Congruent Long-Term Declines in Carbon and Biodiversity Are a Signature of Forest Degradation

Recent global policy initiatives aimed at reducing forest degradation require practical definitions of degradation that are readily monitored. However, consistent approaches for monitoring forest degradation over the long term and at broad scales are lacking. We quantified the long-term effects of intensive wood harvest on above-ground carbon and biodiversity at fine resolutions (30 m²) and broad scales (New Brunswick, Canada; 72,908 km²). Model predictions for above-ground biomass were highly correlated with independent data (r = 0.77). After accounting for carbon stored in wood products, net CO₂ emissions from forests for the region from 1985 to 2020 were 141 CO₂e Tg (4.02 TgCO₂e year^-1; 32% of all reported emissions). We found strong positive correlations between locations with declines in above-ground carbon and habitats for old-forest bird species, which have lost > 20% habitat over 35 years. High congruence between biodiversity and forest carbon offers potential for policy incentives to conserve both objectives simultaneously and slow rates of forest degradation. These methods could be used to track forest degradation for managed forest regions worldwide.