Understanding burn severity sensing in Arctic tundra: exploring vegetation indices, suboptimal assessment timing and the impact of increasing pixel size

Abstract

Little is known about how satellite imagery can be used to describe burn severity in tundra landscapes. The Anaktuvuk River Fire (ARF) in 2007 burned over 1000 km² of tundra on the North Slope of Alaska, creating a mosaic of small (1 m²) to large (>100 m²) patches that differed in burn severity. The ARF scar provided us with an ideal landscape to determine if a single-date spectral vegetation index can be used once vegetation recovery began and to independently determine how pixel size influences burn severity assessment. We determine and explore the sensitivity of several commonly used vegetation indices to variation in burn severity across the ARF scar and the influence of pixel size on the assessment and classification of tundra burn severity. We conducted field surveys of spectral reflectance at the peak of the first growing season post-fire (extended assessment period) at 18 field sites that ranged from high to low burn severity. In comparing single-date indices, we found that the two-band enhanced vegetation index (EVI2) was highly correlated with normalized burn ratio (NBR) and better distinguished among three burn severity classes than both the NBR and the normalized difference vegetation index (NDVI). We also show clear evidence that shortwave infrared (SWIR) reflectivity does not vary as a function of burn severity. By comparing a Quickbird scene (2.4 m pixels) to simulated 30 and 250 m pixel scenes, we are able to confirm that while the moderate spatial resolution of the Landsat Thematic Mapper (TM) sensor (30 m) is sufficient for mapping tundra burn severity, the coarser resolution of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor (250 m) is not well matched to the fine scale of spatial heterogeneity in the ARF burn scar.

Acknowledgements

We thank Toolik Lake Field Station for providing support and logistics; Donnie Bret-Harte, Michelle Mack and Randi Jandt for facilitating access to their CBI survey sites; Benjamin Jones and coauthors, as well as Arctic, Antarctic and Alpine Research, for granting permission to include their burn severity map here; and Christopher Small for providing the spectrometer and for helpful discussions regarding interpretation of reflectance signals. We thank Christopher Noyles at BLM Alaska and Randi Jandt at Alaska Fire Service for providing the Quickbird imagery. We also thank anonymous reviewers for their helpful critiques and comments. This work was funded by NSF grants #0632139 (OPP-AON), #0808789 (OPP-ARCSS SGER), #08292285 (DEB-NEON SGER) and #0423385 (DEB-LTER) to the Marine Biological Laboratory (MBL) and by The Climate Center, Lamont-Doherty Earth Observatory, Columbia University.