Grasses were forged for a different world

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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.


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Bill Edwards    
August, 07, 2013 at 12:09 PM

Craine seems to have bought into the "assumption" that global warming and higher CO2 are caused by man (in step with all the environmentalists). Yet he talks about periods in the past when CO2 has changed affecting climate. What changed the climate in earlier eras before fossil fuel use? I don't see any curiousity about this by our objective scientists. Everyone has quickly decided to blame man and fuels-- the environmentalist line without much thought! Craine seems to be a lab guy from reading a couple articles. Doesn't seem to have much experience in the field. Small, fragmented range mgmt is way different than very large expanses which are quickly disappearing. The lab is about a small & fragmented as it gets. I'd be interested in Savory's thoughts on Craine's articles.

ksdave    
ks  |  August, 07, 2013 at 12:31 PM

I can't imagine the scientific community missing that question (What other sources of CO2 could be causing the level to rise?). So I'm guessing they have looked at CO2 from a global perspective.

Joe Paschal    
South Texas  |  August, 08, 2013 at 12:49 PM

Interesting article. The C4 plants, which my cattle eat, represent only about 5% of the known plant species but store about 30% of the carbon taken by plants. C3 plants are very inefficient in both water use and utilizing photosynthesis since they evolved much earlier than the C4 plants (or plants that use a slightly different system like the cacti) when it was shadier, cooler and wetter. C4 plants tend to be lower in digestibility but do grow well in direct sunlight in more arid areas. If the climate is changing, if I live long enough to see it, the plants will certainly have evolved to survive in it. And if it happens more quickly that that I expect we will find the gene (or genes) to make our C4 plants adapt to higher levels of CO2 just like they are doing in rice. Looking forward to the next segment.

W.E.    
August, 15, 2013 at 02:43 PM

Climate change is a better term than global warming. Yes, people cause it. Machine farming methods to support an ever more urbanized human population are contributing immensely to it. Buy the way, I am a producer of grassfed beef and haven't set foot in a laboratory since my college days forty years ago. Have been observing, responding and adapting to the challenges of management-intensive grazing and finishing beef on pasture since 1989. To fight climate change, the goal should be soil covered with plants 100% of the time. Grazing ruminants can cure many of the ills of climate change. People can return to husbandry of grasses and legumes (and woodlands!), carefully improving soil by managed grazing of ruminant animals. People can stop machine farming of millions of acres to provide the vast quantities of grain grown to keep cattle in feedlots and people in cities. Accomplishing this will require a worldwide concerted effort, education, and a putting down of arms. Our journalists are barking up the wrong tree, making people more fearful of consequences rather than looking for the root of the problem and encouraging people to do something about it. It was poor farming and too much emphasis on warfare that ended the greatest civilizations, including ancient Egypt and the Roman Empire. History should have informed us by now. The plagues of Egypt in the time of Moses were caused by poor farming practices to support a population explosion, allowing desertification of the Fertile Crescent. Human beings are the only creatures who have the luxury of not adapting to natural conditions. If we refuse to balance and adapt our ways in the near future, humanity will pay the largest price it has ever before paid for that luxury.


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