Herbarium Specimens Show Changes in Carbon Dioxide Levels

By Nicole Tarnowsky

Sep 20 2019

It has been documented that since the industrial revolution there has been an increase in carbon dioxide in the atmosphere. This has affected the way plants develop structures in their leaves, and these changes can be seen in herbarium specimens. 

In the first step of photosynthesis, plants absorb carbon dioxide and release oxygen through pores in their leaves called stomates. Each stomate has two kidney shaped guard cells on each side, which open and close the pore. When there is an increase in carbon dioxide, a leaf doesn't need as many stomates for gas exchange and so it produces fewer of them. The density of stomates can be compared on herbarium specimens that were growing in pre-industrial times with lower carbon dioxide levels to those that are now growing in higher levels of carbon dioxide. On average, leaves that were growing in lower levels of carbon dioxide have more stomates.

A simple leaf peel from the surface of a leaf can be put under a microscope to see the stomate cells. Examine photos of the stomates of specimens of Northern Red Oak, growing recently compared to specimens growing over 100 years ago. There are fewer in the recently collected specimens, which correlates to an increase in carbon dioxide in the air.

More about: Climate change


Woodward, F. I. (1987). Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature 327: 617-618.