Department of Geography and Planning, School of Environmental Sciences, University of Liverpool, Liverpool, UK.
Faculty of Biology, Forest Biology Center, Adam Mickiewicz University in Poznań, Poznań, Poland.
Glob Chang Biol. 2024 May;30(5):e17307. doi: 10.1111/gcb.17307.
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.
气候变化对树木繁殖的影响尚未得到充分了解，尽管种群的恢复力依赖于足够的再生来平衡死亡率的增加。形成森林的树种通常会结实，即通过种子产量的逐年同步变化进行繁殖，从而提高授粉效率并减少种子被捕食的几率。然而，最近对欧洲山毛榉的观察表明，当前的气候变化可以抑制种子产量的年际变化和同步性，这种结实的崩溃会极大地降低种子作物的活力。重要的是，目前还不清楚结实崩溃发生的条件以及这种在整个欧洲范围内的物种中崩溃的广泛程度。在这里，我们分析了欧洲山毛榉分布范围内的 50 个长期种群水平种子产量的数据集，并确定了夏季温度升高是结实崩溃的一般驱动因素。具体来说，在物种分布的大部分地区，观察到 6 月和 7 月特定地点的平均最高温度增加，而种群水平种子产量的年际可变性（CVp）下降。CVp 的下降幅度最大的地方，温度上升最快。此外，在过去的四十年中，作物歉收和低种子年的发生频率降低了，表明结实对种子捕食者的饥饿效应发生了改变。值得注意的是，根据夏季平均温度，CVp 在不同地点之间没有差异。相反，结实崩溃是对变暖的局部温度（即相对温度增加）的响应，因此这种风险不仅限于在温暖平均条件下生长的种群。由于 CVp 的降低可能会降低有活力的种子产量，尽管种子数量总体上有所增加，因此我们的研究结果警告说，一种潜在的机制正在进行中，这可能会阻碍欧洲山毛榉在气候变化下的再生潜力，极大地改变森林功能和群落动态。
