Weeds of the globally warmed future, Part II
Researcher sets his sites on perennial weeds after discovering annual weeds could give crops a serious run for their money if CO2 levels increase.

By Mary Woodsen
Posted July12, 2007

Weed ecologist Lew Ziska’s research seems to indicate that the biomass of many annual weeds that cause problems for farmers increases with rising CO2 levels (see Part I).

Perhaps more worrisome is Ziska’s work with two perennial weeds. Quackgrass, an aggressive, spreading grass that can penetrate even plant tubers, is on many states’ noxious weed list. And farmers across North America rank Canada thistle as their worst weed. Both are C3s. Both may be spread by cultivation—especially where growers or gardeners lack the time or know-how to do it right.

Conventional farmers routinely spray a number of times to manage [Canada thistle]. In tomorrow’s world, will they end up spraying more?

Ziska did his quackgrass research in the greenhouse. He wanted to examine how elevated CO2 affects growth rate, photosynthesis and potential Roundup resistance at different points in the quackgrass life cycle. Since glyphosate works best on vigorously growing plants, he trimmed his older cohorts of quackgrass to encourage regrowth. Then, 6-1/2 weeks later, he hit them with Roundup.

Glyphosate decimated all three of the age classes grown at ambient levels. But at elevated CO2, the intermediate and oldest cohorts bounced back. In fact, four weeks after he sprayed the oldest age class, Ziska found that its “dry-weight harvest” tipped the scales at 110 grams, only 30 grams less than that of the unsprayed ambient control. And unlike some plants, weeds as well as crops, quackgrass didn’t acclimate—didn’t stop responding—to elevated CO2 over the long term, in this case 231 days after planting.

Ziska’s work with field-grown Canada thistle sprayed with glyphosate showed similar results. Above-ground growth died back, regardless what CO2 level it had grown at. But the thistle had responded to higher CO2 by developing root systems with double the biomass of plants grown under today’s ambient levels. This extensive root system may have diluted the spray’s effectiveness, helping it bounce back. Ziska’s plants rebounded at about double the rate of ambient plants. In fact, six weeks later you could hardly tell them from the ones that were never sprayed.

Conventional farmers routinely spray a number of times to manage this weed. In tomorrow’s world, will they end up spraying more?

Ambient then, ambient now, ambient down the pike

This wasn’t Ziska’s only look at Canada thistle. To get a feel for how invasive, nonnative perennial weeds have changed under increasing CO2 since the industrial revolution began heating up around at the beginning of the 20th century, he grew six different perennial weeds—Canada thistle, yellow star thistle, field bindweed, leafy spurge, perennial sowthistle and spotted knapweed, C3s all—in three separate “controlled-atmosphere chambers.” One chamber approximated ambient CO2 levels as they were circa 1900, the second was tuned to ambient today, and the third to ambient as it’s likely to be in 100 years.

Most plants probably have already increased their biomass by 22 to 44 percent. Yet Ziska’s six star weeds at ambient-now chalked up biomass increases averaging 110 percent greater than their confreres growing at ambient-then [circa 1900]. That’s about three times the average.

These six weeds aren’t just agronomic bad actors; some have been walloping their way through native plant communities at ever-increasing speed for the past 50 years. Conventional wisdom faults missing parasites and predators: the beneficial organisms that controlled these plants in the lands from whence they came. But what else has changed?

Well—CO2 has gone up. It’s half again as high now as it was 100 years ago. Do all plants respond equally to rising CO2 levels? Have they responded equally during the past 50 to 100 years? Is the rate of response different depending on whether you’re looking back or forward? And at what point on that continuum might you see the strongest response?

The vast majority of studies have looked forward. That’s because it’s easier to add CO2 to a growth chamber than to take it away. Scores of studies on several hundred plants indicate that the boost from ambient-now to tomorrow’s levels is likely to produce plants whose biomass averages 28- to 40-percent greater.

How do Ziska’s six contenders stack up? Er… OK. At tomorrow’s levels their average biomass is on the high end, though not in the extreme. Ziska anticipates them increasing an average of 45 percent. Only Canada thistle at 72 percent and spotted knapweed at 60 percent are likely to show a much stronger-than-average response.

