Won't all plant life on the planet be affected? Wait! Isn'

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Global Warming The green in the greenhouse
effect and the implications for Agriculture
Lewis H. Ziska, PhD, USDA-ARS
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CO2 represents a changing aspect of climate.
380 ppm
CO2 is a principle greenhouse gas.
CO2 also represents the sole source of carbon
for plant photosynthesis and, hence, all
terrestrial life. At present 96 of all
plant species lack optimal CO2
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So what if CO2 goes up? Part I.
No H2O and CO2? Surface temperature would be
18oC. With H2O and CO2? Surface temperature is
15oC.
Adding H2O or CO2 reduces the amount of heat
leaving the atmosphere.
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So what if CO2 goes up? Part II.
BUT! Carbon dioxide is also the source of carbon
for photosynthesis, and consequently for 99
of all life.
CO2 H2O light ? O2 organic C chemical
energy
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Plants are Important.
Plants are necessary for the flow of energy and
carbon through ecosystems. With the exception
of a few subterranean organisms, if plants did
not exist, life would not exist. Plant growth
however is dependent on four abiotic inputs
Sunlight, nutrients water and carbon
dioxide. Any perturbation in these inputs will
alter all living systems.
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CO2 is perturbating
CO2 has risen about 21 since the 1950s.
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SO

• If CO2 is necessary for life to exist but is in
  short supply, and.
• Atmospheric CO2 is increasing as a result of
  human activity
• Wont all plant life on the planet be affected?

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Wait! Isnt more plant growth a good thing?

• We are living in an increasingly lush
  environment of plants and animals as a result of
  the carbon dioxide increase. This is a wonderful
  and unexpected gift from the industrial
  revolution.
• Wall Street Journal, December 4, 1997, similar
  sentiment in Forbes, Greening Earth Society, etc.

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Genetic variability in soybean with increasing CO2
Evaluation of yield response of 9 soybean
cultivars to 710 ppm CO2.
Greenhouse study, USDA
Crop Science 41385-391
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Genetic variability in Spring wheat with
increasing CO2
Evaluation of 4 different lines of Spring wheat
released at different times during the 20th
century
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While there are positive aspects to rising CO2,
not all plants are equally desirable.
Kudzu in North Carolina.
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What are the implications for Weed Biology?
Specifically

• Crop/weed competition
• Invasive weeds
• Weed management.

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Weed/Crop Interactions.
Do all plants respond equally to the same
resource? (i.e. the rise in atmospheric CO2)
There are 45 major crops in the U.S.
There are 410 weed species associated with yield
reductions for those crops.
Weeds account for 7-10 billion dollars in
agricultural losses.
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Wait! Arent most weeds C4 plants, and
therefore wont respond to CO2?

• Although a number of weeds have C4 photosynthesis
  and should, theoretically, not respond to
  increasing CO2, a number of C4 weeds can show a
  strong response.
• C3 and C4 weeds vary with C3 and C4 crops by
  region. Almost all major crops have both C3 and
  C4 weeds in the United States.
• Many of the worst weeds for a given crop are
  simply wild relatives and therefore have the
  same growth habits and photosynthetic pathway
  (e.g. rice and red rice, oat and wild oat).

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Crop/Weed Interactions
Does future elevated CO2 favor the crop or the
weed?
Only for the C4 weed/C3 crop does CO2 favor the
crop. All other combinations favor the weed.
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Crop/Weed Interactions
Does increasing CO2 favor the crop or the weed?
Note No climate model considers impact of weeds
on crop yield
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Does rising carbon dioxide change weed
populations?
Some initial evidence suggesting that C3 weeds
could be preferentially selected.
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Crop/Weed interactions

• Some evidence that agronomic weeds may reduce
  crop yields further in a higher CO2 environment.
• Some evidence suggesting that rising CO2 may be a
  selection factor in weed species dominance.
• A lot we dont know yet.

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Invasive weeds
A weedy species, usually non-native for a given
region, whose introduction results in
wide-spread environmental or species
degradation.

• Financial Cost 13 billion/ year.
• Environmental Cost Loss of diversity Only
  habitat destruction ranks higher.

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Yellow starthistle
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Leafy spurge
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How have invasive weeds responded to recent
changes in atmospheric CO2?
Response is about 3x the average for recent CO2
increases.
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Why such a strong response?
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Canada thistle is a common agronomic weed. What
about other invasives?
Cheatgrass (Bromus tectorum)
Invasive weed of rangelands in Western U.S.
Impacts fire ecology
Dries 4-6 weeks earlier than perennials. Develops
dense stands Grows in 6-22 inch precipitation
area (high fire) Readily ignites.
Overall
Elimination of woody non fire adapted perennials
large monocultures of cheatgrass and reductions
in species diversity.
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Response of three cheat-grass ecotypes to CO2
By end of three months, biomass of all ecotypes
was affected by CO2.
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Cheatgrass and fire ecology CO2 and
combustibility.
Lignin content may also be related to
combustibility. Work is ongoing with Dr. Robert
White of USDA-FS.
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Does elevated CO2 favor invasive species in plant
communities?
System Invasive Species CO2 Favors?
Reference Desert Bromus madritensis
Yes Smith et al. 2000 Prairie Prosopis
glandulosa Yes Polley et al.
1994 Woods Prunus laurocerasus
Yes Hattenschewiler and Korner 2003
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Invasive Weeds and CO2

• Initial evidence indicates that rising CO2 may be
  a factor in the establishment of invasives.
  Increased asexual reproduction?
• Rising CO2 may also effect growth and
  combustibility of cheatgrass, with subsequent
  changes in fire ecology.
• Suggestion that CO2 may select for invasives
  within assemblages of plants.

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Why not just spray them?Humans are not passive!
But! Does increasing CO2 affect chemical efficacy
Increased CO2 improved resistance to glyphosate.
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CO2 and Round-up
As carbon dioxide increases, glyphosate efficacy
is reduced.
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CO2 and Round-up
Ambient CO2
Future CO2
Increasing CO2 reduces herbicide efficacy.
e.g. Ziska et al. Weed Science 52584-588, 2004
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A synopsis of CO2 impacts on herbicide efficacy
Efficacy is reduced in a number of studies. The
basis for the reduction is not entirely known.
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So why not control them mechanically?
NO!
Understanding weed control with increasing CO2 is
still in its infancy.
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CO2 and Plant/Weed Biology Overall.

• Evidence to date indicates rising CO2 per se has,
  or could potentially
• Effect weed populations.
• Effect crop losses due to weeds.
• Effect species diversity
• Effect outbreak of fires
• Effect weed control efforts.

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Where do we go from here?

• Acknowledge that the increase in carbon dioxide
  of and by itself
• will have a significant impact on agriculture.
• Recognize that this impact is both benefit and
  curse.
• (e.g. improved cultivars vs. more aggressive
  weeds)
• Work together to review existing data and
  identify those
• assumptions that are no longer valid.
• 4. Adapt as needed.

KnowledgegtgtIgnorance