A cool thing about warm season grasses

In an earlier blog post, I told a story about how cattle eating grass pumps carbon back into the ground using the same biological processes that put it there.  Today, I want to tell a story about how what you know as “warm season” grasses are (literally) very cool due to their special role in today’s natural carbon cycle.

The earth is 4.5 billion years old.  Photosynthesis was one of nature’s earliest ‘inventions’ arising roughly 2.7 billion years ago.  As most of you likely know, photosynthesis is the process by which energy from sunlight drives the process of combing water with CO2 to create sugar and oxygen.  At first photosynthesis was dominated by species living in water.  Plants began taking off on land only ~440 million years ago and deep-rooted trees didn’t begin thriving until ~390 million years ago. But grasses did not arise until much later.  The earliest grasses in the fossil record occur during the Cretaceous (65 to 145 million years ago).  

But most ‘fossil fuels’ actually come from ‘source rocks’ that originated with photosynthesizing plants in the oceans laid down long before grasses existed.  Although coal is dominated by land plant material, most oil and gas source rocks are actually made up of dead aquatic organic matter that would fall to the sea floor where there was no oxygen around to decompose it.  It got buried, and later ‘cooked’ into the oil and gas that we use today.  Fossil energy is actually thus buried solar energy from eons past – just mostly from oceans.

But nature did not stop innovating.  Up until about 30 million years ago, all photosynthesis was done by the so-called “C3” pathway.  [C3 is so-named because its initial output is a 3-carbon molecule (3-phosphoglycerate).]  But there’s a catch – C3 photosynthesis came about at a time early in Earth’s history when both CO2 and water were very abundant in the surroundings.  After C3 species successfully emerged, CO2 levels on earth fell dramatically – in part due to the success of the C3 plants themselves.  As they succeeded, the C3 mechanism for capturing CO2 in plants thus struggled to get enough CO2 to satisfy its natural appetite.  Plants then developed an increased number of ‘stomata’ – the holes allowing uptake of CO2 from the air.  But this also came at a price – losing water into the air as part of the mechanism for exchange.  

Thus, C3 plants became quite thirsty as they struggled to establish footing on dry land.  To make matters worse, the enzyme that drives photosynthesis in plants (something called ‘Rubisco’) is somewhat unable to tell CO2 from the similar looking oxygen (O2) molecule.  So C3 photosynthesis looses some of its efficiency as the ratio of CO2 to oxygen fell over prehistorical time.  

The paradoxical result is that as C3 plants grew in number, they actually became less efficient at converting CO2 and water into sugars using sunlight.

Then along came a very important and innovative solution allowing plants to thrive better on dry land – the “C4” pathway (Fig 1].  In short, C4 photosynthesis operates as a ‘front end’ to the old C3 photosynthesis system allowing the plant to capture more CO2 while excluding O2, and without loosing so much water via extra stomata. 


Still, C3’s were not put completely out of work.  It turns out that when a leaf is cool (below ~72 degF), C3 plants can still out-produce C4’s.  But when it gets warm and outside, C4 plants really kick it into gear – especially in the presence of over-abundant light [Fig 2].  It is this trait, combined with lower requirement for water, that allowed C4’s to replicate rapidly onto what we now know as places like the “Great Plains.”  The bottom line is that C4 grasses like it warm and sunny, are stingy with water, and have a high appetite for CO2.

C4 photosynthesis emerged roughly 30 million years ago, but didn’t really take off until about 8 million years ago…. pretty much at he same time the first grazing ungulates emerged on the landscape to eat those grasses.  C4 grasses thrived because they could grow faster with less water when it was warm and sunny outside in an environment that was lower in CO2 than when photosynthesis first evolved.  Grasslands rapidly grew to out-compete forests on drier inlands such that they became a dominant biome on planet earth in lands with warm temperatures and more modest rainfall than forests.

One scientist, Greg Retallack, has proposed that this period of co-evolution between grazers and grass was so strong that it not only allowed both grass and grazer to thrive together, but actually drove 3 periods of rapid planetary cooling in the last 30 million years as carbon was taken out of the air and stored in soil. [Fig 3].


Today, C4 plants make up only about 3% of all plant species, but are responsible for taking up a whopping 30% of all CO2 taken up on land.  Roughly 60% of grasses are C4 plants, so it is precisely the global grasslands that are capturing most of the planet’s sunlight (that falls on land) and storing it in carbon-based chemicals – ultimately in the soils.

Even so, the planet’s grasslands are not doing the solar energy conversion work they did just a few centuries ago.  William Clark (of the Lewis & Clark Expedition) made the following field note from the bluffs of eastern Kansas overlooking the Missouri river just over 100 years ago….

“The Plains of this countrey are covered with a Leek Green Grass [big blue stem], well calculated for the sweetest and most norushing hay —interspersed with Cops [copses] of trees, Spreding their lofty branchs over Pools Springs or Brooks of fine water. Groops of Shrubs covered with the most delicious froot is to be seen in every direction, and nature appears to have exerted herself to butify the Senery by the variety of flours Delicately and highly flavered raised above the Grass, which Strikes & profumes the Sensation, and amuses the mind    throws it into Conjecterng the cause of So magnificent a Senerey [several words illegible, crossed out] in a Country thus Situated far removed from the Sivilised world to be enjoyed by nothing but the Buffalo Elk Deer & Bear in which it abounds & [page torn] Savage Indians”
William Clark, near the Missouri River, northeastern Kansas (now Doniphan County) July 4, 1804

Just under 200 miles east, on a tour at the Grassfed Exchange 2014, we saw this same ‘Big Blue Stem’ grass, yes – a C4, reappearing on Greg and Jan Judy’s ranch after many years of ecologically matched grazing.  

To me, that Big Blue Stem grass is a sign that Greg and Jan’s land is turning the carbon pump back on high as it becomes more efficient at capturing incoming sunlight – i.e. ‘putting the cookie back.’  It’s also an example of how both people and planet can prosper together by managing the ecosystem in accordance with how it came to be.  The cocktail of C3 and C4 species best suited to maximizing uptake of solar energy on your land may be different than another place, but wherever you are, it is very likely to work best all year long if there’s some C4 plants in the mix that are exceptionally talented at combining CO2 with water and sunlight to capture today’s sunlight.

I don’t know about you, but think that’s a pretty cool thing about warm season grasses!

Russ Conser
Fulshear, TX


For more reading if interested:

1.    Osborn, Collin and Beerling, David, “Nature’s green revolution: the remarkable evolutionary rise of C4 plants.” Philosophical Transactions of the Royal Society B (November 2005).
2.    Still, Christopher and Berry, Joseph, “Global distribution of C3 and C4 vegetation: Carbon cycle implications.” Global Biogeochemical Cycles Vol 17, No 1 (2003).
3.    Retallack, Greg, “Global Cooling by Grassland Soils of the Geological Past and Near Future.” Annual Review of Earth and Planetary Sciences (2013).
4.    Morton, Oliver, “Eating the Sun: How plants power the planet.” Harper Perennial (2008).