Water vapor is apparently a greenhouse gas. Clouds reflect sunlight and promote global cooling. Trees emit chemicals that turn water vapor into clouds. Therefore trees have at least a threefold direct impact on global warming:
1) They promote cloud formation, which reduces the amount of loose atmospheric water vapor.
2) They absorb CO2 from the atmosphere.
3) They usually reflect more heat than the ground cover they replace.
The new part of this (for me, anyway) is the second half of number 1. It has been confirmed by atmospheric scientists that water vapor is a significant greenhouse gas:
"Specifically, the team found that if Earth warms 1.8 degrees Fahrenheit, the associated increase in water vapor will trap an extra 2 Watts of energy per square meter....
'We now think the water vapor feedback is extraordinarily strong, capable of doubling the warming due to carbon dioxide alone.' "