In most studies of forest succession such as that of Reiners and colleagues (1971) and Oosting (1942), researchers study succession by comparing sites of various ages. This approach is called a "space for time substitution." What are some major assumptions of a space for time substitution? What contribution might the Glacier Bay system make to testing some of those assumptions? Why is this approach often necessary? What advantages for studying succession are offered by systems like Sycamore Creek?