Trees don't release CO2 when they die. They release CO2 when they rot. That might sound like an unimportant distinction, but for example buildings made of wood are a CO2 sink. The wood does not rot, and thus does not release the carbon bound in it. Basically burying trees (or generally biomass) in an oxygen-poor environment will sequester CO2.

Plus, if you do it in bulk, you are creating coal for future people, millions of years from now.

Don’t forget to throw in a few Dinos for the oil!