Two Decades of Phenocam Studies: Progress and Challenges in Tropical Phenology Ecosystems

Phenological monitoring—tracking the timing of biological events such as leaf emergence and senescence—is essential for understanding how ecosystems respond to environmental change. Over the past two decades, phenocameras (automated digital cameras that capture repeated images of vegetation) have transformed phenology research, providing nearly daily, high-resolution data across various ecosystems. Despite major progress worldwide, there is an imbalance in phenocamera coverage between temperate and tropical ecosystems. This gap limits our understanding of tropical vegetation dynamics and affects the accuracy of global vegetation models that inform climate projections. In this review, we synthesize the global use of phenocameras, highlighting their methodological advances, ecological insights, and key applications. We focus on tropical ecosystems, identifying critical gaps, challenges, and opportunities to expand phenocam networks in these biodiverse regions. We conducted a systematic literature review on Web of Science, analyzing studies that used digital cameras for phenological monitoring in natural vegetation. Metadata from the selected articles was compiled, resulting in a total of 196 papers included in the review. Phenocamera applications primarily tracked phenological patterns and developed new methods. Camera-based time series validated orbital sensors, explored leaf phenological drivers, and related to terrestrial productivity proxies. Long-term studies on interannual phenological variation and climate change impacts were limited in both temperate and tropical regions. The global distribution of sites confirmed a concentration of study locations in the northern hemisphere, with most sites in the USA and Western Europe—mainly within temperate biomes such as Boreal forests, Temperate Seasonal Forests, Temperate Woodlands and Shrublands, and temperate grasslands and deserts. Tropical studies mainly focused on seasonally dry ecosystems like seasonally dry tropical forests, scrublands (Caatinga), and savanna woodlands and grasslands. They also included wet forests like the Amazon Rainforest, Atlantic Forest, and tropical mountain grasslands. By addressing these challenges and biases, our review shows a growing monitoring presence in the tropics, promoting a more equitable distribution of phenological research and improving our understanding of climate change effects on biodiversity and ecosystem dynamics worldwide. The insights from our analytical framework can guide future work, helping to develop inclusive phenological monitoring methods and supporting the increasing emphasis on phenology's role in conservation and climate resilience. Addressing these needs is crucial for establishing a more comprehensive, globally balanced phenological monitoring network, which is vital for better ecosystem models and for guiding conservation and climate policies in tropical regions.

Acknowledgements: FAPESP research grants #2021/10639-5, #2022/07735-5, and FAPESP fellowship grants #2024/09117-2 (BA) and #2024/06113-6 (MMPS). CNPQ (428055/2018-4; 306563/2022-3)