I love trees, too, even ones that can't be comfortably hugged. Those who claim that trees don't keep carbon out of the air are guilty of arboreal defamation. In fact, forests and other nonagricultural lands absorb a net of 13 percent of U.S. carbon dioxide emissions, the EPA says. 

 

As you correctly note, it takes a while for dead trees to decay and release the carbon they store. One study estimates that the needles of a dead Douglas fir on the West Coast take more than 10 years to decay completely. It's 15 years for fine roots, 100 for bark, 120 for branches, and 500 for a trunk two feet in diameter. The obvious wild card is forest fires, which can release a whole lot of carbon dioxide in a very short time: around 290 million metric tons a year in the United States, by one government estimate.  

 

Carbon sequestration becomes easier to grasp if you consider a single tree. Plant, say, one silver maple today, and in 25 years—assuming it survives—it will have sequestered about 400 pounds of carbon dioxide, according to the U.S. Energy Information Administration. However, don't feel too righteous if you plant just one tree, because the average U.S. resident emits the equivalent of around 20 tons of CO2 a year.