Ongoing Northern Hemsiphere boreal forest cover change in response to fire and permafrost thawing

Background/Question/Methods

Boreal forest is the most extensive forested biome on earth and it plays important roles in carbon stock and sequestration. At the same time, many studies have suggested that it is also highly sensitive to warming temperatures. Unsustainable forestry practices as well as fires, insect and pathogen outbreaks, and unstable permafrost in synergy with climate change have all been reported to cause boreal forest cover losses, with fire often considered the main driver of loss, while the potential effect of permafrost thawing has only received scant attention in large-scale studies.  We aim to identify if there are regions where boreal forest is undergoing decline due to unstable permafrost by looking at decadal changes in forest cover and their spatial association with permafrost thawing. MODerate Resolution Imaging Spectroradiometer (MODIS) and Landsat satellite data from 2000 to 2010 was used to map forest cover loss in the boreal biome globally. Data on permafrost distribution, temperature anomalies and topographic wetness (estimated by the topographic wetness Index, TWI) was used to assess if boreal forest cover losses reflect effects of permafrost thawing, besides those of known drivers such as fire, insects, pathogens and forestry exploitation. 

Results/Conclusions

Compared to North America, eastern Siberia suffered more high-latitude losses in boreal forest cover. Simultaneous spatial autoregressive models showed along with burned area, permafrost zone was a significant predictor of forest cover change. Although fire emerged as the main driver of boreal forest loss, higher loss was also observed in the (formerly) continuous permafrost areas and, if TWI is high, also in sporadic permafrost areas. In both cases, there was low burnt area, suggesting permafrost thawing to be the cause of boreal forest loss. While further work is needed to elucidate the relative importance of all the factors that may be involved in current boreal forest dynamics and how they vary geographically, these findings show that already now global-warming-induced permafrost thawing is causing strong losses across large areas at a decadal scale.