Problem with GL_SRC_ALPHA in texture_env_combine


i have a problem using an textures alpha value as a mask for rendering a terrain chunk. GL seems to ignore the texture and uses vertex alpha instead. Although texture targets are enabled. I cannot see anything wrong in the tex_env_combine setup.

No gl errors are reported. The setup is here:
The masks texture format is GL_ALPHA and GL_ALPHA8 internally.

Thank you very much for any hints.