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Impact of burn severity on thermokarst initiation and expansion in Arctic tundra ecosystems
  • Yaping Chen,
  • Mark Lara,
  • Fengsheng Hu
Yaping Chen
University of Illinois at Urbana Champaign

Corresponding Author:ychen410@illinois.edu

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Mark Lara
University of Illinois
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Fengsheng Hu
University of Illinois at Urbana Champaign
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Burn severity influences various biophysical and biogeochemical processes, and it is projected to increase in the coming decades across high-latitude ecosystems. However, the impact of burn severity on thermokarst (e.g., land subsidence after ground ice melting) is not well understood. We used time-series image analysis to assess the effects of burn severity and ground ice content on thermokarst processes in the Noatak National Preserve, northwestern Alaska. To extend the temporal depth for illustrating fire-thermokarst linkages, we evaluated eight existing fire indices derived from visible and near-infrared (380-1100 nm) spectral bands and developed burn severity maps of historical fires using Landsat MSS sensors (operation between 1972-1992). Our results reveal that tundra fire is a significant factor in creating thermokarst landforms (p<0.01), and that the magnitude of thermokarst varies with burn severity levels and ground ice content (p<0.05). An abrupt increase in thermokarst occurred one year after fire but the rate of thermokarst decreased after three years. The area of thermokarst three years after fire was highest in high-severity burns (385 ± 47 m2 thermokarst area/ha burned area), followed by moderate- (255 ± 32 m2/ha) and low-severity (201 ± 42 m2/ha) burns. Ground ice content interacted with burn severity to affect thermokarst; the area of thermokarst was twice as large in landscapes with high ground ice (356 ± 67 m2/ha) as in landscapes of low ground ice (167 ± 39 m2/ha) three years after fire. Among the eight fire indices, the Global Environmental Monitoring Index (GEMI) demonstrates the strongest correlation with field-based estimates (R2 = 0.8). Burn-severity maps reconstructed with GEMI reveal that over a 40-year study period, thermokarst expansion occurred more rapidly in high-severe burns than in low-severity or unburned areas. Our results suggest that the projected increase in burn severity may result in abrupt and long-lasting permafrost degradation in tundra ecosystems with potential consequences on Arctic carbon stocks.