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Response of plant species to CO2 levels

By Dennis T. Avery
web posted February 6, 2012

The earth has stopped warming, but the greenhouse gasses continue to accumulate at higher levels in the atmosphere. In fact, it seems certain that the planet will have rising levels of atmospheric CO2  for the foreseeable future. No country has actually produced substantial cuts in its greenhouse emissions, and Asia continues to strongly increase its output of industrial gasses. Nor have any of the "renewable" energy sources been cost-effective enough to survive the coming budget cuts in Europe and the U.S. 

Will the extra CO2 affect the biodiversity of the earth? Some have claimed that C3 plants will now out-compete C4 plants, that weeds will outgrow crop plants, and  tall forest trees will shade out the understory species. (The C3 and C4 plants have different patterns of photosynthesis.)
However, Craig Idso of the Center for CO2 Science says we shouldn't expect much change in our plant diversity due to the expected higher CO2  levels. Idso, trained in agronomy and geography, sees no clear threat to the earth's species richness.

As an example, he notes, C3 plants like corn have shown a larger growth response to higher CO2 levels than C4 plants.  C4 plants, however, appear to gain more in their ability to raise their water use efficiency -- perhaps because they seem to make better use of the mycorrhizal fungi around their roots. The two sets of advantages seem to cancel each other out. Nitrogen-fixing plants seemed to have an advantage over non-nitrogen-fixers -- but that was in studies inside greenhouses. Outdoors that advantage disappeared. 

One study found that weedy mustard was strongly stimulated by more CO2 in the atmosphere -- but no more so than most of the crop plants. On the other hand, says Idso, one of the major British weedy bracken plants seemed to get no stimulus at all from higher CO2 levels -- which could put this invasive weed at a disadvantage in the higher CO2 future.

In the forests, says Idso, there seems little likelihood that the taller trees will shade out the understory species.  Kerstiens reviewed 15 tree studies, and found that the shade-tolerant trees were twice or three times more responsive to added CO2 than the sun-loving tall trees. So, even if far less sunlight got through the CO2-stimulated upper tree canopy, the understory plants would still be vigorous and competitive. On grassland near Basal, Switzerland, elevated CO2 marginally increased the species diversity. 

Hodge found that the increased CO2 levels also induced an increase in soil organic matter, and this seemed to stimulate beneficial soil fungi. Van der Heijden demonstrated that increasing the number of soil fungi species increased ecosystem plant diversity substantially.

None of this reassurance about plant diversity in a world with higher CO2 concentrations should come as startling news, says Idso. After all, most of our plant species evolved originally in much higher atmospheric concentrations of CO2 -- up to ten times as high.   

In terms of temperatures, every species still extant has persisted through 10,000 years of the Eemian Warming before our last Ice Age, which was about 5 degrees C warmer than today, according to the University of Copenhagen.  Each of our species then lived through the Ice Age itself, with a probable drop of 6–10 degrees C that lasted for 90,000 years! That's a range of about 11–16 degrees C just in their "recent" experience. Where did we get the idea that these tough, competitive organisms were fragile?

You could say that the plants now are "just getting back to their roots." The real lesson of this survey of plant responses to changing CO2 concentrations is the resilience of our wild species. Worry about humans in a full blown ice age, not plants happily absorbing CO2. ESR

Dennis T. Avery, a senior fellow for the Hudson Institute in Washington, D.C., is an environmental economist. He was formerly a senior analyst for the Department of State. He is co-author, with S. Fred Singer of Unstoppable Global Warming Every 1500 Years.  





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