Ancient atmosphere was productive.

How could ancient landscapes have provided all the vegetation needed to nourish massive herds of hungry, multiton dinosaurs? New laboratory experiments suggest that in the era just before the dinosaurs went extinct, extra carbon dioxide in the atmosphere may have done the trick, boosting plant productivity to at least three times that of today's ecosystems.

During portions of the Cretaceous period, which ended about 65 million years ago, some regions of western North America supported dense populations of large, plant-eating dinosaurs. In that era, the atmospheric concentration of carbon dioxide ranged as high as 2,000 parts per million (ppm)--more than five times today's values. Oxygen made up as much as 30 percent of the air, in contrast to today's 21 percent. Atmospheric pressure then was about 25 percent higher than it is today.

By growing seedlings of Ginkgo biloba in a hyperbaric chamber, Sara M. Decherd of North Carolina State University in Raleigh and her colleagues investigated the effects such an atmosphere might have had on plants. G. biloba is a still-thriving species whose leaves can be found in the Cretaceous fossil record.

In experiments that lasted 24 hours, plants in atmospheres that contained carbon dioxide concentrations of 2,000 ppm grew five times as fast as those exposed to modem concentrations of the gas. In similar but separate tests, elevated concentrations of oxygen slightly slowed plant growth. When concentrations of both carbon dioxide and oxygen were raised to their Cretaceous levels, at the expense of atmospheric nitrogen, plants grew about four times as fast as they did in current-atmosphere conditions. In monthlong tests, growth slowed after an initial spurt but seedlings still produced three times as much new foliage as did those grown under current conditions, says Decherd.