Can novel pest outbreaks drive ecosystem transitions in northern-boreal birch forest?

1. The boreal biome exhibits distinct alternative ecosystem states with high and low levels of tree-cover. Insect outbreaks facilitated by climate warming could potentially drive transitions from high to low tree-cover states. We investigated whether two key premises for such outbreak-induced transitions – critical thresholds (tipping points) and positive feedbacks that could maintain alternative states – are present in the northern-boreal mountain birch forest of Fennoscandia. Here, climate warming has promoted range expansions of defoliating geometrid moths, resulting in novel, severe multispecies outbreaks, most recently during 2002-2010. 2. We conducted regional-scale field surveys of forest damage and recovery in 280 mountain birch stands in a northeast Norway immediately after the outbreak (2010) and six years later (2016). Satellite-derived time series of the normalized difference vegetation index (NDVI) provided an index of stand defoliation during the outbreak period. 3. The proportion of dead stems per stand displayed a bimodal distribution, with stands generally being either lightly or severely damaged. This was due to a critical threshold in the relationship between defoliation and stem mortality, with mortality rates increasing abruptly in stands experiencing a mean drop in NDVI of more than 4 % during the outbreak. The two key forest regenerative pathways – basal sprouting and sapling production – both displayed positive feedbacks with surviving stems and trees, so that regeneration success declined with increasing damage to the mature tree layer. These feedbacks imply that stands which have been forced across critical defoliation thresholds and suffered collapses of living tree cover may struggle to recover, especially if the loss of positive regenerative feedbacks is compounded by ungulate browsing on birch recruits. 4. Synthesis. The north Fennoscandian mountain birch forest displays critical thresholds and positive feedbacks that conform to theoretical expectations for a system that could be vulnerable to abrupt and persistent changes of state in the face of novel, climatically facilitated insect outbreaks. These findings deepen the understanding of the persistent losses of tree-cover that have occasionally been observed after outbreaks in this system in the past, and adds to the list of mechanisms that could help explain the bistability of tree-cover across the boreal biome.

1. 寒带生物群区（boreal biome）呈现出树木覆盖度高低迥异的交替生态系统状态（alternative ecosystem states）。气候变暖助推的昆虫暴发（insect outbreaks）或可推动林分从高树木覆盖度状态向低覆盖度状态转变。我们针对芬诺斯坎迪亚（Fennoscandia）北部寒带山地桦木林，探究了这类昆虫暴发诱导的状态转变所需的两个核心前提是否成立：即临界阈值（critical thresholds/tipping points）与可维持交替生态系统状态的正反馈（positive feedbacks）。当前该区域因气候变暖，食叶尺蛾（defoliating geometrid moths）的分布范围持续扩张，引发了新型、严重的多物种昆虫暴发，最近一次暴发发生于2002-2010年。
2. 我们在此次暴发结束后（2010年）及暴发后6年（2016年），对挪威东北部280个山地桦木林分开展了区域尺度野外调查（regional-scale field surveys）以评估森林受损与恢复情况。研究借助卫星获取的归一化差分植被指数（NDVI）时间序列数据，表征暴发期内各林分的落叶程度（stand defoliation）。
3. 各林分的枯死茎秆占比呈现双峰分布（bimodal distribution），林分整体多表现为轻度或重度受损两种状态。这一现象源于落叶程度与茎秆死亡率（stem mortality）之间存在临界阈值：当林分的归一化差分植被指数在暴发期内平均下降超过4%时，茎秆死亡率会骤然升高。森林的两种核心再生途径——基部分蘖（basal sprouting）与幼树生成（sapling production）——均与存活茎秆及林木呈现正反馈关系：成熟林层受损程度越高，再生成功率越低。这类正反馈表明，若林分越过临界落叶阈值并出现活立木覆盖度骤降，其恢复难度将大幅提升；若再叠加有蹄类啃食（ungulate browsing）桦木幼苗（birch recruits）所造成的再生反馈缺失，恢复困境将进一步加剧。
4. 综合分析。芬诺斯坎迪亚北部山地桦木林存在临界阈值与正反馈机制，符合理论预期：在气候变暖驱动的新型昆虫暴发冲击下，该生态系统极易发生突发且持久的状态转变。本研究结果加深了我们对该生态系统过往偶发昆虫暴发后树木覆盖度持续流失现象的理解，同时补充了可用于解释寒带生物群区树木覆盖度双稳态（bistability）的机制列表。