But what if you look back instead of ahead, if you consider how much these plants’ biomass may have already gone up with increasing CO2 from 1900 to today? Ziska did just that, reviewing a handful of previous studies that examined changing biomass among plants grown at ambient circa 1900 and plants grown at ambient-now.

Those previous studies produced results similar to that first batch: Most plants probably have already increased their biomass by 22 to 44 percent.

Yet Ziska’s six star weeds at ambient-now chalked up biomass increases averaging 110 percent greater than their confreres growing at ambient-then. That’s about three times the average. The least responsive among them, spotted knapweed, clocked a respectable 80 percent increase. And Canada thistle blew out the stops at 180 percent.

Proxy for a planet

Ziska put another spin on his research in 2001. He decided to use inner-city Baltimore as a low-cost alternative to the “free air CO2 exchange” units many researchers use to simulate future levels of CO2. Baltimore, like every big city, has as much carbon dioxide in its air as the rest of the planet will have in 50 years. Ziska wondered if the city might serve as a living lab—a harbinger of the landscape of the future.

Ziska planted a uniform seed bank in plots along a transect running from Baltimore’s inner core through a suburb to an organic farm 40 miles out. He made sure the subsoil and topsoil, as well as rainfall, were the same in every plot. (What he didn’t also control was the temperature, which tended to be a couple of degrees higher in the city.)

The first year he saw just what he thought he would: country weeds 3-feet tall; suburban weeds 6-feet tall; city weeds—the same seed bank, soil, and subsoil—up to 12-feet tall.

The second year blew his mind. For then Ziska saw the trees come in. Five years later, the first-year weeds in Baltimore were nearly gone. Instead, trees stood 8- to 15-feet tall. In December 2006 Ziska cut those trees, since they shaded the weeds so much he could no longer see how their ecology would change over time. Meanwhile, the rural site has barely changed; its trees stand 1- to 3-feet tall.

A harder row to hoe

Much of Ziska’s work looks at how weeds might respond to herbicides in the future and suggests that chemical control may require more sprays or higher doses, increasing both economic and environmental costs. While he hasn’t looked at the effect of that mainstay of organic farming, cultivation, on weed dispersion, he can’t help but think about it.

Cultivation breaks plants and their roots into small pieces—great for getting rid of some weeds, less so for others. Ziska’s work on Canada thistle indicates that at future-ambient, Canada thistle (and perhaps quackgrass) may steeply increase the biomass of plant roots relative to top growth. Will the same be true of similar plants? No one’s looked. But if so, then what? Such plants often propagate from tiny pieces of root. The greater the root biomass, the greater the potential for poorly planned or badly executed tillage to spread rather than hinder such plants.

On the other hand, although Ziska’s work may mean that farmers dealing with weeds could face practical challenges—some of them profound—in decades to come, organic growers may be better able than conventional growers to adapt. John Teasdale, an agronomist with the USDA, says that rising CO2 won’t be the only force causing selection.

Good organic farmers watch their fields, Teasdale says, mixing and matching strategies that impose a wide range of selective pressures on weeds. The more different kinds of selection that are going on, the harder it is for weeds to adapt. While organic farmers won’t have any silver bullets, they’ll use lots of little hammers—farmer-driven selective pressures acting at different times and in different ways—to help keep weed populations from building up.

Part of doing science is appreciating what you don’t know. There’s a world of difference between could be come more difficult and will become more difficult, and Ziska would be the first to say so. His research can only indicate what might happen. What he’s got: some evidence suggesting that weeds may reduce crop yields more in a higher-CO2 world than they do today. Some evidence suggesting that rising CO2 may be a selection factor in which weeds do well in the future. Some evidence suggesting that CO2 may select for invasives within assemblages of plants. And a lot of uncertainties.

What Ziska is confident of: Only for the combination of C4 weeds and C3 crops does CO2 favor the crop. All other combinations favor the weeds. Yet we lack a climate model for the future that considers the impact of weeds on crop yields.