Berry production, shoot growth and biomass of E. hermaphroditum, V. vitis-idaea and V. myrtillis at Abisco ANS

Extreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short-lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2-10 °C for 2-14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week-long extreme winter warming events - using infrared heating lamps and soil warming cables - for 3 consecutive years in a sub-Arctic heathland dominated by the dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis-idaea (both evergreen) and Vaccinium myrtillus (deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week for E. hermaphroditum and V. myrtillus, and berry production reduced by 11-75% and 52-95% for E. hermaphroditum and V. myrtillus, respectively. Greater shoot mortality occurred in E. hermaphroditum (up to 52%), V. vitis-idaea (51%), and V. myrtillus (80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long-term summer warming simulations and the 'greening' seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.极端天气事件若突破物种致死阈值，会对物种存活与群落结构产生强烈负面影响。北极地区的极端短期冬季增温事件会快速融雪，使生态系统暴露于反常的暖空气环境中（例如2~14天内气温达2~10℃）；但当冬季气候恢复常态后，失去保温积雪的生态系统将面临更为严寒的温度。已有研究表明单次此类事件会降低植物繁殖率并增加枝条死亡率，但对于多次事件的影响，以及其对群落结构、生长、碳平衡与养分循环的更广泛影响，目前仍知之甚少。为解答上述科学问题，我们在以矮灌木帚石楠（Empetrum hermaphroditum）、越橘（Vaccinium vitis-idaea，常绿）与欧洲越橘（Vaccinium myrtillus，落叶）为优势物种的亚北极灌丛中，连续3年开展为期一周的极端冬季增温模拟实验，实验采用红外加热灯与土壤加温电缆进行施温。在第二次和第三次冬季增温事件后的生长季，帚石楠与欧洲越橘的春季芽萌动期最多延迟一周；帚石楠和欧洲越橘的结实量分别下降11%~75%与52%~95%。帚石楠（最高达52%）、越橘（51%）与欧洲越橘（80%）的枝条死亡率均显著上升。根系生长量下降超过25%，但土壤养分有效性未受显著影响。第三次模拟增温后的夏季，生态系统总初级生产力下降超过50%。总体而言，此次极端冬季增温事件造成的破坏程度可观，且关键的植物响应方向与长期夏季增温模拟实验中观察到的生长提升，以及部分北极地区出现的‘变绿’现象截然相反。鉴于北极冬季增温幅度高于夏季，且极端事件发生频率预计将进一步提升，这使得我们当前对北极生态系统响应气候变化的认知存在较大不确定性。