Post-fire vegetation succession in the Siberian subarctic tundra over 45 years

Wildfires are relatively rare in subarctic tundra ecosystems, but they can
strongly change ecosystem properties. Short-term fire effects on subarctic
tundra vegetation are well documented, but long-term vegetation recovery
has been studied less. The frequency of tundra fires will increase with
climate warming. Understanding the long-term effects of fire is necessary
to predict future ecosystem changes. We used a space-for-time approach to
assess vegetation recovery after fire over more than four decades. We
studied soil and vegetation patterns on three large fire scars
(>44, 28 and 12 years old) in dry, lichen-dominated forest tundra
in Western Siberia. On 60 plots, we determined soil temperature and
permafrost thaw depth, sampled vegetation and measured plant functional
traits. We assessed trends in NDVI to support the field-based results on
vegetation recovery.Soil temperature, permafrost thaw depth and total
vegetation cover had recovered to pre-fire levels after >44 years,
as well as total vegetation cover. In contrast, after >44 years,
functional groups had not recovered to the pre-fire state. Burnt areas had
lower lichen and higher bryophyte and shrub cover. The dominating shrub
species, Betula nana, exhibited a higher vitality (higher specific leaf
area and plant height) on burnt compared with control plots, suggesting a
fire legacy effect in shrub growth. Our results confirm patterns of shrub
encroachment after fire that were detected before in other parts of the
Arctic and Subarctic. In the so far poorly studied Western Siberian forest
tundra we demonstrate for the first time, long-term fire-legacies on the
functional composition of relatively dry shrub- and lichen-dominated
vegetation.

亚北极冻原生态系统中的野火相对罕见，但其可显著改变生态系统特性。目前针对野火对亚北极冻原植被的短期影响已有充分研究记录，但关于植被长期恢复的探讨仍相对不足。随着气候变暖，冻原野火发生频率将有所提升。明晰野火的长期影响，是预测未来生态系统变化的必要前提。本研究采用空间替代时间法（space-for-time approach），对逾四十年间的野火后植被恢复情况进行评估。我们以西伯利亚西部以地衣为主的干旱森林冻原为研究区域，对三处过火年限分别超44年、28年及12年的大型火烧迹地的土壤与植被特征展开研究。我们在60个样地中测定了土壤温度与永久冻土融化深度，采集植被样本并测量了植物功能性状。我们通过分析归一化植被指数（NDVI）的变化趋势，为野外观测得到的植被恢复结果提供佐证。过火逾44年后，土壤温度、永久冻土融化深度以及总植被覆盖度均已恢复至野火前水平，总植被覆盖度亦恢复至野火前水平。与之形成对比的是，过火逾44年后，植被功能群仍未恢复至野火前的状态。过火区域的地衣覆盖度更低，而苔藓植物与灌木的覆盖度更高。相较于对照样地，过火区域的优势灌木物种矮桦（Betula nana）表现出更高的活力（比叶面积与植株高度均更高），这表明灌木生长存在野火遗留效应。本研究结果证实了此前在北极与亚北极其他区域观测到的野火后灌木扩张现象。在此前研究相对匮乏的西伯利亚西部森林冻原区域，本研究首次证实，野火会对以灌木与地衣为主的干旱植被的功能组成产生长期遗留效应。