Preliminary results of chromatic vegetation indices in a temperate European wetland

Understanding seasonal vegetation dynamics in wetlands is crucial for tracking ecosystem functioning in the face of climate change. RGB cameras installed at ground level offer a cost-effective and automated approach to monitoring vegetation phenology through chromatic indices derived from digital images. In this study, we analysed three years (2022 – 2024) of high-frequency imagery from a peatland site in the Biebrza National Park (NE Poland), focusing on three chromatic vegetation indices: red (r_cc), green (g_cc), and blue (b_cc) chromatic coordinates.

Image data were collected using a fixed RGB camera (StarDot NetCam SC) mounted above a sedge- and reed-dominated vegetation canopy. A consistent region of interest (ROI) was defined to capture the seasonal greening patterns without sky interference. Chromatic indices were extracted using the phenopix package in R and filtered to reduce noise caused by changes in illumination. We observed seasonally repeatable trajectories, with a springtime decline in r_cc accompanied by a rapid increase in g_cc. This pattern aligns with the increase in chlorophyll content in leaf tissues and the seasonal activation of the xanthophyll cycle. Conversely, b_cc reached its maximum during the dormant season, particularly in midwinter, when vegetation activity is minimal and atmospheric scattering dominates the scene brightness.

Although these phenological signals were generally consistent between years, notable differences emerged in the timing and intensity of index transitions – particularly during the onset of greening and the summer peak of activity. These variations likely reflect interannual differences in environmental conditions. Ongoing analyses aim to identify the relationships between chromatic index dynamics and abiotic drivers such as air temperature, solar radiation, water table level, and vapor pressure deficit (VPD).

Our results demonstrate the effectiveness of RGB-based phenological monitoring to reveal fine-scale seasonal processes in wetland vegetation. Further integration with micrometeorological and ecophysiological datasets will allow a more complete understanding of the climatic controls shaping phenological responses in peatland ecosystems.

Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219. The authors thank the authorities of the Biebrza National Park for allowing continuous measurements in the area of the Park.