Carbon storage in floodplains and riparian forests: Effects of damming and historical land use changes

Riparian forests and floodplains are fundamental carbon (C) sinks to mitigate climate change, particularly in water-stressed regions like Southwestern Europe. These areas have undergone major changes in land use and land cover (LULC), as well as alterations in streamflow by dam hydrological regulation, with consequences for regional and local C stocks. We assessed temporal and spatial patterns of C stock change in two regulated riverine floodplains in Portugal subject to contrasting hydrological alteration (moderate vs high). Floodplain C stocks were estimated by combining LULC mapping from pre-dam (1965) and post-dam (2013) airborne imagery with riparian field surveys and literature search. Total C stock across the study area increased from 4.64 to 9.30 ktC due to hydrological alteration, agricultural land abandonment and the expansion of riparian and managed forests. Riparian forests exhibited the highest C values (163.0 tC/ha), accounting for c. 50% of the total estimated carbon. Carbon gains were larger in the floodplain with high hydrological alteration, consistent with the riparian tree encroachment into the former active channel; however, floodplain differences were also shaped by contrasting LULC trajectories and should be interpreted as case-study evidence. These results highlight the strong sensitivity of riparian forests and floodplains to interacting hydrological and landscape changes over the past five decades. From a management perspective, increasing C stocks represents an opportunity for climate-change mitigation, but it may also involve ecological trade-offs; therefore, it should be pursued alongside native biodiversity conservation to maintain ecosystem functioning.