When trees grow, they absorb atmospheric carbon dioxide. During the burning of wood, the carbon dioxide that was absorbed is released back into the atmosphere.
In essence, atmospheric carbon dioxide (CO2) in the atmosphere is not increasing. If the tree "dies" naturally, it will still decay and eventually release the same amount of carbon dioxide (CO2) into the atmosphere.
Burning for example wood pellets and briquettes simply speeds up this process. This process is known as the Carbon Cycle.