What processes enables lumber to be a self-maintaining renewable resource?

Lumber can be reused again and again. If required they can be reshaped and redesigned

Explanation:

Lumber being a processed part of wood is an example of renewable natural resources.  

All natural objects which are able to provide energy in some form or can be used again and again with ability to regrow and redevelop it are known as renewable source of energy.

Trees can regrow itself from seed and all its parts can be utilized up to the maximum extent without waste generation. Even the left overs can be used to produce lumber, cardboard etc, and they too are renewable as they can be reused again and again and if required can be reshaped and redesigned.