Will plants grow us out of trouble in a greenhouse world?

By Gary Raham
Nature Writer and Illustrator

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Although not as sure as death and taxes, two bets look pretty sound in the next 50 years: 1. The amount of carbon dioxide (CO2) in the air will increase. 2. The world will continue a feverish trend, with average global temperatures increasing by 3 to 10 degrees F.

If the first bet comes true, the second is almost certain to follow, as CO2 is a gas that helps trap the sun's heat. We often hear about the wilder weather and rising sea levels such a warming might cause, but is there a good side to this trend? After all, CO2 serves as a raw material for plant photosynthesis and a planetwide greenhouse might use up lots of carbon dioxide as plants flourish. The scientific reports on this so-called "fertilization effect" have been mixed.

Scientists have plotted a rise in CO2 concentration from a pre-industrial 280 parts per million (ppm) to a current 350 ppm - the highest concentration in 20 million years or so. The surprise is that the rise hasn't been steeper. Each year humans pump about 8 billion metric tons of carbon into the biosphere, but only about 3.2 bmt hang around in the air. Scientists believe the oceans dissolve half of the "missing" carbon. Land plants may serve as the "sink" for the rest. This makes what plants do with carbon dioxide important.

Average global temperature has risen 1 degree over the past century, which doesn't seem like much to sweat about. But 80 percent of Glacier National Park's glaciers have disappeared since 1910 and the rest have shrunk by two thirds. Eighty percent of the snows of Kilimanjaro have melted during the same period, and Arctic Sea ice has declined by 10 percent in the last 30 years. The Maldives, a country consisting entirely of a collection of islands that only rise 8 feet above sea level, could easily sink beneath a rising sea.

Could land plants grow so quickly that they inhale yet more CO2 and become a deeper "carbon sink" to help offset some of these consequences? Some early studies pointed in that direction, but current research indicates we may be expecting too much of green plants. Plant productivity, it turns out, is only partly correlated with the rate of photosynthesis.

Biochemically speaking, plants fall into two large categories based on how they make sugars and respire or "burn" them for their own use. So called C3 plants get more of a boost from increased CO2 because they respire less. These plants include nearly all forest trees and some major crops like rice, wheat, potatoes and beans. C4 plants, including corn, sorghum and sugarcane, don't get the same CO2 boost. But even plants that do enjoy increased rates of photosynthesis with high CO2 decline back to preexisting rates over time, for reasons that are still unclear.

Studies performed on abandoned farmlands in the Midwest have shown that the effects of water, habitat fertility and light will trump any advantage conferred by high CO2 levels. Lack of adequate light and nutrients proved to cancel nearly all the benefits of increased carbon dioxide. Scientists obtained similar results studying tundra grasslands under high CO2 conditions.

On the other hand, sedges in marshlands near Chesapeake Bay with lots of water and nutrients showed excellent growth with air enriched to 700 ppm of CO2. Under similar conditions, pure stands of timber in New England grew well. However, mixed stands of deciduous trees, even though all were C3 plants, didn't do as well. Apparently competition for space, nutrients and light overshadowed the positive affects of CO2.

Some U.S. Department of Agriculture studies have shown crop increases averaging 34 percent under high CO2 conditions, but the yields were dependent on plenty of water and fertilizers. C3 crops like soybeans may do well, especially when their weeds are C4 grasses, but important C4 crops like corn may suffer in relation to C4-style weeds.

Other possible complications abound. Nutrient cycling could dramatically change, for example, affecting the abundance of nitrogen-fixing legumes, like peas. This would, in turn, impact the nutritional value of plant material, both for various animal consumers and the insects that pollinate the plants. Rate of decomposition for plant litter with less nitrogen also decreases, which would alter nutrient recycling.

In short, some plants will like a greenhouse planet and others will not. The exact results of an atmosphere rich in CO2 are unknown. Humans, all six billion of us, are collectively conducting an experiment of global proportions every time we drive to the store, barbecue a burger, or pass a little gas. (Methane is a greenhouse gas, too.) Let's hope that all those leafy green veggies and trees out there will help keep us out of ecological trouble a little while longer.