Harnessing foresters' engagement for climate change adaptation: the emerging tool of next-generation citizen science

Observations and experiments in ecology and evolutionary biology often suffer from context dependence, limiting the generality of findings, the development of theories, and the building of predictive models. Systematic reviews and meta-analyses, as well as data mining, are commonly used to find general patterns in nature; however, issues of data homogeneity often arise. Our thorough review of past and ongoing citizen science (CS) projects reveals that CS targeting the general public, equipped with smartphones, is increasingly used to amplify the spatial and temporal scale of data collection in biodiversity research. While this approach has provided an extraordinary boost to data collection, we propose that CS targeting members of the public with specialized knowledge, such as farmers, gardeners, foresters, birdwatchers, or divers, has high yet untapped potential to increase the scale of experiments and create generalizable knowledge. Here, we describe the recruitment strategy, experimental design, and lessons learned in such a "next-generation" CS project: MyGardenOfTrees. It involved recruiting over 300 foresters across Europe, each of whom agreed to set up and follow (for five years) a small unit of a continent-scale transplant experiment. This unprecedented experiment, run exclusively by foresters and for foresters, covers the whole species range of two ecologically and economically key temperate forest tree species. It aims to understand the spatial extent of local adaptation: a fundamental assumption of all assisted migration programs. We conclude by identifying other research areas in ecology and evolution where an experimental CS approach could help resolve context dependence in the context of global change.