I am making this Christmas Tree but the problem is that I can't render the whole scene in one go because of too many particles. So I have rendered my tree and now I want to render only tinsels wrapped around the tree but I want it as png with alpha so that I can add them together as layers. So how can I do this? Thanks.
It should work if you replace the transparent with a holdout shader. However that doesn't solve the Memory issue. There's a fairly easy solution, which of course doesn't work in all cases. In post you can use the Z-pass in order to combine renderlayers. Compositor -> SHIFT A -> Color -> Z-Combine. It is intended to do what you want, but it will produce strange results when MB or DoF is involved.