Weeds of the globally warmed future
Researcher looks at the potential impact of rising CO2 levels on weed populations, yesterday, today and tomorrow

By Mary Woodsen
Posted June 15, 2007

Some voices in the ag sector hail the benefits of rising atmospheric carbon dioxide—higher yielding, more vigorous crops—as an unexpected boon of the industrial revolution. This is because most plants evolved back when atmospheric CO2 levels were quadruple what they are today. Carbon levels tumbled when the ice ages kicked in, and most green plants have operated at half throttle ever since.

Studies show that wheat, rice and soybeans—not to mention many vegetable crops—could benefit from higher levels of CO2. For starters, these crops could grow more quickly, produce higher yields, and become inherently more drought-resistant.

But what about the weeds? Will they sit back and watch while agriculture’s big boys take over?

Compare the adaptability of weeds and crops, and crops aren’t even in the running, says weed ecologist Lew Ziska of the United States Department of Agriculture’s Agricultural Research Station in Beltsville, Maryland. It’s weeds that thrive under widely ranging and changing conditions. In fact, in tomorrow’s world, some agricultural weeds may prove more difficult and costly to control using today’s two most relied-on means—herbicide sprays and cultivation.

Since spraying herbicides outstrips all other short-term means of dealing with weeds today, Ziska has looked at how some of our nastiest weeds respond to herbicide regimes at future CO2 levels. He homed in on Monsanto’s Roundup for the simple reason that this herbicide, as well as companion Roundup Ready crops, account for such a large share of the agricultural marketplace.

Built-in resistance, all on their own

Roundup Ready soybeans and corn are tweaked at the core of their being, their DNA, to provide built-in resistance to glyphosate, the essential ingredient in Roundup. This means conventional growers can spray their soybeans and corn after both the crops—and many of the weeds in the neighborhood—have sprouted. The crops live. The weeds die.

Or at least they did.

In just the past decade, some weeds have become glyphosate-resistant all on their own. Meanwhile, other more-invasive weeds may have become more competitive with crops. Coincidentally perhaps, atmospheric CO2 has begun increasing at ever-accelerating rates.

And coincidence it may be. But Ziska’s research suggests that in some cases the connection might be causal. He has tested how some of North American’s most notorious annual and perennial weeds might respond to glyphosate at the CO2 levels anticipated planet-wide by 2100. He has looked at the larger context, too, to get a feel for whether rising atmospheric CO2 gives invasive weeds their edge.

To understand the different ways these weeds have responded, it helps to understand what botanists mean when they toss around phrases like “C3” and “C4” plants. These labels tell how certain types of plants assimilate carbon in photosynthesis: C3 plants form a pair of three carbon-atom molecules, and C4 plants form four carbon-atom molecules.

Still poking along

"Back in the day, before weed scientists took a closer look, it made perfect sense: Overall, crops will do better, not worse, in the war with weeds. When you dig deeper, though, you notice that while farmers grow about 45 major crops, 410 weeds reduce yields in these crops. That’s a lot of weeds."

C3 plants are the ones that, since carbon levels plummeted, are still managing at half throttle. They get stoked when CO2 goes up. C4 plants—there’s only a handful of them, about 4 percent of all plants—have evolved ways to concentrate CO2. Being well-adapted to today’s CO2 levels tends to give them a competitive edge with crops. By and large, they don’t get much of a boost off higher CO2.

The Greening Earth Society—an industry-backed think-tank that questions the relationship between CO2 and global warming—will be the first to tell you that most crops are C3 plants and most weeds are C4 plants, and they have a point. Back in the day, before weed scientists took a closer look, it made perfect sense: Overall, crops will do better, not worse, in the war with weeds. And early research bore out those assumptions.

When you dig deeper, though, you notice that while farmers grow about 45 major crops, 410 weeds reduce yields in these crops. That’s a lot of weeds, a big enough number that C3 plants don’t need to be in the majority to outclass the competition, particularly when you consider who’s on the team. What’s your worst weed nightmare? Canada thistle or bindweed, perhaps? Star thistle or cheatgrass? Quackgrass, lambsquarter, spotted knapweed? C3s all.

