In Word Ways 2005-103ff., Eric Iverson presented some 4-letter trees, and some trees with 5-letter words. As invited by the editor, I take up the search from there. Throughout, I assume that all words in a tree must be different. The reader should sympathize if there are any errors, for tracing a vast number of almost identical words through many trees is even worse than proofreading word ladders, and repeatedly reminded me of the famous motif from Zork: "You are in a maze of twisty little passages, all alike".
"Larger Letter Trees,"
Word Ways: Vol. 38
, Article 3.
Available at: http://digitalcommons.butler.edu/wordways/vol38/iss3/3