15 December 2009

What Does More Atmospheric Carbon Mean for Plants?

Posted by Michael McFadden

Image from the Missouri Department of ConservationPlants assemble themselves with a common set of building blocks: light, water, a carbon source, and a variety of other nutrients, like nitrogen in nitrate form. There’s usually plenty of carbon in the atmosphere, and as more carbon is pumped out into the atmosphere that reservoir only increases. The idea that photosynthetic organisms, either forests or phytoplankton, will simply ramp up their photosynthetic rates and act as giant sponges for atmospheric carbon has been bandied around, especially in the media. And wouldn’t that be nice? It’s a comforting idea, that green things will mitigate the effects of our carbon excess.

But photosynthetic organisms obviously function in a more complex way, and adding more of one building block doesn’t mean they can automatically grow faster: it’s like giving a construction company a truckload of cement without also giving them extra steel beams and framing material—they can’t build a skyscraper with just cement, no matter how hard they try.

So what’s happened in the plant world? Have plants—trees in particular—increased their carbon uptake now that there’s more carbon available in the atmosphere, or have they been limited by nitrogen availability? The general hypothesis is yeah, sort of, up to a point—but evidence has been hard to come by.

Christy Goodale, from Cornell, said today in the B23G: Role of Climate, Carbon, and Limiting Nutrient Cycles and Human Activities in Terrestrial Ecosystems, II session that there’s marginally increased carbon sequestration at higher rates of nitrogen deposition, but that the carbon is often sequestered into soils, not plant matter. In some cases, increased nitrogen deposition even decreased carbon sequestration. But in the majority of cases, young stands of trees respond most clearly to nitrogen deposition, increasing their carbon uptake, while older trees are much less effective at sequestering it away.

And what’s going to happen in the future? Will trees be able to keep up with carbon fluxes—will there be enough atmospheric nitrogen deposition to keep up?

Scott Ollinger, presenting right after Dr. Goodale, showed an interesting result: Plots he and his colleagues studied across North America showed an increase in albedo with increased nitrogen availability in underlying soils (but they couldn’t find a corresponding leaf level response! Oh, how much we have yet to understand…). So in areas with higher nitrogen, the trees were more reflective, which might be a factor in a potential cooling feedback mechanism. That means that in a high-carbon, high-nitrogen world, increased reflectance by canopy-level plants might actually be stabilizing the climate system—just another example of how complex predicting our climate future may be.

–Ale Borunda, Columbia University Earth and Environmental Sciences Journalism Graduate Student