Reconstructing fire regimes in tundra ecosystems to inform a management-oriented ecosystem model

Sheng Hu, Feng; Barnes, Jennifer; Joint Fire Science Program

We reconstructed fire history in one of the most flammable tundra ecosystems in Alaska, the Noatak River watershed, and interpreted records in the context of local vegetation change and re- gional climate. We also developed models link- ing monthly weather conditions to annual area burned in Alaskan tundra from 1950-2008. Both objectives served the additional goal of improv- ing the Boreal ALFRESCO model, an ecosys- tem model representing vegetation change as a function of climate and disturbance by fire. Fossil pollen records indicate that tundra veg- etation in the Noatak study area showed sub- tle shifts over the past 6000 years, likely in re- sponse to previously-described millennial-scale variations in relative moisture (precipitation - evaporation). Different patterns between sites suggest that local factors modified the impacts of regional dryness ca. 4000 years ago and subsequent increases in relative moisture. Es- timated fire return intervals (FRIs) since 6000 years before present (yr BP) varied from 30 to 720 years, with mean FRIs (summarized over 2000-yr periods) varying from 120 to ca. 500 years. These millennial-scale changes in mean FRIs were significantly linked to changes in vegetation, suggesting that white spruce, shrub birch, and grasses are associated with increased fire risk, whereas alder is associated with de- creased fire risk. The taxonomic makeup of fu- ture tundra ecosystems, therefore, could have important impacts on fire risk. When charac- terized over the past 2000 years, estimated FRIs were shorter (average 134 yr [95% CI 109-162]) at warmer sites with greater tussock-shrub tun- dra abundance than at cooler, up-valley sites with a greater abundance of low shrub-tundra (mean FRI 295 yr [189-415]). Results from modern fire-climate analyses in- dicate that annual area burned can be largely ex- plained with climate variables representing sum- mer temperature and precipitation (r 2 > 0.83). Models linking historical climate with tundra area burned in combination with estimated FRIs from the paleo data were used to parametrize tundra regions represented by the Boreal AL- FRESCO model. Comparisons between histor- ical simulation and the paleo record from the Noatak study suggest that the newly-informed Boreal ALFRESCO model provides improved estimates of tundra fire occurrence in the Noatak as compared to previous versions