Woodland Trust page [9] is from a conservation charity and is rated Medium-high.
All sources
[1] Parts of a tree
Minnesota Department of Natural Resources (Forest Treasures) · Reliability: High
Type: State natural resources agency (education page)
The crown is the branches and leaves; leaves are 'tiny factories' that make food using water from the roots and carbon dioxide from the air. Evergreen leaves are needles. Roots anchor the tree and absorb water and minerals; they often spread well beyond the crown, with large roots mostly in the top 12 inches (about 30 cm) of soil. Trunk layers: outer bark (hard dead tissue that protects the living inner parts), inner bark or phloem (pipelines that carry food from the leaves), cambium (thin growing layer that makes trunk, branches and roots thicker), xylem or sapwood (carries water and minerals from the roots towards the leaves), heartwood (non-conducting wood in the centre that gives support).
Britannica Kids (Encyclopaedia Britannica) · Reliability: High
Type: Encyclopedia for young readers
Trees are tall, woody plants with a main stem, the trunk; most are over about 3 m tall; some live for hundreds or thousands of years. Conifers make seeds in cones and usually have needle-like leaves; broad-leaved trees make seeds in fruits or pods. Evergreens keep their leaves all year; deciduous trees lose their leaves in autumn and winter and grow new ones in spring. Leaves make food by photosynthesis. Trunks and branches get thicker as the tree ages, and rings show its age. Trees release oxygen, absorb carbon dioxide, return nutrients to the soil through dead leaves and twigs, and their roots help prevent soil erosion. Birds, monkeys, sloths, frogs and squirrels live in trees.
Overall: six carbon dioxide and six water molecules produce one sugar molecule and six oxygen molecules (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), using light energy. Photosynthesis takes place in chloroplasts; chlorophyll absorbs light. Carbon dioxide enters and oxygen leaves through small, regulated openings called stomata, usually on the lower surface of the leaf.
UCAR Center for Science Education · Reliability: High
Type: University research consortium (education page)
Each year a tree adds a ring: a light layer formed in spring and early summer and a dark layer formed in late summer and autumn. Where water limits growth, trees make wider rings in wet, cool years than in hot, dry years; narrow rings can come from drought or a severe winter. The study of tree rings is dendrochronology. By matching living trees with old dead wood, records go back more than 10,000 years. Scientists usually take a core about 4 mm wide with a borer instead of cutting the tree down.
U.S. Geological Survey, Water Science School · Reliability: High
Type: National science agency (education page)
Transpiration is the release of water vapour from plant leaves: roots take up water from the soil, it moves through the plant and leaves release it through their stomata. A large oak tree can transpire 40,000 gallons (151,000 litres) of water per year.
Clemson Cooperative Extension, Home & Garden Information Center · Reliability: High
Type: University extension service (fact sheet)
Chlorophylls are the main pigments that turn sunlight into sugars; they break down constantly and must be replaced. Yellow and orange carotenoids are there all season but masked by the green chlorophyll. Red and purple anthocyanins are made from glucose trapped in the leaf. Each leaf has an abscission (separation) layer at the base of its stalk; in autumn it seals off water flow into the leaf and sugar flow out, chlorophyll declines, and the layer eventually lets the leaf drop. Changes in light and temperature bring on autumn colour.
[7] About the trees (Redwood National and State Parks)
U.S. National Park Service · Reliability: High
Type: National park agency (education page)
Coast redwoods may grow to 367 feet (112 m). They can reach 2,000 years old and regularly reach 600. Fog provides about 40% of their moisture. Their roots go only 3–4 m deep before spreading 20–27 m outwards. They grow from a seed no bigger than a tomato seed; thick bark helps protect them from most fires.
U.S. National Park Service, Great Basin National Park · Reliability: High
Type: National park agency (history page)
The Great Basin bristlecone pine called Prometheus had 4,862 growth rings; because harsh conditions meant not every year produced a ring, its age was estimated at about 4,900 years. It was cut down in 1964 after core samples were taken. A bristlecone pine in California's White Mountains was later dated (2012) at 5,065 years old.
Trees remove carbon dioxide from the atmosphere and release oxygen. Oaks have been found to support over 280 species of insects, which feed many birds and other animals. Trees give nesting places for birds, homes for small mammals among roots and roosts for bats. Trees near rivers reduce flooding, help prevent soil erosion, trap air pollution on leaves and bark, and give shade.
Measure the trunk circumference 1.3 m above the ground; read an estimated dry weight from the circumference table. About half of the dry weight is carbon, so divide by 2. Multiply the carbon by 3.67 to get the carbon dioxide absorbed over the tree's life. Worked example: an oak of about 220 cm circumference has a dry weight of about 5,771 kg, so about 2,886 kg of carbon, and 2,885.5 × 3.67 ≈ 10,590 kg of CO₂.