Global Warming - The “green” in the greenhouse effect

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					Global Warming: The “green” in the greenhouse effect and the implications for Agriculture

Lewis H. Ziska, PhD, USDA-ARS

CO2 represents a changing aspect of climate.
~380 ppm

*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

CO2 is a principle greenhouse gas.

So what if CO2 goes up? Part I.
Gas Nitrogen (N2) % 78.1

Oxygen (O2)
Argon (Ar) Carbon Dioxide (CO2) Water (H2O)

20.1
0.93 0.038

0.1 to 1

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.

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

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.

CO2 is “perturbating”

CO2 has risen about 21% since the 1950s.

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…

• Won’t all plant life on the planet be affected?

Wait! Isn’t 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.

Genetic variability in soybean with increasing CO2

MANDARIN WILLIAMS MANCHU HARROW MUKDEN CNS ARKSOY S-100 DUNFIELD

* * * * * * * X= 40.5% * *
0 20 40 60 80 100

Evaluation of yield response of 9
soybean cultivars to 710 ppm CO2.

Greenhouse study, USDA Crop Science 41:385-391

Genetic variability in Spring wheat with increasing CO2
Response of wheat cultivars to rising CO2

Average seed yield (g per plant)

35

21

Percent protein in flour for Spring wheat

30 25 20 15
Marquis, Released 1903

20 19 18

Marquis Thatcher

Chris

17 16 15 200
Oxen

10 5 0 300 400 500

Thatcher, Released 1921 Chris, Released 1965 Oxen, Released 1996

600

700

300

400

500

600

700

800

Atmospheric carbon dioxide

CO2 concentration

Evaluation of 4 different lines of Spring wheat released at different times during the 20th century

While there are positive aspects to rising CO2, not all plants are equally desirable.

Kudzu in North Carolina.

What are the implications for Weed Biology? Specifically:
 Crop/weed competition

 Invasive weeds

 Weed management.

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.

Wait! Aren’t most weeds “C4” plants, and therefore won’t 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).

Crop/Weed Interactions
Sorghum
0 -10
Velvetleaf

Soybean

Percent reduction in seed yield from weeds.

-20 -30

Red-root pigweed

Lambsquarters

-40 -50
Red-root pigweed

Does future elevated CO2 favor the crop or the weed?

-60

Ambient Carbon Dioxide

Sorghum
0 -10 -20
Velvetleaf

Soybean

-30 -40 -50 -60

*

Red-root pigweed

* *
Elevated Carbon Dioxide

Red-root pigweed Lambsquarters

Only for the C4 weed/C3 crop does CO2 favor the crop. All other combinations favor the weed.

Weed
Grasses (C4 )

Crop/Weed Favored? Environ. Reference Interactions Crop
Does increasing CO2 favor the crop or the weed?
Lucerne Soybean Sorghum Soybean Lucerne Sorghum Grasses Crop Crop Weed Weed Weed Weed Weed Field Field Field Field Field Field Field Bunce, 1993 Ziska, 2000 Ziska, 2003 Ziska, 2000 Bunce 1995 Ziska, 2003 Potvin and Vasseur, 1997

Amaranthus retroflexus (C4 ) Amaranthus retroflexus (C4 ) Chenopodium album (C3 ) 3 Taraxacum officinale (C3 ) 3 Albutilon theophrasti (C3 ) Taraxacum and Plantago (C3 ) 3

Note: No climate model considers impact of weeds on crop yield

Does rising carbon dioxide change weed populations?
2004
700

Not Sprayed

Weed Biomass (g m-2)

600 500 400 300 200 100 0

**
p = 0.07

Broadleaf weeds (C3)

Broadleaf weeds (C4)

Grassy weeds (C4)

Ambient CO2 Elevated CO2

Some initial evidence suggesting that C3 weeds could be preferentially selected.

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 don’t know yet.

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.

Yellow starthistle

Leafy spurge

How have invasive weeds responded to recent changes in atmospheric CO2?

Response is about 3x the average for recent CO2 increases.

Why such a strong response?
Canada Thistle
a
3.0

2000

2001

Elevated / Ambient

a

2003

*
3.0 2.5

*
2.5

2.0

*
b

b

2.0

*
1.5

1.5

*

1.0

1.0

0.5

0.5

0.0

0.0

Shoot

Root

Shoot

Root

Shoot

Root

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.

Response of three cheat-grass ecotypes to CO2
140 120 100 80 60 40 270 ppm CO2 320 ppm CO2 370 ppm CO2 420 ppm CO2

Total plant biomass (g plant-1)

20 0 140 120 100 80 60 40 20 0 140 120 100 80 60 40 20 0 20 30 40 50 60 70
Col 1 vs Col 2 Plot 1 Regr Col 1 vs Col 4 Plot 2 Regr Col 1 vs Col 6 Plot 3 Regr Col 1 vs Col 8 Plot 4 Regr

1220 m

By end of three months, biomass of all ecotypes was affected by CO2.
1737 m

2171 m 80 90

Days after sowing

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.

Does elevated CO2 favor invasive species in plant communities?

System
Desert Prairie Woods

Invasive Species
Bromus madritensis Prosopis glandulosa Prunus laurocerasus

CO2 Favors?
Yes Yes Yes

Reference
Smith et al. 2000 Polley et al. 1994 Hattenschewiler and Korner 2003

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.

Why not just spray them? Humans are not passive!
But! Does increasing CO2 affect chemical efficacy

Increased CO2 improved resistance to glyphosate.

CO2 and Round-up

2004
700

2004
Not Sprayed
60

Weed Biomass (g m -2)

600 500 400 300 200 100 0

Weed Biomass (g m-2)

**
p = 0.07

**

Sprayed

50 40 30 20 10 0

Broadleaf weeds (C3)

Broadleaf weeds (C4)

Grassy weeds (C4)

Broadleaf weeds (C 3)

Broadleaf weeds (C 4)

Grassy weeds (C 4)

Ambient CO2 Elevated CO2

Ambient CO2 Elevated CO 2

As carbon dioxide increases, glyphosate efficacy is reduced.

CO2 and Round-up
Ambient CO2 Future CO2

Increasing CO2 reduces herbicide efficacy.
e.g. Ziska et al. Weed Science 52:584-588, 2004

Growth rate (g / day)
0.0
Canada thistle Canada thistle 1 Canada thistle 1 Canada thistle Lambsquarters Lambsquarters (death) Red-root pigweed Red-root pigweed (death) Quackgrass Quackgrass (death)

0.5

1.0

1.5

2.0

0.0

0.5

1.0

1.5

2.0

A synopsis of CO2 impacts on herbicide efficacy

B.

A.

Efficacy is reduced in a number of studies. The basis for the reduction is not entirely known.

So why not control them mechanically?

NO!

Understanding weed control with increasing CO2 is still in its infancy.

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.

Where do we go from here?
1. Acknowledge that the increase in carbon dioxide of and by itself will have a significant impact on agriculture. 2. Recognize that this impact is both benefit and curse. (e.g. improved cultivars vs. more aggressive weeds) 3. Work together to review existing data and identify those assumptions that are no longer valid. 4. Adapt as needed.

Knowledge>>Ignorance


				
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