what kind of tree absorb the most carbon dioxide?

Green plants absorb carbon dioxide, helping to reduce global warming.
But it is difficult to say what kind of trees absorb the most carbon dioxide.

Some scientists have calculated how much carbon dioxide a tree can absorb in a year. It is difficult to generalize because some trees are bigger than others, and younger trees require more nutrients than older ones. However, on average it is estimated that one tree will remove 3 kilograms of CO2 from the air over the course of one year.

Professor Robert Teskey at the Warnell School of Forest Resources, University of Georgia, USA, believes that as trees grow larger, their ability to recycle carbon increases. Professor Teskey's laboratory is measuring the amounts of carbon dioxide recycled by trees of different sizes by using small electrodes to directly measure the level of CO2 (carbon dioxide) found in tree sap.

There is an article called Soil fertility limits forests capacity to absorb excess CO2.
Do go to http://www.eurekalert.org/pub_releases/2001-05/UoM-Sflf-2205101.php
and read it yourself.

2007-05-11 21:02:59 補充：
Dave Nowak, a researcher at the U.S. Forest Service’s Northern Research Station in Syracuse, New York has studied the use of trees for carbon sequestration in urban settings across the United States.Reference: http://environment.about.com/od/whatyoucando/a/best_trees.htm

2007-05-11 21:04:21 補充：
A study he co-authored lists the Common Horse-chestnut, Black Walnut, American Sweetgum, Ponderosa Pine, Red Pine, White Pine, London Plane, Hispaniolan Pine, Douglas Fir, Scarlet Oak, Red Oak, Virginia Live Oak and Bald Cypress as examples of trees especially good at absorbing and storing CO2.

Young trees can absorb more carbon dioxide.
A single mature tree can absorb carbon dioxide at a rate of 48 lbs./year and release enough oxygen back into the atmosphere to support 2 human beings.

Reference: http://www.coloradotrees.org/benefits.htm