Tussocks Enduring or Shrubs Greening: Alternate Responses to Changing Fire Regimes in the Noatak River Valley, Alaska

As the Arctic warms, tundra wildfires are expected to become more frequent and severe. Assessing how the most flammable regions of the tundra respond to burning can inform us about how the rest of the Arctic may be affected by climate change. Here we describe ecosystem responses to tundra fires in the Noatak River watershed of northwestern Alaska using shrub dendrochronology, active-layer depth monitoring, and remotely sensed vegetation productivity. Results show that relatively productive tundra is more likely to experience fires and to burn more severely, suggesting that fuel loads currently limit tundra fire distribution in the Noatak Valley. Within three years of burning, most alder shrubs sampled had either germinated or resprouted, and vegetation productivity inside 60 burn perimeters had recovered to prefire values. Tundra fires resulted in two phases of increased primary productivity as manifested by increased landscape greening. Phase one occurred in most burned areas 3–10 years after fires, and phase two occurred 16–44 years after fire at sites where tundra fires triggered near-surface permafrost thaw resulting in shrub proliferation. A fire-shrub-greening positive feedback is currently operating in the Noatak Valley and this feedback could expand northward as air temperatures, fire frequencies, and permafrost degradation increase. This feedback will not occur at all locations. In the Noatak Valley, the fire-shrub-greening process is relatively limited in tussock tundra communities, where low-severity fires and shallow active layers exclude shrub proliferation. Climate warming and enhanced fire occurrence will likely shift fire-poor landscapes into either the tussock tundra or erect-shrub-tundra ecological attractor states that now dominate the fire-rich Noatak Valley.