This study explores the long-term impact of citizen science projects on knowledge production, focusing on the creation and sustainability of knowledge commons. It examines how participatory science projects generate and disseminate knowledge, particularly in relation to open access, preservation and editorialisation. Using the ecological project “Tree Bodyguards” as a case study, the article identifies various outputs, including scientific articles, media documents and protocols. A methodological grid is introduced to analyse these outputs as “traces,” utilising concepts such as hints, remains and memory. The findings reveal that while a portion of the knowledge produced is accessible to the public, a significant amount remains restricted or disappears over time due to inadequate preservation and institutional support. The study highlights the importance of ensuring that co-produced knowledge in citizen science projects is not only shared but also preserved as a common good, accessible to both participants and the broader public. It emphasises the need for institutions to commit to the long-term sustainability of this knowledge to foster true collaboration and societal benefit.