This paper provides an empirical account of the problem of interdisciplinarity in the field sciences, considering it as a driver of ontological change. Our case study is an ongoing interdisciplinary research project in environmental science. Its objective is to trace the long-term histories of European old-growth forests. To account for the mechanisms involved when researchers seek to do interdisciplinary science in the field, we describe 1/ four research practices that take advantage of the spatial order of the study site in order to make forests temporal processes knowable, thereby producing a field site crisscrossed by multiple spatiotemporal orders; 2/ those practices geared towards articulating these spatiotemporal orders and the limits faced by the consortium towards their complete integration; 3/ how such articulation transforms the conception of old-growth forests as spaces shaped by historical processes integrating human activities and valued ecological processes. We argue that interdisciplinary research practice in environmental field sciences does not lead to a synthesis of pre-existing domains of knowledge production. Rather, it does tend to transform both the object of study and the disciplines involved. The field, as both an object of study and a research place, becomes a broker toward ontological changes.