Trees absorb carbon dioxide (when talking about a "carbon sink", it means the carbon as any form).

Decaying vegetation is actually the opposite of a carbon sink. A carbon sink is something that takes carbon out its natural cycle and stores in for an extended period of time. Vegetation, particularly trees, absorb carbon, and thus they act as stores. However, decaying vegetation releases the carbon back into the air as it decays. Therefore, it is not a sink.

Trees absorb carbon dioxide (when talking about a "carbon sink", it means the carbon as any form).

carbon source

heat sink is when heat is absorbed into any type of environment, including the aquatic ecosystem; and carbon sink is when CO2 is absorbed into any type of environment including the terrestrial ecosystem.