Made of Air

a tree is built from the sky, not the soil.

a story in seven chapters: where the wood of a tree comes from. not the soil but the air, carbon dioxide joined with water by sunlight; the oxygen it gives off is the water's, the whole planet breathes with the seasons, and burning gives it all back

A tree is built mostly from air. Asked where a tree's mass comes from, most people say the soil: in Minds of Our Own: Lessons from Thin Air (Harvard-Smithsonian / Annenberg, 1997), university graduates mostly said the soil. Jan van Helmont (who died in 1644) grew a willow in a pot of 200 lb of dried, weighed soil for five years: the tree went from 5 lb to 169 lb 3 oz (about 2 kg to 77 kg), and the soil lost only about 2 oz, some 57 grams (Ortus Medicinae, 1648). He guessed the water made the wood; the missing part is the air. Leaves take in carbon dioxide through tiny pores, stomata: tens to hundreds in a square millimetre (5 to 1,000 across plants; Hetherington and Woodward, Nature, 2003), and while a pore is open, water vapour gets out too. Sunlight powers photosynthesis: carbon dioxide + water + light -> sugar + oxygen. The oxygen given off comes from the water, not the carbon dioxide: in 1941 Ruben, Randall, Kamen and Hyde gave the alga Chlorella water tagged with heavy oxygen (18O), and the oxygen it gave off carried the tag (J. Am. Chem. Soc. 63, 877). The whole planet breathes with the northern seasons: at Mauna Loa, NOAA's record of the sky's carbon dioxide (about 420 ppm) peaks each May and falls about 6 ppm by early autumn as the northern forests grow, then climbs again over winter; the trace on the page is a smooth model of that shape, not the measured data, and the record's rise from year to year is left out. Sugars are built into wood, and dry wood is about half carbon by mass (USDA Forest Service: 47.4% on average), so 1 kg of dry wood holds the carbon of about 1.7 to 1.8 kg of carbon dioxide. Burn it and about 99% goes back into the air as carbon dioxide and water; the ash, about 0.5 to 1%, is the minerals the roots took from the soil, which matches van Helmont's few grams. The light and heat are the sunlight the leaves stored. Coal is the swamp forest of the Carboniferous, about 359 to 299 million years ago, buried before it rotted (UC Berkeley UCMP), so burning it lets out sunlight about 300 million years old. The molecules are drawn as characters, one standing for very many, and nothing is to scale; the climb of the numbers between van Helmont's weighings, the pore's count and the burning are animations. The framing, trees come out of the air and fire lets the sun back out, is Richard Feynman's (Fun to Imagine, BBC, 1983).

Concepts: biology, chemistry, ecology.

An interactive you play with one thumb, on Wundr.