Now usually zmjehzd comes in to tell where somewhere along the line from PIE* deru- "oak" to tree, the d became t. ( I always like that part)

From tsuwms etymology-list of words it seems that the Old Germanics, Frisians, Sweeds, Norse and Goths turned the deru-word from d to t > to the "tree" direction. The east European languages stayed close to the first and oldest word "deru". (as did Old Irish and Welsh) I like those things.
Enough stuff to make up a nice story for grandaughters. :-)