To test that hypothesis, scientists at Duke's FACE site mounted pipes and valves on three separate rings of towers to release extra gas onto the forested test plots the towers surround. The computerized system maintains CO2 concentrations of 1 times today's levels, regardless of wind or weather conditions. Three additional tower rings surround similar test plots but emit no gas, thus serving as experimental "controls." Scientists measure effects of the extra carbon dioxide by comparing results from the active and control sites.
The FACE tower rings are located in Duke Forest, a research reserve created out of former agricultural land that was replanted with trees. "Essentially there's no topsoil on that site," Schlesinger said. "The land was probably exhausted by cotton and tobacco farming in the 1800s and early 1900s."
Scientists at FACE had an unanticipated opportunity to assess how drought affects trees growing in a CO2 enriched atmosphere when 2002 proved to be one of the driest years on record in North Carolina, Schlesinger said. By comparison, 2003 was one of the area's wettest years.
"As the experiment has continued, we realized just how hard it is to see what ultimately controls tree growth -- whether CO2, water or soil nutrients," he added.
Apart from the impact of nitrogen deficiency and drought, the scientists have found some indication that pine tree growth declined over the years at the high CO2 levels, he said. The trees bathed in high CO2 also added more fine roots, which Schlesinger suggests
Contact: Monte Basgall