2.1.1 Wildfire occurrence detection in Alaskan tundra
We defined the extent of Arctic tundra in Alaska with the commonly used
Circumpolar Arctic Vegetation Map (Walker et al., 2009). MODIS Thermal
Anomalies/Fire locations product (MCD14ML; Giglio et al., 2003) was
chosen to determine the locations and dates of fire occurrences. We
first identified individual fire events with MCD14ML data based on its
consistent information of active fire points. We designed a
spatiotemporal clustering method designed based on the Density-Based
Spatial Clustering of Applications with Noise (DBSCAN; Ester et al.,
1996) algorithm (Text S1; Figure S1). The maximum distance between two
neighboring fire points in a cluster was set to 2.5 km (Loboda and
Csiszar, 2007). Since fire events that occurred during different time
periods could be grouped into the same cluster, we further separated
points of different fire events in a spatial cluster with a temporal gap
of 4 days, as suggested by Loboda and Csiszar (2007). The locations and
dates of the active fire points with the earliest acquisition time were
then extracted to represent the tundra fire occurrences.