Thanks for your tips,
Incappucciato d'Ombra. PC version doesn't seem to give you any help -- no automatic tree-morphing! I also tried slowing everything (in-game speed and DOSbox speed) way down, which made the game crawl but didn't fix the problem. I ended up going back to a saved game, just before the first time the Sheriff's men swept the forest. Rather than going to the Ancient Oak (where the Green Man lives), as I did the first time, I went to the Ash Grove SW of the widow's cottage. Played the game through to the second sweep, returning to the Ash Grove, and had time to run, access cursors, and turn into a tree again. Problem solved! Must've just been a weird bug/glitch I ran into. Hopefully this will help anybody else who
might have this same problem.
Thank goodness for Abandonia and its forums!