Posted 08/23/23
We analyzed circumboreal ecoregion level trends (1983-2020) in growing season (Mar-Nov) burned area and severe wildfire weather to determine if either have increased, and if change in severe wildfire weather has influence trends in burned area. We found no significant trends in burned area within any circumboreal ecoregion. However severe wildfire weather appears to be worsening across most Eurasian boreal ecoregions and may drive future increases in burned area within Eastern Siberian boreal forests.
The boreal forests of Russia, Canada, Alaska, and Scandinavia, collectively known as the circumboreal region, contain 25% of global forests and 50% of the world’s soil carbon pool. Wildfire is an important disturbance agent within the circumboreal region and plays a key role in facilitating its nutrients cycling and succession. Climate change is warming the circumboreal region at twice the rate of the global average creating longer, hotter, and drier wildfire seasons. This in turn has warmed and dried the circumboreal growing season, stressed vegetation, and may have contributed to the severe 2019-2021 wildfire seasons. The specter of increasing burned area extent within the circumboreal region is concerning because it may volatilize the region’s enormous soil carbon pool, exacerbating climate warming.
Ground-based mapping of burned area within the circumboreal region is challenging due to its remote and inaccessible nature. Instead, burned area is typically mapped using satellite-based remotely sensed datasets. Mapping burned area within the Eurasian boreal region prior to the year 2000 is difficult due to a lack of available Landsat and MODIS data. Only the coarse resolution AVHRR archive provides data capable of exhaustively mapping wildfire throughout the circumboreal region over multi-decadal timescales. Mapping wildfire with AVHRR data is complicated by sensor-based limitations that, if unaccounted for, will degrade the quality of resulting burned area maps. An advantage of statistical models over machine learning algorithms typically used to map burned area is that they allow known sources of variation to be explicitly quantified. This makes statistical models well suited for wildfire detection with AVHRR data as the archive’s sources of noise are well documented. As such, we developed a methodology which uses autoregressive timeseries analysis to map circumboreal burned area with AVHRR data (Figure 1).
High wildfire years within the circumboreal region are usually facilitated by unseasonably hot and dry summertime conditions. Climate change is predicted to increase the severity of wildfire weather within the circumboreal region, which in turn is expected to increase annual burned area. However, it is unclear which circumboreal ecoregions are currently experiencing worsening wildfire weather, and to what degree this is affecting burned area extent within those ecoregions. We tested for long term trends in ecoregion level severe wildfire weather by conducting a timeseries analysis on circumboreal annual (1983-2020) growing season (Mar.-Nov.) 95th percentile Canadian Fire Weather Index value (Figure 2). We performed this timeseries analysis using a methodology known as remotePARTS which accounts for spatial and temporal autocorrelation in remotely sensed datasets. We then determined the impact of trends in severe wildfire weather on annual ecoregion level burned using autoregressive timeseries analysis.
We found no significant long-term trends in severe wildfire weather within North American boreal ecoregions. However, wildfire weather appears to be worsening more rapidly in northern latitudes at the continental scale. Conversely, wildfire weather is worsening across all Eurasian boreal ecoregions with the Trans-Baikal conifer forests experiencing the greatest change. Trends in severe wildfire weather were also shown to influence interannual variability in burned area with the Northern Canadian Shield Taiga (50%) and Trans-Baikal Conifer Forests (30%) most sensitive to this relationship (Figures 3, 4).
Climate change is causing the circumboreal wildfire season to become longer, drier, and warmer. Our findings highlight the ecoregion level differences in the intensity of this change. While no significant ecoregion level trends in burned area have yet been detected, the sensitivity of circumboreal wildfire regimes to wildfire weather makes them vulnerable to changing fire seasons. In particular, fire regimes of Eastern Eurasian boreal forests which are experiencing worsening wildfire weather are particularly vulnerable. Further study of climate and wildfire interaction within these ecoregions is required to understand how these fire regimes are currently changing, and how they may change in the future.
Story by Stephens, Connor