The brown is the live vein, the white is deadwood.
As LB said, it is bleached by limesulfur.
This is in order to make the tree look aged and as if it's had a hard life (seriously).
As Bonsailearner said, this is to create an impression of an old tree, that has been fighting the elements for many years, where the exposed part died off and was sculpted and bleached by wind and sand.
