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
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
||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
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
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.