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

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.