Data from: Reproduction as a bottleneck to treeline advance across the circumarctic forest tundra ecotone

The fundamental niche of many species is shifting with climate change, especially in sub-arctic ecosystems with pronounced recent warming. Ongoing warming in sub-arctic regions should lessen environmental constraints on tree growth and reproduction, leading to increased success of trees colonising tundra. Nevertheless, variable responses of treeline ecotones have been documented in association with warming temperatures. One explanation for time lags between increasingly favourable environmental conditions and treeline ecotone movement is reproductive limitations caused by low seed availability. Our objective was to assess the reproductive constraints of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures of trees (cones and catkins) and stand attributes across circumarctic treeline ecotones. We used generalized linear mixed models to estimate the sensitivity of seed production and the availability of viable seed to regional climate, stand structure, and species-specific characteristics. Both seed production and viability of available seed were strongly driven by specific, sequential seasonal climatic conditions, but in different ways. Seed production was greatest when growing seasons with more growing degree days coincided with years with high precipitation. Two consecutive years with more growing degree days and low precipitation resulted in low seed production. Seasonal climate effects on the viability of available seed depended on the physical characteristics of the reproductive structures. Large-coned and -seeded species take more time to develop mature embryos and were therefore more sensitive to increases in growing degree days in the year of flowering and embryo development. Our findings suggest that both moisture stress and abbreviated growing seasons can have a notable negative influence on the production and viability of available seed at treeline. Our synthesis revealed that constraints on predispersal reproduction within the treeline ecotone might create a considerable time lag for range expansion of tree populations into tundra ecosystems.许多物种的基础生态位（fundamental niche）正随着气候变化发生偏移，在近期增温显著的亚北极生态系统中这一现象尤为突出。亚北极区域的持续升温本应缓解树木生长与繁殖所受的环境限制，提升树木定植苔原的成功率。然而，已有研究记录到林线交错带（treeline ecotones）对气候变暖呈现出差异化响应。针对环境条件日趋适宜与林线交错带迁移之间存在的时间滞后现象，现有解释之一为低种子可获得性引发的繁殖限制。本研究旨在评估环北极北部林线交错带优势树种所面临的繁殖限制。我们对环北极林线交错带内的树木繁殖结构（球果与柔荑花序）及林分特征进行了采样调查。我们借助广义线性混合模型（generalized linear mixed models），估算了种子产量与有活力种子可获得性对区域气候、林分结构及物种特异性特征的敏感性。种子产量与现有有活力种子的活力均受特定连续季节气候条件的强烈调控，但二者的响应模式存在显著差异。当生长度日数更高的生长季与降水充沛的年份重合时，种子产量达到峰值；若连续两年生长度日数偏高但降水偏少，则会导致种子产量显著降低。季节气候对现有有活力种子活力的影响，取决于繁殖结构的物理特性。大球果、大种子物种需要更长时间完成成熟胚的发育，因此在开花与胚发育年份，其对生长度日数升高的敏感性更强。本研究结果表明，水分胁迫与缩短的生长季均会对林线区域的种子产量与有活力种子活力产生显著负面影响。综合分析显示，林线交错带内扩散前繁殖过程受到的限制，可能会显著延缓树木种群向苔原生态系统扩张的进程。