There is not a way in the GUI to set up Requirement permissions for each folder. However, you may be able to utilize the data hiding filter (Tools - Customization - Groups and Permissions - Data-Hiding tab) to limit the fields that certain groups have access to. This can be filtered to specific folders, requirement types, etc. This will restrict the fields views and changes that can be made to a requirement, but will not completely hide the requirement from the tree.