Monitoring succession after a non-cleared windthrow in a Norway spruce mountain forest using webcam, satellite vegetation indices and turbulent CO2 exchange

Forests cover approximately 30% of the world's land area and are responsible for 75% of terrestrial gross primary production. Disturbances, such as fire, storm or insect outbreaks alter the dynamics and functioning of forest ecosystems with consequences, in terms of species distribution and/or gross primary production, not fully understood. Large forest areas are intensively managed and natural disturbances are yet rare events but expected to increase with climate change. Here, we used digital repeat photography to observe the ecological succession in a windthrow disturbed forest in the Bavarian Forest National Park (Germany) and compared it to satellite-derived vegetation indices (NDVI, EVI, and PPI) as well as turbulent CO2 exchange. A data-driven clustering of the webcam images identified three regions of interest: spruce, grass and a transition region that showed grass in the beginning and became successively overgrown by spruce. The succession was mirrored in trends of annual maxima of gross primary production (GPP), satellite vegetation indices and derived image greenness (green chromatic coordinate, GCC) in the transition region. These trends were also responsible for a positive link between seasonal GPP and proxies. Start and end of growing season were estimated from GCC, NDVI, EVI, PPI, and GPP, compared to each other, and were linked partly to climatological growing season indices and phenological observations. This study demonstrates the suitability and benefits of a webcam in monitoring forest recovery after a severe windthrow event, thus offering a versatile tool that helps to understand successional and phenological processes after a disturbance.