On 80 acres of wettest land in Catahoula Parish in Louisiana, an experimental crop of hardwood saplings is seizing carbon dioxide from the atmosphere and locking it up in wooden cells. On each acre, the trees probably take in as much carbon dioxide each year as a typical automobile puts out, according to the scientists at Louisiana Tech University who manage the project. So while there are skeptics, this experiment in what scientists call the sequestration of carbon, if it can be replicated on a large scale, could represent a significant weapon against the risk of global warming caused by manmade carbon dioxide and other greenhouse gases.