Weeds don’t share well

CO2 is one of four critical resources plants need. Water, sunlight, and nutrients are the others.

What if you change the amount of water a plant gets by 25 percent, Ziska asks. Will it change its growth? What about 25 percent more or less sunlight or nutrients? Will there be winners, losers? Well, yes.

"The thing is, we’ve had enough experience with many plants to predict if they’ll do better or worse with more or less sunlight or water or nutrients. We haven’t had the same depth of experience with changing CO2 levels."

Ditto with carbon dioxide, which has increased about 25 percent since the beginning of the industrial revolution. The thing is, we’ve had enough experience with many plants to predict if they’ll do better or worse with more or less sunlight or water or nutrients. We haven’t had the same depth of experience with changing CO2 levels. We can’t predict ahead of time just how each species will respond—or how those physiological responses may change as each species acclimates to rising CO2. But we can look back to research done decades ago to help us understand the relative differences between crops and weeds.

It was the 1950s, and fertilizer was cheap. Scientists decided to see if dosing fields with extra nitrogen would reduce competition from weeds. Their premise: crops and weeds that got their fill of nitrogen would be relieved of competing for the same limited resource, and yields would go up even if weed densities stayed the same. Share the wealth, in other words. But weed densities didn’t stay the same. The weeds proved better able to use the extra nitrogen, leaving less available for crops. Yields took a nosedive.

“Companion plants”

One of Ziska’s first projects, inspired by that nitrogen research, looked at how field-grown soybeans, a C3 crop, would do at ambient (today’s) and elevated (tomorrow’s) CO2 levels with lambsquarter (a C3), and redroot pigweed (a C4)—two of North America’s most problematic annual weeds—along for the ride. “Companion plants,” you might call them.

Ziska set up several “open-top chambers” in a field at the research station. The chambers were set up so he could add extra CO2, following the daytime-nighttime cycles CO2 naturally travels. Some chambers got elevated CO2. Others didn’t.

Ziska seeded soybeans in early June. Lambsquarter or pigweed seedlings, planted in greenhouse flats at elevated or ambient CO2 on that same day, were transplanted after the soybeans emerged and spaced at two weeds per meter of row. All other weeds were removed every week. And, of course, he had a control: chambers of soybeans kept entirely weed-free at either ambient or elevated CO2.

The soybeans in the elevated, weed-free control did great, increasing seed yield by 23 percent and biomass by 32 percent. But those “companion plants” took a chunk out of yields.

C4 pigweed reduced soybean yields at ambient CO2 by 45 percent. At elevated CO2, it reduced yields by only 30 percent: much less, but still ahead of weed-free soybeans’ 23-percent yield bump.

And while C3 lambsquarter reduced yields by a mere 28 percent at ambient, at elevated levels soybean yields fell by 39 percent.

In a related project, Ziska looked at how greenhouse-grown pigweed and lambsquarter behaved if you zap them with glysphosate at today’s CO2 levels and those anticipated by the end of this century.

Predictably, ramping up CO2 did nothing for pigweed survival after the herbicide hit. It died in every replication. The same with seedling lambsquarter. Yet 4-week-old lambsquarter pulled back from the brink, if barely, putting out new growth—and even flowering and setting seed—after getting Round Up-ed at the higher seed level.

Bad weather doubles research yield

How might soybeans behave when you’re not trying to set them up, but just want to watch how they grow with today’s conventional protocols, only at tomorrow’s CO2 levels? Ziska did this for two years running. The first year—2003, the wettest in Maryland since record keeping began in 1895—the only weeds were shallow-rooted C4 grasses favored by heavy rains and standing water. Elevated CO2 had no effect on weed biomass at maturity, and glyphosate provided 100-percent control.

But in 2004, with near-normal rainfall, a range of C3 and C4 weeds popped up, including C3s lambsquarter, velvetleaf and Virginia copperleaf, along with C4 pigweed and those C4 grasses. The elevated CO2 plots produced C3 broadleaves with five times the biomass of those in the ambient plots. Even after a hit of herbicide, about 6 percent of these C3s survived. It’s in the marriage of such changed conditions and survival rates that the potential for genetic resistance is born.