Quirks and Quarks

Which trees provide the most oxygen over the course of a year, deciduous or evergreen?

Deciduous and evergreen trees can produce similar amounts of oxygen each year, as long as they also have a similar total leaf area.

What matters is total leaf area.

Deciduous trees turning yellow (in the fall) among a backdrop of evergreen trees in Alberta (Sean Michaletz)

This week's question comes from Holly Irving in Lake Echo Nova Scotia. She asks:

Which trees of the same age or size provide us with the most oxygen over the course of a year, deciduous or evergreen?  Do evergreen trees make oxygen in the winter?

Sean Michaletz, assistant professor of ecology at the University of British Columbia, says the answer to this question has to do with photosynthesis. This occurs in plant leaves and uses carbon dioxide, water and light energy from the Sun to produce sugars that feed the plant, but also yield oxygen as a by-product.

Photosynthesis occurs across the surface of a leaf, and the total oxygen production of a tree depends upon its total leaf area. This changes with the age and size of a tree. Old trees produce more oxygen and young trees. 

Leaf area also changes dramatically from season to season. Deciduous trees produce new leaves each spring that are dropped each fall. They generally have a shorter growing season for photosynthesis than evergreen trees that retain their leaves year round. 

Evergreen trees can photosynthesize through the winter as long as they are not frozen and have access to water. 

However, given the cold climate in Canada for example, it is unlikely that evergreen trees here produce much oxygen during the winter.   

Deciduous and evergreen trees vary in their speeds of photosynthesis. Deciduous trees photosynthesize much faster. There is a trade-off then between speed of photosynthesis and length of growing season. But data suggests that deciduous and evergreen trees produce similar amounts of oxygen over the course of a whole year, provided they also have a similar total leaf area.