I can relate to your situation and here’s why: We purchased and shipped a Fox Trotter/TWH from Tennessee to Redmond, WA. He came from wide-open spaces to Western Washington trees! He was a spooky mess for the year we had him there. Wind and rain in the trees didn’t help either. We tried different calming supplements (didn’t help much) as well as lots of natural horsemanship methods (Clinton Anderson, etc.). His temperament/personality was already very sensitive (didn’t take much to ask a little for a lot), but the enclosed environment of all the trees made him feel trapped and claustrophobic. You could see him thinking “There must be a horse-eating boogie-something in that forest!” It’s just a different “world” out on a trail, too, than an arena. He actually performed much better in a covered arena. After that first year of owning him in WA, we moved to southwest Idaho, with high desert & wide open spaces, and he changed overnight. Although he is still a sensitive horse, he is more calm and connected to the rider in this environment. My answer for western WA: Ground work in the trees. Get him used to the thinking side of his brain (instead of reactionary side) to get his focus on and trust in you when he is surrounded by trees (or anything scary). The Clinton Anderson Method is great for this. Also the use of a “dead broke” horse to read off of is a great idea. Whether or not the trees are his issue, I hope the best for you and your TB!