Monitoring coniferous forest biomass change using a Landsat trajectory-based approach

Forest biomass is a major store of carbon and thus plays an important role in the regional and global carbon cycle. Accurate forest carbon sequestration assessment requires estimation of both forest biomass and forest biomass dynamics over time. Forest dynamics are characterized by disturbances and recovery, key processes affecting site productivity and the forest carbon cycle. Thus, spatially and temporally explicit knowledge of these processes and their drivers are critical for understanding regional carbon cycles. Here, we present a new method that uses satellite data to estimate changes in forest aboveground biomass associated with forest disturbances and recovery at annual time steps. First, yearly maps of aboveground biomass between 1985 and 2010 based on Landsat time series and field data were created. Then, we applied a trajectory-based segmentation and fitting algorithm to the yearly biomass maps to reconstruct the forest disturbance and recovery history over the last 25 years. We tested the method over a coniferous forest region in the Western Carpathian Mountains, which experienced long-term environmental changes. Overall, 55% (~ 30,700 ha) of the total coniferous forest experienced a loss of biomass over the observation period, while ~ 30% showed severe or complete removal of forest biomass. At the same time, 11.2% of the area was reforested or regenerated on previously damaged forest stands. The total coniferous biomass dropped by 15% between 1985 and 2010, indicating negative balance between the losses and the gains. Disturbance hotspots indicate high insect infestation levels in many areas and reveal strong interactions between biomass loss and climate conditions. Our study demonstrates how spatial and temporal estimates of biomass help to understand regional forest dynamics and derive degradation trends in regard to regional climate change.