Timber and timbre have distinctive definitions. They are not simply the result of Americanization (or Americanisation, if you like) of the same word.
Gray vs. grey
Canceled vs. cancelled
Affect vs. effect
Learned vs. learnt
Its vs. it’s
Favorite vs. favourite
Practice vs. practise
Fish vs. fishes
Realise vs. realize
Alumna, alumnae, alumni, alumnus
Than vs. then
Center vs. centre
Adviser vs. advisor
Timber and timbre have distinctive definitions. They are not simply the result of Americanization (or Americanisation, if you like) of the same word.