The trees that can be felled have red leaves and an easily identifiable profile, so they can be seen at a distance. If you are a beginning player and haven't had a chance to gain Crafting experience to train skills which allow you cut down high level trees, you should start in a low level area. You should find some trees in the Eastern Perennial Trail right near Soltown. If you go southwest to Ardoris, you will find a boat on the overland map just to the west of Ardoris which will take you to the island of the Hidden Vale. On that island, some good beginner areas with trees are North Valeway, South Valeway and Kingsroad. If you already have good forestry skills, you can try the higher level areas. In the Hidden Vale you will find a lot of trees in Greymark Forest (in the open areas) and in the North, South and Deep Ravenwoods. The Verdantis Shardfall (PVP) has a good number of high level trees, but it is a long walk to get there. Good luck!
Howdy Pallyp! Welcome to Shroud of the Avatar! I understand your confusion about harvestable trees. I didn't figure it out until I happen to see someone cut one down. If someone has come by and lumbered, then there might not be any available trees to harvest in that section for awhile and some areas do not have many harvestable trees. Look for the trees like the one Mata showed you. They tend to stand out compared to the lovely green ones. Besides taking a boat to Hidden Vale from Ardoris there is another way at this time (it may not be active in other future releases). You can use a moongate! Go northward from Soltown to Solace Bridge. In Solace Bridge, beyond the destroyed fort, there is a moongate. This moongate will take you to the town of Owl's Head in Hidden Vale. You can also use the moongate to get back to Solace Bridge. Personally I enjoy cutting down this trees because I pretend they have been beetle killed and I'm improving the health of the forest.
I have found a few from getting close, but I definitely can't tell them apart. That explains why it was hard for me to find them. It's either that, or I'm coming after someone else already got them. Do we have a lot of lumberjacks?
Hmmm.... well another way is eventually the resource will have sparkles coming from it. Sometimes you have to wait a bit and sometimes you have to be close to the resource. I guess Twilight vampires like to rub up against ore veins, harvestable trees, dead animals, and various herb plants.
Currently the trees are very easy to spot regardless of color, because of their signature profile, as seen in the screenshot that @Mata posted. They always stand apart from other trees, so it is easy to see their outline even at night, when the colors are not recognizeable.
They also have regular ridges on the bark and very straight trunks; as mentioned they're normally apart from the mass of trees. Try Greymark Forest, in the centre where a lot of them stand until you've got a rough feel for what they look like. And invest a lot of points into lumberjacking to increase both how much you get and make the sparkles more obvious. But yes, it is frustrating when you first start, as you'd assume the trees would actually be in the forests themselves, but they're largely not.
Notice, in Mata's post, the picture of the tree. Look for that silhouette on the sky line, along ridges, etc. All choppable trees have that look. That's one of the easiest way to find them in, for instance, Greymark Forest. Good Luck.