Does fire always accelerate shrub expansion in Arctic tundra? Examining a novel grass-dominated successional trajectory on the Seward Peninsula

Over the last century in the circumpolar north, notable terrestrial ecosystem changes include shrub expansion and an intensifying wildfire regime. Shrub invasion into tundra may be further accelerated by wildfire disturbance, which creates opportunities for establishment where recruitment is otherwise rare. The Seward Peninsula currently experiences more frequent and larger fires than other tundra regions in Alaska. There are areas of overlapping burn scars dating back to the 1950s. Using a chronosequence approach, we examined vegetation and ecosystem dynamics in tussock tundra. Increasing burn severity and fire frequency corresponded with an increase in grass cover and a decrease in shrub basal area. We used multivariate ordination analysis to create a single integrator variable of fire effect that accounted for time after fire, burn severity, and number of times burned. This fire effect was significantly associated with decreases in soil organic layer thickness and overall plant biomass. Unlike previous studies in Arctic Alaska tundra, we found that increases in fire frequency and severity did not increase shrub cover and biomass. Instead, intensifying fire disturbance, and particularly repeat fires, led to grass dominance. Our findings support the hypothesis that intensifying tundra fire regimes initiate alternative post-fire trajectories that are not shrub dominated and that are structurally and functionally quite different from sedge or shrub-dominated tundra.

在过去的百年间，北极地区显著的地表生态系统变化包括灌木扩张和野火频发的加剧。灌木对苔原的入侵可能因野火的干扰而进一步加速，野火创造了在其他情况下罕见的新生机会。塞沃德半岛目前经历的火灾比阿拉斯加其他苔原地区更为频繁且规模更大。存在自20世纪50年代以来的重叠烧痕区域。采用年代序列方法，我们研究了苔原草丘地区的植被和生态系统动态。火灾严重程度和频率的增加与草地覆盖面积的扩大以及灌木基面积减少相一致。我们运用多元排序分析创建了一个单一的综合变量，用以衡量火灾效应，该变量考虑了火灾后的时间、火灾严重程度以及火灾次数。这种火灾效应与土壤有机层厚度和整体植物生物量的减少显著相关。与阿拉斯加北极苔原地区的先前研究不同，我们发现火灾频率和严重程度的增加并未导致灌木覆盖面积和生物量的增加。相反，增强的火灾干扰，尤其是重复火灾，导致了草地的优势地位。我们的研究结果支持了以下假说：加剧的苔原火灾制度引发了非灌木主导的、在结构和功能上与莎草或灌木主导的苔原截然不同的替代火灾后轨迹。