My new yard has a lot of trees, probably 20-30. There are apple, pear, peach, pine, plum, and a few others. Most of them are fairly old (50+ years). Some of them don't seem to be very healthy (dead branches, withering/yellow leaves, etc.). Are there any tips or tricks for helping an unhealthy tree? How do I figure out what is wrong with them?
Generally speaking, established trees like it just the way it is. Grade changes, drainage changes, more foot or car traffic causing compaction; these are things that trees don't like.
That sounds like a lot of trees for a backyard. I've seen cases where previous owners liked trees and planted a lot of them. "They're so cute when they're small." After twenty years it can get a bit crowded as the trees fill in.
Things that you can do from easiest to harder: