A common misapprehension..

As originally derived in 1907, Einstein's famous equation E=mc2 says that the liberation of an energy E by any system will cause it to lose a mass m equal to E divided by c2, the square of the speed of light. This is just as true of a burning tree as of a nuclear bomb, except that the energy released in ordinary burning is too small to allow a measurement of the decrease in mass of the products of combustion. If one insists on saying that mass is converted to energy in nuclear reactions, then one must say the same of ordinary burning. The true source of the energy released when a tree burns is the sunlight absorbed during the tree's life, and the true source of the energy that is released in a uranium fission is the energy stored in the nuclears when it was formed in a supernova explosion long before the earth condensed from the interstellar medium.
This explains when tree burns..it just change it's state into Co2 and some by products.
However there must be some very small amount of loss in mass..at subatomic levels.