Exploding trees occur when stresses in a tree trunk increase due to extreme cold or lightning, causing it to split suddenly.
Cold weather will cause some trees to shatter by freezing the sap, because it contains water, which expands as it freezes, creating a sound like a gunshot. The sound is produced as the tree bark splits, with the wood contracting as the sap expands. John Claudius Loudon described this effect of cold on trees in his Encyclopaedia of Gardening, in the entry for frosts, as follows1:
The history of frosts furnishes very extraordinary facts. The trees are often scorched and burnt up, as with the most excessive heat, in consequence of the separation of water from the air, which is therefore very drying. In the great frost in 1683, the trunks of oak, ash, walnut, and other trees, were miserably split and cleft, so that they might be seen through, and the cracks often attended with dreadful noises like the explosion of fire-arms. In the frost of 1837–8 large bushes of heath had their stems split by the frost into shreds, and the wood of the evergreen oak and that of the sweet bay was cracked and split in a similar manner.
Henry Ward Beecher records anecdotal evidence of the wood from which instrument cases and carrying boxes were splitting in temperatures of −70 °F (−57 °C) in Captain Bach's travels near the Great Slave Lake. Linda Runyon, author of books on wilderness living, recounts her experience of the effect of cold on maple trees as follows:
I was relaxing in front of a fire in the crispness of early morning when Crack! A sound like an explosion came from behind me in the woods. I scanned the trees and saw that a maple tree had "exploded". The explosion caused a big crack in the tree about three feet high. When a winter wind stirs the frozen trees, they sometimes appear to burst vertically. When it was 40 degrees below zero at night, I lay awake and listened to the trees explode. That's a true wilderness thermometer!— Linda Runyon, The Essential Wild Food Survival Guide 
We saw 47 below on our porch, and we didn't look again. I would hear these bangs and I blamed it on the house expanding or contracting, or whatever, from the cold, but it was the trees exploding. It was the bark bursting, and you could hear it. That's how wild it was.
Tree sap is a supercooled liquid in cold temperatures. John Hunter observed, in his Treatise on the Blood, that tree sap within a tree freezes some 17 degrees Fahrenheit below its nominal freezing point.
Trees can explode when struck by lightning. The strong electric current is carried mostly by the water-conducting sapwood below the bark, heating it up and boiling the water. The pressure of the steam can make the trunk burst. This happens especially with trees whose trunks are already dying or rotting. The more usual result of lightning striking a tree, however, is a lightning scar, running down the bark, or simply root damage, whose only visible sign above ground is branches that were fed by the root dying back.
Exploding trees may occur during a wildfire, but there's apparently no evidence of this.. The myth of exploding trees may have originated with a classic film about wildfires, Red Skies of Montana, which showed firefighters being harassed by exploding trees, thanks to movie magic.
April fools' hoax
Exploding trees were the subject of a 2005 April Fools' Day hoax in the USA, covered by National Public Radio, stating that maple trees in New England had been exploding due to a failure to collect their sap, causing pressure to build from the inside. The root pressure in a maple tree is approximately 0.1MPa, one standard atmosphere, which is nowhere near enough to cause a tree to explode.
- ^1 Similar text can be found in the entry for Frost in Charles Hutton's 1795 Mathematical and Philosophical Dictionary
- Judith Levin (2004). Life at a High Altitude. Life in extreme environments. The Rosen Publishing Group. p. 10. ISBN 0-8239-3987-1.
- Jonathan Dorn (May 2000). "Pop Goes the Forest". Backpacker Magazine. Active Interest Media, Inc. 28 (186, number 4): 72. ISSN 0277-867X.
- Holladay, April (2007-02-07). "Buying genetic pets; Exploding sap trees; Non-blinking cows". WonderQuest.
- Henry Ward Beecher (1859). Plain and pleasant talk about fruits, flowers and farming. New York: Derby & Jackson. p. 100.
- Charles Annandale, ed. (1901). "Frost". The New Popular Encyclopedia. VI. London and Glasgow: The Gresham Publishing Company. p. 37.
