Sorry to break the news to you, but you will have to use Photoshop. The alpha channel should be pulled from the tens of thousands of textures you have lying around the house and placed in the 'transparancy' channel of the shader, eh, tree. When you import your humongous amounts of textures in the color channel it just takes from the bitmap what it says, the color. In photoshop you could make an action, I think, to process small batches of your biotanical collections. An action would leaf (sorry, leave) you with a new document per texture, that should be saved by a describing name (like 'SouthChineseScholarTree-ALPHA') which I think is not possible in PS. What need to be done is that in the 'Channels' tab you select the channel, choose 'Duplicatie Channel' from the tab menu and choose 'NEW' in the following dialog box. This creates a new document that can be saved as a grayscale bitmap, to use as an alpha. Because there are so many textures involved you should maybe only use the ones you are actually going to need. Unless you are making an Encyclopedia Of Trees Around The World the workload should be overseeable. Good luck with it!