Grass defines them both.
If we can keep our land dominated by grass, there should always be a spot for grazers.
This is not news. We spend a lot of effort trying to keep out plants that aren’t grass. Weeds get sprayed. Trees are burned or cleared.
Grasses have always coexisted with other species, but in our changing world, is grass under threat? Is something changing that can make it harder for grasses to grow and compete against plants that aren’t grasses like broadleaf weeds and trees?
As a global change scientist, I spend a lot of time thinking about what makes different plants unique so we can help predict the future for those plants.
Grasses are almost instantly recognizable, but how much do we know about grass? What makes grass unique? What environmental conditions favor grasses?
The first thing that people think about with grasses is their growing points. “Typical” grass has its growing point at or below the ground surface for much of its life cycle.
Yet, a lot of other species in grasslands do, too. Think about stiff goldenrod, which is pretty grassy for a wildflower. Its leaves grow from the same position as grasses, but they don’t dominate.
It’s not the growing points that will define the future of grasses.
Instead, it might be how they photosynthesize when carbon dioxide concentrations are low.
I discussed earlier how CO2 pushes down protein concentrations in leaves. When CO2 concentrations are high, plants gain a lot more C, which dilutes out protein. The plants also add more carbon to the soil, which reduces how much nitrogen plants can get to make protein**
**The unique chemistry of proteins is their nitrogen. Sugars don’t have it. Proteins do.
But elevated CO2 pushes down protein in almost all plants.
Why is CO2 a threat to grasses specifically?
One of the reasons is how they photosynthesize. To understand that we need to look way back into the past.
Grasses have been present since the time of the dinosaurs, more than 70 million years ago. Dinosaurs ate grasses (among a lot of other things). But the grasses didn’t come together to form grasslands until about 15 million years ago**. Before then, they were mixed in underneath trees and in open spots here and there.
**Give or take a few million depending on where in the world we focus.
When we look at the evolutionary history of grasses, they first evolved when CO2 concentrations were high, but really diverged into over 11,000 species largely during times when CO2 concentrations were low.
What is it about the 11,000 grasses that help them take over when CO2 concentrations are low?
Grasses have a couple adaptations that allow them to prosper (relative to other species) when CO2 concentrations are low.
One adaptation that helped grasses expand was how they photosynthesize.
Photosynthesis is a complex series of reactions and all land plants more or less photosynthesize the same way. With photosynthesis, plants use a special enzyme to take the energy from light and turn CO2 into sugars. Under some conditions, this main enzyme doesn’t make new sugars—it breaks them apart. Bad idea for a plant.
What are those conditions? High temperatures. Low CO2 concentrations.
When it’s hot and CO2 concentrations are low, photosynthesis runs in reverse and plants start to starve.
Starving is bad evolutionarily. So, some of the grasses evolved a trick to minimize the enzyme not working right.
It’s called C4 photosynthesis.**
**What does the C4 mean? What is this trick? I’m going to defer to Wikipedia or YouTube to explain this. They essentially recreate the atmosphere of the dinosaurs within the leaves.
Grasses that use C4 photosynthesis--like big bluestem, buffalo grass, and Bermuda grass--make sure that the photosynthetic enzymes don’t work in reverse. Even under low CO2 concentrations, the C4 grasses do just fine. About half of the grasses of the world are C4 species**.
**We call most of the other species C3 species. Again, check out Wikipedia to understand why.
If C4 photosynthesis helps plants make sugars when CO2 concentrations are low, then why would rising CO2 concentrations favor these species? CO2 isn’t toxic. It can only help plants grow more.
It turns out there is a cost making sure photosynthesis doesn’t run in reverse. The cost is worth it when CO2 concentrations are low. But when they are high, it is enough to favor other species**.
**The cost is about 10% of all the carbon it takes in. Doesn’t seem like much, but try to compete with your neighbor when you are losing 10% of gross budget and they aren’t. Note: always make sure your neighbors pay their taxes.
When we look to the future, CO2 concentrations are going to continue to go up for awhile.
When we think about the penalty that C4 grasses pay, they are going to have a harder time competing against those that don’t pay the penalty.
Is this penalty enough to threaten grasslands?
Only about half of the world’s grasses are C4. The rest of the grasses don’t pay the penalty. If C4 grasses do worse, the other grasses could expand. Rising CO2 concentrations are also going to make the world even warmer, and that helps favor C4 grasses.
Case solved? Nothing to worry about?
The issue is complex, but for all we learned so far (and the too few experiments we’ve run), it’s not obvious how rising CO2 concentrations will impact species with different ways of photosynthesizing.
But, it turns out there is another trick that makes grasses different. It didn’t evolve when CO2 concentrations were low, but it really came in handy then. And with CO2 concentrations rising, this trick might no longer be helpful.
And it’s why global change scientists think a lot about silica.
Joseph Craine studies the functioning of nutrient-limited grasslands. At the heart of it all is understanding grass: how it's built and how it works. Craine earned his B.S. from Ohio State University and his Ph.D. from the University of California, Berkeley.View All Blogs »