I agree with you with being upset or pissed - They have no right to remove trees/brush on the line and on your side. I'm in the middle of a "new" neighbor that knew the fence line was off from the survey. When the land was surveyed, when the land was settled around here = the western and northern portions of the sections were short by todays surveying equipment. By the new survey, I'll be shorted 3.75 acres from the old fence line (that stood for over 100 yrs). The neighbor knew this when he purchased the property and still took out the fence and all trees (all 50-100 yrs old). Yes, they sucked moisture out 50ft on each side, but the wildlife liked it, it acted as a shelterbelt with snow blowing in the winter and the trees branches were trimmed up and maintained well above modern equipment. It kept trespassers off each others property (now snowmobiles and sidexsides and atvs track where they want, when no crops are in. 
Hopefully you have a survey that will land the new fence in the same spot as the old fence. I'm not willing to give up 3.75 acres of my property without compensation. Lucky for me, I took pic's before and when purchased, the surveyor showed where the old fence stood, as compared to where the survey shows today. I did have it surveyed to where the old fence should be and have posts there every 200ft. 
Hope it all works out.