I don't see an overal sepia tone in this photo. I actually see alot of green in the photo. Aside from that, the photo looks too clustered and you miss the whole beautiful day scene. It would be better if you could actually see through the tree or if the angle was different.
While this does have some brown tones in it, you also have greens and yellows. Sepia tone refers to the coloring of a black and white photographic print that has been toned with a sepia toner to simulate the faded brownish color of some early photographs.
Aside from that, the highlights are a bit blown out by the harsh sunlight also.