Renewal of postdoctoral appointment I noticed during my application process for post-doc jobs that several say "1-2 year appointment" or "possible renewal for second year". I understand from this answer that it is in everyone's benefit to not be "stuck" in a postdoc situation for too many years for fear of starving oneself.

My question is, if not mentioned, what is the general criteria for a renewal of postdoc appointment to a second year (or perhaps to a third)? Most of the applications I have put in to labs/univs. are not clear at all on renewal criteria. Is it based on need? performance? Mutual understanding? Does anyone have examples on some of these situations?
My feeling is that unless there are funding issues, there would be some resistance to firing a postdoc unless there were real problems: the startup cost/time for a new postdoc can be high. 

Having posted ads like these myself before, I can say that one non-funding-based reason to mention a 1-2 year postdoc is to allow for a hedge in case the hired person turns out to be really bad. In that case, you have the option of doing a first-year review and dismissing the person (i.e the default bit is set to NO and some activation energy is required to make it a YES)

In the reverse case of a two-year postdoc, the default bit is now YES and significant activation energy is required to make it a NO. If not done right, this could even lead to charges of unfair dismissal and so on. 

So since there's a glut of postdocs and a short supply of positions, the "buyers" have some power to shape the position to suit their needs.