Nice start to the tree, but with a little more research it could be a great tree. To remove the halo around your leaves try expanding your alpha and go over the edge of the leaves by a pixel or two.
Have a look at the shape of the trunk of the tree especially the base and compare them with some google images. Older trees rarely are cylindrical. I knocked up a very quick tree stump to show what I mean.
They also twist a little as well.
As for the branches I would add a few more forks into them if you can spare the poly count, have a look on google for some winter trees and see how the branches fork out, smaller branches should be done on the alpha map plains as you have done but I would add some more shape definning branches.
Dont let me put you off, you have made a good model but with a few tweeks this could be a great model.
A mate of mine has done some tutorials on foliage and tree creation for max but you should find them just as useful no matter what software you use.
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