Next, I tried to apply this to the leafs of a tree, but this doesn't work. I think it is because all the leafs are just one object. So, is there a way to separate all the leafs so they can be adressed as many objects?

Thanks,

Kyraia

Firstly select your single leaf object that consists of all the trees leaves.

Go into edit mode and press "A" to select all the faces of all the leaves.

Press "P" which brings up the Separate selection box.

Choose "By Loose Parts".  This will separate all the leaves from each other.

When doing this with an object like the leaves on a tree be prepared to wait awhile for your computer to process this command as it can take some time due to the number of objects that need separating.  Also I suggest that when you select your leaf object right at the start you move this object to its own layer as you will have hundreds of individual leaf objects at the end of this process.  This will help when you follow the steps in the video you linked too.  

Cool! It worked. Thanks a lot.

When doing this with an object like the leaves on a tree be prepared to wait awhile for your computer to process this command as it can take some time due to the number of objects that need separating.  

Actually, I wanted to use this for an autumn or winter tree which doesn't have many leafs.

Kyraia