Wildfire impacts on seedbank and vegetation dynamics in Calluna heath

Moorlands dominated by Calluna vulgaris (hereafter Calluna) are globally rare and under increasing threat of wildfires due to climate change. The soil seedbank is important for community resilience, but research on its contribution to regenerating vegetation after moorland fires has, to date, focused on prescribed fire or on the short-term (≤ 2 year) impacts of wildfire. To address the role of the seedbank in long-term vegetation regeneration, we studied a chronosequence of six wildfire sites within a Scottish moorland catchment, ranging from 2 to 64 years since burning. We recorded vegetation composition and sampled the seedbank. Calluna and Erica spp. comprised 66 % and 27 % of germinated seeds, respectively, and the majority of vegetation species were not represented in the seedbank. Canonical Correspondence Analysis (CCA) revealed that time since fire was a strong predictor of vegetation species composition, but it had no effect on seedbank composition, suggesting that the same species dominate the seedbank regardless of burning. Whilst total seed density was not affected by time since wildfire, the effect on the non-Calluna seedbank was more notable. The proportion of non-Calluna species decreased with time in both vegetation and seedbank as Calluna cover was re-established and the seedbank of other species became gradually depleted. Whilst relative non-Calluna cover in the vegetation was at a maximum in recently burnt areas and then declined with time since fire, the species richness and proportion of non-Calluna in the seedbank followed a unimodal pattern after fire and reached a peak after approximately 35 years. Our results contribute to the knowledge of how moorland ecosystems regenerate after severe disturbances, which are likely to become more common as the climate changes.

以帚石楠（Calluna vulgaris，下称Calluna）为优势种的荒野生境在全球范围内属于珍稀生态系统，且正因气候变化面临愈发严峻的野火威胁。土壤种子库对群落恢复力至关重要，但迄今为止，有关其在荒野火灾后植被重建中作用的研究，多聚焦于计划火烧，或是野火发生后≤2年的短期影响。为探明种子库在植被长期恢复中的作用，我们针对苏格兰某荒野流域内的6个野火样地开展了时间序列研究，样地的火烧后年限跨度为2年至64年。我们记录了植被组成，并对种子库进行了采样。萌发的种子中，帚石楠与欧石楠属（Erica spp.）物种分别占萌发种子总数的66%与27%，且多数植被物种并未在种子库中被检测到。典范对应分析（Canonical Correspondence Analysis, CCA）结果显示，火烧后时长是植被物种组成的强预测因子，但对种子库组成无显著影响，这表明无论火烧事件如何，种子库中的优势物种始终保持一致。尽管总种子密度不受野火后时长的影响，但非帚石楠类群的种子库所受影响更为显著。随着帚石楠覆盖度逐步恢复，其他物种的种子库逐渐枯竭，因此植被与种子库中的非帚石楠物种占比均随时间推移而下降。植被中非帚石楠物种的相对盖度在新近过火区域达到峰值，随后随火烧后时长增加而降低；而种子库中非帚石楠物种的物种丰富度与占比则呈现出单峰模式，在火烧后约35年达到峰值。本研究结果增进了我们对荒野生态系统在严重干扰后恢复过程的认知，而随着气候变化，这类严重干扰事件的发生频率大概率会进一步提升。