What if people started editing this etymological tree, adding/removing things as they deem sensible, for their own variations? We could even map the edits into a tree of etymological trees for the word "tree"!

Serious now. Something I love about etymological trees for words instead of whole languages is how much it shows hidden details about the life of the languages. Like that Classical Persian ⟨دارو⟩ dārū "medicine, drug" showing that, in the past, people were making some medicine out of some local tree; or Greek ⟨δρυάς⟩ dryás, that ⟨-άς⟩ -ás suffix hints dryads were originally seen as some sort of wood critter (see *κέμος *kémos "hornless" → ⟨κεμάς⟩ kemás "young deer").

I just wish they tagged the language for each entry. I can't be the only one who thought ⟨𐎭𐎠𐎽𐎢𐎺⟩ was Hittite instead of Old Persian. (The /d/ gave it away.)

Latin ⟨dūrus⟩ hard, though, harsh

Personally I don't buy it.

The root form of this PIE word is probably *derw- ~ *drew-. You'd need to go with the first to explain why Latin has a vowel between the /d/ and /r/; and it would be the o-grade, so you'd end with *dóru, the basic form for the noun "tree".

However:

• PIE *o doesn't typically become Latin /u/, except through the humī rule (unaccented *o before *m) or if Latin would interpret it as the case suffix. Neither is the case here.
• vowel length popping up out of nowhere. It usually signals a coda consonant being deleted.

Instead I think it's easier to explain ⟨dūrus⟩ as coming from *duh₂-rós, the zero-grade of *dweh₂-rós "long, distant". I have no idea what that *-rós is supposed to mean, but it pops up a lot in adjectives, it's called a Caland suffix. But then you're changing the root from *derw- (tree, stiff) to *dweh₂- (far, distant).