Reproductive data Fagus sylvatica: Widespread masting breakdown in beech

Climate change effects on tree reproduction are poorly understood even though the resilience of populations relies on sufficient regeneration to balance increasing rates of mortality. Forest-forming tree species often mast, i.e. reproduce through synchronised year-to-year variation in seed production, which improves pollination and reduces seed predation. Recent observations in European beech show, however, that current climate change can dampen interannual variation and synchrony of seed production, and that this masting breakdown drastically reduces the viability of seed crops. Importantly, it is unclear under which conditions masting breakdown occurs, and how widespread breakdown is in this pan-European species. Here, we analysed 50 long-term datasets of population-level seed production, sampled across the distribution of European beech, and identified increasing summer temperatures as the general driver of masting breakdown. Specifically, increases in site-specific mean maximum temperatures during June and July were observed across most of the species range, while the interannual variability of population-level seed production (CVp) decreased. The declines in CVp were greatest where temperatures increased most rapidly. Additionally, the occurrence of crop failures and low-seed years has decreased during the last four decades, signalling altered starvation effects of masting on seed predators. Notably, CVp did not vary among sites according to site mean summer temperature. Instead, masting breakdown occurs in response to warming local temperatures (i.e. increasing relative temperatures), such that the risk is not restricted to populations growing in warm average conditions. As lowered CVp can reduce viable seed production despite the overall increase in seed count, our results warn that a covert mechanism is underway that may hinder the regeneration potential of European beech under climate change, with great potential to alter forest functioning and community dynamics.

尽管森林种群的恢复力依赖于充足的自然更新以抵消不断攀升的死亡率，但人们对气候变化对树木繁殖的影响仍知之甚少。形成森林的乔木树种通常会出现结实周期性（即通过种子产量的逐年同步波动完成繁殖，该策略可提升授粉效率并降低种子被捕食的风险）。然而，针对欧洲山毛榉的近期观测结果显示，当前的气候变化会削弱种子产量的年际波动与同步性，而这种结实周期性的崩溃会大幅降低种子的活力。至关重要的是，目前尚不清楚结实周期性崩溃发生的具体条件，以及这一现象在该泛欧洲分布树种中的普及程度。本研究分析了覆盖欧洲山毛榉整个分布范围的50份种群水平种子产量长期数据集，并明确夏季气温升高是导致结实周期性崩溃的普遍驱动因素。具体而言，在该树种分布的绝大多数区域，样地6月和7月的平均最高气温均出现上升，与此同时种群水平种子产量的年际变异系数（CVp）有所下降。年际变异系数的下降幅度在气温上升最快的样地最为显著。此外，在过去四十年间，种子绝收与低产年份的出现频率有所降低，这表明结实周期性对种子捕食者的饥饿效应已发生改变。值得注意的是，年际变异系数并未随样地夏季平均气温的不同而出现差异。相反，结实周期性崩溃是对当地气温升高（即相对气温上升）的响应，因此该风险并不局限于生长在平均气温较高环境中的种群。尽管种子总产量整体有所提升，但年际变异系数的降低会减少可育种子的产量。本研究结果警示，一种潜在的隐蔽机制正在发挥作用，可能会削弱气候变化背景下欧洲山毛榉的自然更新潜力，进而极大地改变森林生态系统功能与群落动态。