Tracking vegetation phenology for a deciduous black locust plantation with the use of a phenology camera, satellite vegetation indices and eddy flux measurements – advantages and limitations

The accurate assessment of vegetation phenology, i.e. the seasonal changes in plant development status, is a critical step for the study of ecosystems’ performance and their response to the ongoing climate change. Satellite products have been used for a quite long time for this purpose, frequently with the combination of data from eddy flux towers. In recent years, the use of digital photography from properly parameterized cameras (phenology cameras), has been introduced as a very promising low-cost methodology for precise monitoring of vegetation phenology.

In this study, we provide an assessment of vegetation phenology for a five-year period (2019 – 2024) of a black locust plantation, located in the restored areas of the Lignite Center of Western Macedonia, Greece. During the study period, repeated RGB images in half – hourly time steps were automatically taken with the use of a phenology camera installed on the top of an eddy flux tower in the study site and timeseries of the Green Chromatic Coordinate (GCC) index were extracted. Furthermore, for the same period, high resolution Sentinel 2 satellite products were used for the estimation of four commonly used vegetation indices (NDVI, EVI, LSWI and SAVI). Finally, carbon flux phenology was assessed from the respective measurements of the eddy flux tower.

Our results indicate that the use of phenology cameras can provide an explicit representation of vegetation phenology and estimation of the respective phenological indices (start, end, peak and length of the growing season). GCC is well correlated with all the studied satellite vegetation indices, however it provides the advantage of the continuous measurements, as the results are not affected by the weather conditions, in contrary to satellite products.  Additionally, it can be used for the distinction of overstory and understory vegetation phenological status, which is very critical especially in deciduous ecosystems, but it cannot be assessed with the use of satellite products. Furthermore, it can provide valuable information for other phenological parameters, such as the start and length of the blooming period, which is also difficult to assess by other methods. Concerning the representation of carbon flux, GCC does not seem to provide any further direct advantages compared to satellite vegetation indices, however the ability of the distinction of understory and overstory vegetation phenology can provide other benefits, such as the more efficient parameterization of productivity models.