Good grief, rguru! Is there any GL forum you “didn’t” spam? Don’t cross-post! Your post has nothing to do with OpenGL Shading Language, Linux, Math and Algorithms, etc. This question is either “Beginners” or “User Hardware, Software, & Gaming Help”, depending on whether you’re a GL developer or a GL user.
Now to your problem…
That’s a standard alpha blending problem.
You’re not drawing your alpha-blend geometry in a proper back-to-front ordering. Consequently, the alpha-blend region of the tree texture is blending with sky, ground, or some such – whatever was behind it when you drew it, because it was drawn “before” some objects behind it.
Solution is to sort them properly, or use multisample alpha (aka screen-door transparency, aka alpha to coverage).
And finally, please do not cross-post. That kind of spam is annoying and discourages folks from wanting to help you. Find the most appropriate forum and post it once. Most of us read all the non-platform-specific forums anyway (e.g. I read everything but Windows and Mac).