Virtual Museum of Canada
Jardin botanique de Montréal 
Centre for Forest Research


The roots have another good reason to stay close to the surface: oxygen can't penetrate any deeper than the first few centimetres of soil. The roots "breathe". That's right – even though they don't photosynthesize, they need to exchange gases, by absorbing oxygen and releasing carbon dioxide.

Redwoods, which grow in places where mudslides are common, can even be "smothered" to death! A heavy mudslide cuts their roots off from the oxygen dissolved in the upper soil layers. The result is a race against the clock: the trees' roots grow upward as quickly as possible, some of them even breaking through the surface before they stop.

Photo of a Siberian elm (Ulmus pumila) in an urban setting, completely surrounded by asphalt
Ulmus pumila
© Natural Resources Canada, Canadian Forest Service
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To keep trees healthy, park visitors are often asked to stay on marked trails. That's because if too many people walk around the base of a tree, the soil will become compacted and unable to absorb oxygen. The roots will die and the tree could suffer badly. How healthy do you think the roots of this urban elm are?