Is it best to hire professionals to cut down small trees?

I wouldn't if you have any kind of heart problems. With clippers you're going to be straining a lot and that could put a lot of stress on you. Other than that, if you're in decent shape I'd say you could handle SMALL plants/trees. I actually just did some of that recently and it wasn't too bad. Just don't "overextend" and try to do more than you think you can handle; I eventually had to stop MYSELF because I tried cutting down a tree that was just too wide for my own clippers! That is when I called in professionals.