By crossdating ring growth between trees, dendrochronologists can assign the true year of formation for every ring of each sample, and this chronology information then allows for various other analyses of past environmental and/or human events. In the example above, the living tree (right side) has a known bark date--the present year--which may be taken as the starting point for the chronology. After crossdating samples from living trees, a dendrochronology will extend from the present back in time to some earlier year, say AD 1500.
Then, dendrochronologists often find and sample wood from dead trees (middle), either as standing snags or fallen logs, that both lived for a long time and died long ago. If the dead trees lived concurrently with the living trees, then the outer ring-growth pattern of the dead trees will match (crossdate with) that of the inner portions of the living trees. Thus, the dendrochronology will be extended further back in time to an even earlier year, say AD 1200.
Lastly, dendroarcheologists often collect samples from wooden beams, uprights, and other structural parts of dwellings that were constructed, lived in, and abandoned long ago (left side). If the trees that provided the structural parts lived concurrently with the dead snags or logs, then the outer ring-growth pattern of the structural samples will match (crossdate with) that of the inner portions of the dead trees. Thus, the dendrochronology will be extended further back in time to an even earlier year, say AD 800, and the human behavioral events of the past can be reconstructed for further archeological analysis.
Without knowing the actual year dates of formations of all rings of these samples, this application of dendrochronology would not exist. With crossdating, however, this application is possible and, indeed, dendroarcheology is a major subfield of dendrochronology. Likewise, with crossdating to know the exact year of formation of tree rings we may study past wildland fires (dendroecology), earthquakes (dendrogeomorphology), and climate changes (dendroclimatology) as well as many other environmental processes.