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）为优势种的高沼地在全球范围内均属稀有生境，且因气候变化正面临愈发严峻的野火威胁。土壤种子库（soil seedbank）对于高沼地群落的恢复力至关重要，但迄今为止，有关其在高沼地火烧后植被更新中作用的研究，多聚焦于计划火烧，或仅关注野火发生后2年以内的短期影响。为厘清种子库在植被长期更新中的作用，本研究针对苏格兰某高沼地集水区内的6个野火样地开展了时间序列（chronosequence）调查，样地的火烧后年限跨度为2至64年。研究人员记录了各样地的植被组成，并对种子库进行了采样。萌发种子中，帚石楠与欧石楠属（Erica）物种分别占66%与27%，且绝大多数植被物种并未在种子库中得到体现。典范对应分析（Canonical Correspondence Analysis，CCA）结果显示，火烧后年限是植被物种组成的显著预测因子，但对种子库组成并无显著影响，这表明无论火烧发生与否，种子库均由相同的优势物种主导。尽管种子总密度不受火烧后年限的影响，但对非帚石楠种子库的影响则更为显著。随着帚石楠盖度逐渐恢复，其他物种的种子库逐渐枯竭，非帚石楠物种在植被与种子库中的占比均随火烧后年限的增加而下降。尽管植被中非帚石楠的相对盖度在新近火烧样地中达到峰值，随后随火烧后年限增加而下降，但种子库中非帚石楠物种的物种丰富度与占比则在火烧后呈现单峰模式，约在35年时达到峰值。本研究结果有助于深化对高沼地生态系统在剧烈干扰后恢复过程的认知，而这类干扰事件在气候变化背景下或将愈发频发。