Hi,
I have a Niederlassungserlaubnis. My Wife is moving to the US for pursuing a Masters Degree and would probably not come back to Germany for the next 3 years or more. I will probably be here in Germany.

When she leaves, should I surrender her Aufenthaltstitel which is valid till 2017? Or is there any way she can maintain that while I am in Germany? In case she needs to visit me for a few weeks, it will be nice to have the card around.

Also how can one maintain a Niederlassungserlaubnis after relocating to the US?

Thanks!

I think, as long as the husband has a legal residence permit then wife can keep residence permit as dependent (and vice versa). So she can keep it as long as your permit is valid assuming the following requirements. She must apply to renew it before expiratation, regularly. But note that her residence permit will probably become invalid if she satys 6 months or longer out of Germany. So she needs to visit germany at least every 6 months to keep the card (There may be exceptions of which I am not aware, but this is what I know).

In the same way, you will loose your Niederlassung if you stay out of Germany for 6 months or longer. If you get german nationality before you move to US, you will of course not have this problem any more. 

 I know that exceptions can be made to this rule of 6 months. You have to enquire in the AB and they will give you a form which you need to fillout. They will ask for the reason and the normal processing time is 3 weeks for that.

I know of this because I was about to move to another country also and had similar issues. I was about to seek an exception for 2 years with valid reason. I got this form from AB but never submitted it  because I had to put things on hold for family reasons.
best