I did a search on stack and found this post  http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=001716#000000

In the post one person mentions that stack is caused by design and material. From the other explanations I get the design part, but what role does material play in stacking?

IMHO the 'kind' of wood or other stack elements isn't particularly important.  Some 80% of the power of a bow comes from the two strips of fiberglass and how far apart they are kept by the stack in between.  Even thickness of glass isn't particularly important except in the sense that thinner glass will bend around extreme curves easier.

Obviously Balsa and probably Aspen wouldn't work well as laminations, as they would no doubt crush when flexing, or even during glue-up, and that would change the space between the two pieces of glass.

Don't use cypress either. It will fail after very little use.

Good to know...

The core does play a role but as long as the core can handle the compression on the belly and tension on the back you can use whatever you want. Of course now its just a matter of limb mass for your choices.

Simply put stacking is when the limb tips begin to bend past 90 degrees.