Step-by-step explanation:
The law of conservation of mass and energy according to thermodynamics is NOT violated. The ash of the wood weighs less than the initial wood because during the burning some of the mass was converted to gases that escaped into the atmosphere. Some of the carbon in the wood, for example, turned to carbon dioxide. Water in the wood also turned to gas (water vapour) and escaped.
If it is possible to capture all these gases and weigh them, then ideally it would be observable that the law of conservation is NOT violated.