I love the old RSPB system for birds which starts with 'is it bigger than a duck'. and then drills down from there. So for trees I guess we would start with two folders 'is it deciduous or not'; and then subfolders for general shape, bark type etc. So if you have no idea what tree you have, it's a bit of an adventure finding out that way. This would be a lot of work to set up though, so I think browse by name is my second choice.