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.
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