Tussocks enduring or shrubs greening: Alternate responses to changing fire regimes in the Noatak River Valley, Alaska, 2017-2020

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 northwest 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 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 re-sprouted, and vegetation productivity inside sixty burn perimeters recovered to pre-fire 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 in situations 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 North as air temperatures, fire frequencies, and permafrost degradation increase. This feedback will not have indiscriminate expansion though; in the Noatak Valley, the fire-shrub-greening process is relatively limited in tussock tundra, where low-severity fires and shallow active layers exclude shrub proliferation. 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.

随着北极变暖，苔原（tundra）野火预计将变得更为频繁且剧烈。评估苔原最易燃区域对燃烧的响应，有助于我们了解北极其余区域可能受到的气候变化影响。本研究借助灌木树轮年代学（shrub dendrochronology）、活动层厚度（active-layer depth）监测与遥感植被生产力分析，刻画了阿拉斯加西北部诺阿塔克河流域内苔原野火对生态系统的影响。研究结果表明，生产力相对较高的苔原更易发生野火且燃烧烈度更高，这意味着当前燃料负荷限制了诺阿塔克谷内苔原野火的分布格局。野火发生后的三年内，绝大多数采样的桤木灌木要么完成萌发，要么重新萌枝，且60处火烧迹地内的植被生产力已恢复至火前水平。苔原野火会引发两个阶段的初级生产力提升，具体表现为地表绿化程度增加：第一阶段出现在多数火烧区域野火发生后的3至10年；第二阶段则出现在野火触发近地表永久冻土（permafrost）融化、进而推动灌木扩张的区域，时间跨度为火后16至44年。目前诺阿塔克谷内正运行着“野火-灌木-绿化”的正反馈循环，随着气温升高、野火频次增加与永久冻土退化加剧，该反馈循环可能向北扩张。但该扩张并非无差别进行：在诺阿塔克谷内，“野火-灌木-绿化”过程在草丛苔原中相对受限——这类区域火灾烈度较低且活动层较浅，无法支撑灌木扩张。气候变暖与野火发生频次的提升，可能会将原本少火区域转变为两种以火为适应的生态吸引态（ecological attractor states）：即当前在多火的诺阿塔克谷占据主导地位的草丛苔原与直立灌木苔原。