© J. Dirk Nies, Ph.D.
Global warming, climate change, sea level rise, drought, floods, greenhouse gases, carbon pollution; these are phrases we use to characterize the effects of elevated levels carbon dioxide in the earth’s atmosphere. As important as these effects are or may turn out to be, they represent only part of this very complex story.
In this series of articles, I wish to shine a spotlight on different, seldom discussed aspects of elevated CO2 in the air. To do so, I will shift away from physics toward biology and change our frame of reference from climate science and oceanography to agronomy and plant science by describing how plants, trees and agricultural crops are responding to the increased availability of CO2 in the air. Keep in mind as you read, these are just preliminary chapters of the fuller story, a story that we are continuing to write and to comprehend.
Photosynthetic plants construct the molecules of life using atmospheric carbon dioxide as their starting point. All of the calories we consume each day come directly or indirectly from plants. Foods are solid and liquid forms of sunlight; they contain the electromagnetic energy of the sun converted and stored as chemical energy in proteins, carbohydrates and fats.
With more CO2 available in the air (35 percent more in my lifetime) brought about by our burning of fossil fuels, plant productivity is changing right now on our farms, in our gardens, out on the prairies and within the earth’s forests. And more changes are likely to emerge as CO2 levels increase.
First, a little background information before we proceed. Plants capture the carbon dioxide they need to grow through microscopic pores on the surface of their leaves and stems called stomata. CO2 diffuses from the air through the stomata into plant cells where the photosynthetic process incorporates CO2 into organic biochemicals. While CO2 is coming in, water is simultaneously diffusing out through the stomata to the atmosphere in a process called transpiration.
In hot and dry conditions, plants tend to partially close their stomata to reduce transpiration and conserve water. However, partially closed stomata diminish a plant’s ability to absorb CO2 from the air, which in turn can slow down photosynthesis and stunt growth.
As my first example I have chosen scientific results published in the journal Global Change Biology obtained by researchers at the Experimental Research Station of the University of Illinois who investigated the response of the cassava plant to elevated levels of CO2 in the atmosphere.
Cassava ranks second—after potatoes—as the most important root crop in the world. Like the potato, cassava is a native of South America, where the plant is more commonly known as yuca (not to be confused with the spiky yucca shrub, which is a different plant). Today, cassava is grown most extensively in Africa and Asia (where it is also known as manioc) and it has become the third largest source of food carbohydrates in the tropics, after rice and corn. Cassava is the principal food for more than half a billion people, providing almost two-thirds of human caloric intake in sub-Saharan Africa. The importance of cassava to many Africans is epitomized in their name for the plant, agbeli, meaning ‘there is life.’ Many North Americans are more familiar with cassava, after it is treated and dried to a powder, as tapioca.
To make their experiments more representative of what happens in the real world, the University of Illinois researchers left their laboratories and greenhouses and examined cassava grown under field conditions.
They exposed one set of plants to elevated levels of CO2(585 ppm) in the open air, while the other control set was grown under ambient CO2 levels. After three and half months, they harvested the plants. They found that for those plants grown in the CO2-enriched atmosphere, the cassava root tuber dry mass increased more than 100 percent compared to plants grown at ambient CO2 levels. This is astounding. Without genetic engineering, without selecting for those varieties that do well in elevated CO2, without adding any more natural or chemical fertilizers, water, warmth or sunshine, edible yields doubled! Additionally, the cassava’s biomass above ground also increased, but not as dramatically (30 percent).
The authors of the study concluded that “High photosynthetic rates and photosynthetic stimulation by elevated [CO2], larger canopies, and a large sink capacity all contributed to cassava’s growth and yield stimulation.” [CO2] means the concentration of CO2 in air. By ‘larger canopies’ they mean more and/or larger leaves on each plant. ‘A large sink capacity’ refers to the bulky and growing tubers, which are excellent places (sinks) to store the output of all that increased photosynthetic activity occurring above ground in the larger leaf canopy. In other words, plants grown in the presence of extra CO2 were bigger above ground, and even more so below ground.
The team of researchers noted that before they performed their field study, “The potential for cassava to enhance food security in an elevated [CO2] world is uncertain as greenhouse and open top chamber (OTC) study reports are ambiguous. Studies have yielded misleading results in the past regarding the effect of elevated [CO2] on crop productivity, particularly in cases where pots restricted sink growth.”
Cassava tubers and its leaves—which also are edible—frequently contain trace levels of compounds that, in the presence of water, can release toxic hydrogen cyanide (HCN). Cassava must be treated and prepared properly prior to human or animal consumption. The study authors noted “Importantly, and in contrast to a greenhouse study, they found no evidence of increased leaf N or total cyanide concentration in elevated [CO2].”
This study highlights several important points to remember about how plants respond to increasing enrichment of CO2 (carbon pollution) in the air.
First, increasing levels of CO2 in the atmosphere can lead to substantial increases in the yields of important food crops. This is called the ‘CO2 fertilization effect.’ In this experiment, crop yields more than doubled when atmospheric CO2 concentrations were increased two-thirds over ambient levels. This field study suggests that farmers and gardeners may see greater yields in those crops that have the natural capacity and inclination to store the “fruits” of enhanced photosynthetic activity in edible leaves, berries, fruit, roots and tubers.
Second, experiments such as this one performed under field conditions employing free-air CO2 enrichment (FACE) technology can lead to surprising and even contradictory findings compared to what occurs in the laboratory or in controlled greenhouse environments. To emphasize in a humorous way the unexpected turn of events, the authors titled their paper, Cassava about-FACE: Greater than expected yield stimulation of cassava (Manihot esculenta) by future CO2 levels.
Third, EPA projects atmospheric levels of CO2 reaching 585 ppm (the level investigated in this study) by the 2050s, only four decades from now, so significant increases in some agricultural and horticultural yields may be in store as the 21st century unfolds.
There is so much more of this multipart, multifaceted story to tell. My purpose in writing is to broaden our understanding of the impact that rising levels of CO2 is having directly on plants. This information can help us better comprehend the ramifications our greenhouse gas emissions are having on the planet we share.