ABSTRACT

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.

摘要

在上个世纪的极地北极，陆地生态系统的显着变化包括灌木丛的扩张和野火制度的加剧。野火干扰可能会进一步加速灌木入侵苔原，这为在其他情况下很少招募的定居创造了机会。与阿拉斯加的其他苔原地区相比，苏厄德半岛目前遭受的火灾更频繁，更大。在1950年代，有烧伤疤痕重叠的区域。使用时间序列方法，我们研究了草丛苔原中的植被和生态系统动态。烧伤严重程度和着火频率的增加与草地覆盖面积的增加以及灌木基部面积的减少相对应。我们使用了多变量排序分析来创建火灾效果的单个积分变量，该变量说明火灾后的时间，燃烧严重性，和燃烧的次数。这种火灾的影响与土壤有机层厚度的减少和植物总生物量的减少显着相关。与先前在北极阿拉斯加冻原上的研究不同，我们发现火势发生频率和强度的增加并未增加灌木的覆盖率和生物量。相反，加剧的火势干扰，尤其是反复发生的火势，导致了草的优势。我们的发现支持以下假设：强化苔原火势会引发火灾后的其他轨迹，这些轨迹不是灌木为主的，并且在结构和功能上与莎草或灌木为主的苔原完全不同。我们发现，着火频率和严重程度的增加并未增加灌木的覆盖率和生物量。相反，加剧的火势干扰，尤其是反复发生的火势，导致了草的优势。我们的发现支持以下假设：强化苔原火势会引发火灾后的其他轨迹，这些轨迹不是灌木为主的，并且在结构和功能上与莎草或灌木为主的苔原完全不同。我们发现，着火频率和严重程度的增加并未增加灌木的覆盖率和生物量。相反，加剧的火势干扰，尤其是反复发生的火势，导致了草的优势。我们的发现支持以下假设：强化苔原火势会引发火灾后的其他轨迹，这些轨迹不是灌木为主的，并且在结构和功能上与莎草或灌木为主的苔原完全不同。