The future of our grasslands is in doubt.
Grasslands are not necessarily imperiled today, but will face only more threats in the future.
In the future, I can talk about things like drought and invasive species, but for now I’m going to focus more on carbon dioxide.
CO2 concentrations in the atmosphere just passed 400 parts per million (ppm). That’s an amazing number for many of us that remember 350 ppm as not too long ago. Half the people reading this probably were born before it was 320. CO2 concentrations are only likely to go up. Projections vary, but they are likely to surpass 700 ppm by the end of this century.
As a global change scientist, one of our jobs is to understand what is likely to happen to grasslands as CO2 concentrations continue to climb.
From experiments and monitoring, we know that as CO2 concentrations climb, protein concentrations in grass decline. That can only hurt weight gain in cattle, but it isn’t enough to threaten grasslands.
Looking more closely, we learn that grass is a plant that was forged when CO2 concentrations in the atmosphere were low. If grass is particularly adapted to low CO2 concentrations, high CO2 concentrations might not be good for them.
Almost half of the world’s grasses use a trick used to keep photosynthesizing under low CO2 concentrations, something called C4 photosynthesis. Grasses that use this trick keep the CO2 concentrations high in their leaves so photosynthesis always goes forward. There is a cost to this trick though, which makes it likely that rising CO2 concentrations would not be favorable to these grasses. But, there are still many grasses that don’t use this trick and rising temperatures could still make C4 photosynthesis favorable.
When we look more closely at grasses, there is another trick they use that would be helpful when CO2 concentrations are low. When CO2 concentrations are low, sugars are harder to produce for many plants. Sugars provide energy, but they also are used to make compounds like cellulose and lignin that provides structure for the grasses.
When atmospheric CO2 concentrations were low, carbon was expensive. And it became even more expensive to for plants to build cell walls. Grasses evolved another trick to help build stiff cell walls without using as much carbon. They used silica.**
**A number of other species developed this trick, too. Horsetails are called scouring rushes because they were Nature’s Brillo pads. They are packed with silica and scrub well. Cucumbers are prickly in part because they have silica, too.
Silica is silicon dioxide. It forms hard crystals. Quartz is a big silica crystal. Silica can be embedded in cell walls of plants and used instead of carbon-based molecules to make stems and leaves rigid. When carbon is expensive, this is a great innovation. Silica is cheap. It doesn’t need energy or carbon. Think of it as an additive.
The consequences of using silica set off a million-year arms race with grazers. When grasslands began to expand millions of years ago, the browsers of the world set out to eat grass. Unlike the soft leaves of the forest, grass was impregnated with silica. Not necessarily to deter herbivores, but it certainly had that effect. Their teeth weren’t always up for the job and got worn down. That set off the arms race for higher and higher crowns of teeth.**
**Horse teeth still amaze me.
It wasn’t just the teeth. Jaws had to become a lot stronger. Silica in soft tissues is more than an irritant. And when silica builds up in the body, mammals can get urolithiasis. Silica stones get deposited in the urethra and urination can become reduced or blocked. That’s why it’s also called water belly**.
**Water belly is not a pretty term to google.
The effects of being able to use silica in their cell walls impacted the ability of grazers to eat grass, but evolutionarily using silica to deter herbivores was likely secondary to building stiff cell walls when carbon starved.
We don’t have too many experiments on how much silica helps plants grow, but one experiment grew cucumbers in solutions with and without silica. The plants without silica just didn’t grow as well. Now, all the hydroponic cucumber growers make sure that their solutions have a lot of silica in it.
One researcher tried the same thing with grasses. Same result. Greater growth. Stiffer stems. More flowering.
In general, when CO2 concentrations are low, silica is a great partial substitute for carbon.
Again, like C4 photosynthesis, not all grasses use silica. And a lot of plants that aren’t grasses do, too. But, like C4 photosynthesis, using silica is a trick that is most helpful when CO2 concentrations are low.
When they rise, that trick no longer pays the same dividends. Rising CO2 won’t directly hurt the grasses, but other plants benefit a lot more than the grasses do.
If grasses were forged when CO2 concentrations were low, it is a good first guess that they likely not to do well when CO2 concentrations become high. Something else will do better.
But who is this?
When we look out in the plant world, which are the species that are likely to benefit the most?
Most likely, the mortal enemy of grasses: Trees.
Next time, I’ll discuss why trees are likely to become an even a greater threat to grasses as CO2 concentrations continue to rise.