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Does Increased CO2 Increase Weeds?

By   /  May 20, 2013  /  1 Comment

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This graph and description is from this website by Richard Deems: http://www.godandscience.org/apologetics/global_warming.html
The atmospheric concentration of CO2 has varied greatly over time, from a low of about 180 parts per million (ppm) during several periods of glaciation over the past 650,000 years to this year’s high of 400 ppm (Figure 1). As can be seen, CO2 levels go through cycles of increase and decrease over 100,000 year intervals, which correspond to eccentricity changes in the earth’s orbit. The atmospheric concentration of CO2 was relatively constant at about 280 ppm for ~1,000 years before 1750. Since 1750, the CO2 concentration has risen, reaching ~377 ppm in 2005. The inset (small graph in upper right) of figure 1 shows the atmospheric levels of CO2 for the last 50 year that produce the sharp spike at the far right of the main graph. Although the cycles of CO2 levels (approximately 100,000 years apart) are naturally on a rise from the last ice age, this spike is dramatically higher than any levels seen over the last 650,000 years.

A few years back I went to a meeting in Denver sponsored by the Colorado Cattlemen’s Society.  I don’t remember the exact title, but it was something about how ranchers could adapt to a changing climate.  Scientists explained what their research was telling them about plant responses to increasing CO2 in the atmosphere, and shared computer models of how the weather might change in different parts of the country.  As is normal for scientists, their talks were laced with the words, “probable,” “possible,” and “further research is needed.”  That’s because, as we all know, the more you learn, the more you realize that there are always a lot of factors affecting what’s happening on a day to day basis, making it difficult to be “sure” of anything.  During breaks the cattlemen shook their heads and laughed.  It’s hard to make decisions about such long-term potentials when you’re trying to solve today’s very real problems.

I was reminded of that meeting when I came across a 2008 presentation to the Northeastern Weed Science Society, by USDA-ARS scientist Lewis Ziska describing his research on how increasing levels of atmospheric CO2 might affect agriculture and the weeds that so many folks combat.  For those who think of pasture weeds as problems, the results of research aren’t good news.  Yes, CO2 does increase your pasture weeds, and the weeds seem to do better than our grasses.

In his presentation, Ziska shared results of research showing that:

• Initial evidence indicates that warmer winters are a factor in the northward spread of Kudzu.  His work shows that it is likely to spread as far north as the Great Lakes region by 2019.  He said that “Data suggest that the link between global warming and the spread of an invasive species is real, not hypothetical.”

• For those of us in the west where cheatgrass is expanding its range, data shows that higher levels of atmospheric CO2 make cheatgrass more combustible.  This lowers the point at which cheatgrass becomes a fire fuel and increases fire intensity.  Though additional research needs to be done, Ziska said that, overall, the likelihood of fire in cheatgrass has increased as a function of recent increases in CO2.  Increased CO2 also reduces the digestibility of many grasses, including cheatgrass.  In regions where the spring flush of cheatgrass is the primary forage for cattle, this is not good news.

• Ziska’s work with Canada thistle and CO2 indicates that Canada thistle does quite well as atmospheric CO2 increases.  The plant, which can spread by roots (rhizomes) increases root growth as CO2 increases whether nitrogen is present in the soil or not.

RoundUp-and-CO2• Finally, increasing CO2 appears to reduce the effectiveness of glyphosate (Roundup).

He concluded his presentation by saying that we need new management strategies for weeds and that we “Can no longer assume that what worked in the past will work in the future.”

None of that feels like very good news if we keep on thinking of pastures as we’ve always thought of them.  But if we change our ideas about pastures and think of “weeds” as forage, the future could still be pretty bright.

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About the author

editor and contributor

Kathy worked with the Bureau of Land Management for 12 years before founding Livestock for Landscapes in 2004. Her twelve years at the agency allowed her to pursue her goal of helping communities find ways to live profitably AND sustainably in their environment. She has been researching and working with livestock as a land management tool for over a decade. When she's not helping farmers, ranchers and land managers on-site, she writes articles, and books, and edits videos to help others turn their livestock into landscape managers.

1 Comment

  1. Bill Elkins says:

    Last summer in 3rd yr of holistic grazing I first noticed a very definite increase in patches of smooth Brome and Canadian T may also be expanding. Our cattle love both, and I attribute this development to our grazing program, which might be described as “mini-mob, relaxed Holistic version of Greg Judy’s” in style.
    This spring the Brome patches are even more prominent, and they get grazed down to 1″ within an hour or two Happily they are regenerating noticeably within days. If it weren’t for winter, I’d be worrying whether my Ky31 toxic fescue will get pushed out.
    After reading the above, it occurs that the story may be more complicated than first thought. As you know Cheat is a cousin of Smooth Brome. Moreover I noticed a patch of very lush Brome just outside our perimeter which has never been grazed. Maybe CO2 is the prime mover here too?

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