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Quantifying shifts in plant phenology in response to climate change represents an ongoing challenge, particularly in mountain ecosystems. Because climate change and phenological responses vary in space and time, we need long-term observations collected at a broad spatial scale. While data collection by volunteers is a promising approach to achieve this goal, one major concern with citizen science programs is the quality and reliability of data. Using a citizen science program (Phenoclim) carried out in the western European Alps, the goals of this study were to analyze (1) factors influencing participant retention rates, (2) the efficacy of a citizen science program for detecting temporal changes in the phenology of mountain trees, (3) differences in budburst date trends among different observer categories, and (4) the precision of trends quantified by different categories of participants. We used 12years of annual tree phenology measurements recorded by volunteers (schools and private individuals) and professionals within the Phenoclim program. We found decadal-scale shifts in budburst date consistent with the results from other studies, including significant advances in budburst date for the common birch and European ash (-4.0 and -6.5days per decade respectively). In addition, for three of six species, volunteers and professionals detected consistent directional trends. Finally, we show how differences in precision among the categories of participants are determined by the number of years of participation in the program, the number of sites surveyed, and the variability in trends among sites. Overall, our results suggest that participants with a wide range of backgrounds are capable of collecting data that can significantly contribute to the study of the impacts of climate change on mountain plant phenology.