Tussocks enduring or shrubs greening: Alternate responses to changing fire regimes in the Noatak River Valley, Alaska

Abstract:

As the Arctic warms, wildfires in the tundra biome are expected to become more frequent and severe. Assessing how the most flammable regions of the Alaskan tundra respond to burning can inform us about how the rest of the Arctic may be affected by climate change. Here we describe vegetation responses to tundra fires in the Noatak River watershed of northwest Alaska using shrub dendrochronology, active-layer depth monitoring, and remotely sensed vegetation productivity. Results show that tundra fires result in net increases in primary productivity as manifested by increased landscape greening. Within three years of burning, most alder shrubs sampled from two fires had either germinated or re-sprouted and vegetation productivity inside sixty burn perimeters recovered to pre-fire values. Three to ten years after fire, burned areas were ~8% greener than before fires, possibly because fire increases nutrient availability. A second greening phase occurred 16-44 years after fire in situations where tundra fires triggered near-surface permafrost thaw resulting in shrub proliferation and involved a 14% increase in greening relative to pre-fire values. Interestingly, within vegetation type, relatively productive tundra is more likely to experience fires and burn more severely, suggesting that fuel loads currently limit tundra fire distribution in the Noatak Valley. This pattern, in combination with expectations of more tundra fire in a warmer Arctic, suggests that a fire-shrub-greening positive feedback is currently operating in most of the fire-prone regions of the Noatak Valley and this feedback could expand North as air temperatures, fire frequencies, and permafrost degradation increase. This feedback is relatively limited in tussock tundra, where low-severity fires and shallow active layers exclude shrub proliferation. Escape routes from the fire-shrub-greening positive feedback occur when there is a lack of seed sources for post-fire shrub expansion, when tundra vegetation shifts to boreal forest, and during long fire-free periods that lead to paludification. Climate warming and enhanced fire occurrence will likely shift fire-poor regions into either the tussock tundra or erect shrub tundra fire-adapted ecological attractor states that now dominate the fire-rich Noatak Valley.