Fire severity is a key driver shaping the ecological structure and function of North American boreal ecosystems, a biome dominated by large, high-intensity wildfires. Satellite-derived burn severity maps have been an important tool in these remote landscapes for both fire and resource management. The conventional methodology to produce satellite-inferred fire severity maps generally involves comparing imagery from one year before and one year after a fire. However, environmental conditions unique to boreal forests - including intense productivity of post-fire colonizing plants and persistent cloud cover - have limited the accuracy of resulting products. 

We introduce an alternative method – the ‘hybrid composite’ – based on deriving a mean severity (i.e. delta Normalized Burn Ratio and Relativized Burn Ratio) over time on a per-pixel basis within the cloud-computing environment of Google Earth Engine. Compositing is a relatively new approach where all clear pixels (i.e. without clouds, smoke, or shadows) across a date range of interest are stacked, and a summary statistic (e.g. mean) is calculated for each pixel in a stack. Our ‘hybrid composite’ method constructs the post-fire image from Landsat satellite data composited from images acquired in the time period immediately after fire through the early growing season of the following year. We compare this approach to the traditional paired-scene approach (single-date Landsat scenes from one year before and one year after fire) and another composite approach where the post-fire time period is from one year after fire.

• Validation statistics based on field-derived data for 52 fires across Alaska and Canada indicate that the hybrid composite method outperforms the other approaches for measuring fire severity.
• This approach presents an efficient and cost-effective means to monitor and explore trends and patterns across broad spatial domains, and could be applied to fires in other regions, especially those with frequent cloud cover or rapid vegetation recovery.

Improved fire severity mapping in the North American boreal forest using a hybrid composite method

Holsinger, Lisa M. ; Parks, Sean A. ; Saperstein, Lisa B. ; Loehman, Rachel A. ; Whitman, Ellen ; Barnes, Jennifer ; Parisien, Marc‐André , 2021