- Linda Runyon (2007). The Essential Wild Food Survival Guide. Lulu.com. p. 97. ISBN 978-0-936699-10-3.
- Geraldine Warner (1996-02-01). "Freezes are becoming a distant memory". Good Fruit Grower. 47 (3).[permanent dead link]
- Joseph Kinsey Howard (1994). Strange empire: a narrative of the Northwest. Borealis Books. Minnesota Historical Society Press. p. 43. ISBN 0-87351-298-7.
- Edmund Morris (2001). The rise of Theodore Roosevelt. Modern Library Paperbacks Series. Modern Library. p. 365. ISBN 0-375-75678-7.
- James Earl Sherow (2007). The grasslands of the United States: an environmental history. Nature and human societies. ABC-CLIO. p. 105. ISBN 978-1-85109-720-3.
- Sidney Kirkpatrick (2006). The revenge of Thomas Eakins. Henry McBride series in modernism and modernity. Yale University Press. p. 337. ISBN 0-300-10855-9.
- Alain Haché (2002). The physics of hockey. JHU Press. p. 8. ISBN 0-8018-7071-2.
- David Ames Wells (1856). Familiar science, or, the scientific explanation of the principles of natural and physical science: and their practical and familiar applications to the employments and necessities of common life. Philadelphia: Childs & Peterson. pp. 129–130.
- John Hunter (1835). James F. Palmer (ed.). The Works of John Hunter: with notes. III. London: Longman, Rees, Orme, Brown, Green, and Longman. p. 107.
- "Sequoiadendron giganteum — A 120 years old tree exploded by lightning". Arboretum de Villardebelle. 2001-02-22.
- Michael Bath (2006-02-12). "Funnel cloud observed and lightning explodes a tree in the Lismore area". Storm News and Chasing. Michael Bath and Jimmy Deguara.
- George W. Dunne; Roland F. Eisenbeis (1972-05-20). "Nature Bulletin No. 458-A". Forest Preserve District of Cook County. Cite journal requires
- "Tree, nature's lightning rod". West Virginia Lightning. Archived from the original on 2008-05-14. Retrieved 2009-09-27.
- Ira Wolfert (August 1959). "The Awesome Miracle of Lightning". Popular Science. Bonnier Corporation. 175 (2): 186. ISSN 0161-7370.
- Barbara W. Ellis; Fern Marshall Bradley; Helen Atthowe (1996). The organic gardener's handbook of natural insect and disease control. Rodale. pp. 392. ISBN 9780875967530.
- Rolf. E. Johnson, ed. (January 2002). Rain Forests of the World. New York: Marshell Cavendish. p. 238. ISBN 978-0-7614-7254-4. Retrieved 2009-09-25.
- The National Geographic Magazine. 134. 1968. Missing or empty
- Weick, Karl E. (1993). "The collapse of sensemaking in organizations: the Mann Gulch disaster". Administrative Science Quarterly. 38 (4): 628–652. doi:10.2307/2393339. JSTOR 2393339.
- Clint Willis, ed. (2002). Fire Fighters: Stories of Survival from the Front Lines of Firefighting. Da Capo Press.
- Robert L. Santos (1997). "The Eucalyptus of California — Section Three: Problems, Cares, Economics, and Species". Denair, California: Alley-Cass Publications. Archived from the original on 2010-06-02. Cite journal requires
- Ted Williams (January–February 2002). "Eucalytus Roulette (con't) Excerpted from America's Largest Weed". Audubon Magazine. Robert Sward. Archived from the original on 2009-09-09. Retrieved 2009-09-27.
- Robert Siegel (2005-04-01). "April Fool's: New England Suffers Maple Woes". National Public Radio.
- Webb, David T. "Transpiration". BOT 311 Spring 2006 Syllabus. University of Hawaii at Manoa. Archived from the original on 2009-09-20. Retrieved 2009-09-27.
- Charles Hutton (1795). "Frost". Mathematical and Philosophical Dictionary. London: J. Johnson. p. 520.
- YouTube video with cold weather and bursting tree bark at 43:53, Wild Russia, Episode 6, Primeval Valleys