In Western Europe, primary forests were long ago replaced with secondary woodlands worked and managed by human beings. "Sylvicultural cycles" have taken the place of natural "sylvigenetic cycles". Prunus serotina, a species introduced from South America, is becoming dominant in the Compiègne forest landscape. However locals and even foresters do not seem to perceive of this situation as an invasion. By taking the forest to be a mental construct specific to stakeholders, the cognitive framework and practices available to foresters are studied as they work in an uncertain world in the throes of "global" changes. This approach raises questions about how our relations with our environment are now being redefined.