The seasonal timing of life history transitions is often critical to fitness, and many organisms rely upon environmental cues to match life cycle events with favorable conditions. In plants, the timing of seed germination is mediated by seasonal cues such as rainfall and temperature. Variation in cue responses among species can reflect evolutionary processes and adaptation to local climate and can affect vulnerability to changing conditions. Indeed, climate change is altering the timing of precipitation, and germination responses to such change can have consequences for individual fitness, population dynamics, and species distributions. Here, we assessed responses to the seasonal timing of germination‐triggering rains for eleven species spanning the Streptanthus/Caulanthus clade (Brassicaceae). To do so, we experimentally manipulated the onset date of rainfall events, measured effects on germination fraction, and evaluated whether responses were constrained by evolutionary relationships across the phylogeny. We then explored the possible consequences of these responses to contemporary shifts in precipitation timing. Germination fractions decreased with later onset of rains and cooler temperatures for all but three Caulanthus species. Species' germination responses to the timing of rainfall and seasonal temperatures were phylogenetically constrained, with Caulanthus species appearing less responsive. Further, four species are likely already experiencing significant decreases in germination fractions with observed climate change, which has shifted the timing of rainfall towards the cooler, winter months in California. Overall, our findings emphasize the sensitivity of germination to seasonal conditions, underscore the importance of interacting environmental cues, and highlight vulnerability to shifting precipitation patterns with climate change, particularly in more northern, mesic species.

中文翻译：

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对变化降雨时间的发芽反应揭示了野花分支的潜在气候脆弱性


生活史转变的季节性时间通常对适应性至关重要，许多生物体依靠环境线索将生命周期事件与有利条件相匹配。在植物中，种子发芽的时间是由降雨量和温度等季节性线索介导的。物种之间线索反应的差异可以反映进化过程和对当地气候的适应，并可能影响对不断变化的条件的脆弱性。事实上，气候变化正在改变降水的时间，而发芽对这种变化的反应会对个体适应性、种群动态和物种分布产生影响。在这里，我们评估了跨越 Streptanthus/Caulanthus 分支（十字花科）的 11 个物种对发芽触发降雨的季节性时间的反应。为此，我们实验操纵了降雨事件的开始日期，测量了对发芽分数的影响，并评估了响应是否受到整个系统发育的进化关系的限制。然后，我们探讨了这些响应对当代降水时间变化的可能影响。除 3 种 Caulanthus 物种外，所有 Caulanthus 物种的发芽分数都随着降雨的延迟和较低的温度而降低。物种对降雨时间和季节性温度的发芽反应受到系统发育限制，Caulanthus 物种的反应似乎较慢。此外，随着观察到的气候变化，四个物种的发芽率可能已经显着下降，这已将加利福尼亚的降雨时间转向较凉爽的冬季。 总体而言，我们的研究结果强调了发芽对季节性条件的敏感性，强调了相互作用的环境线索的重要性，并强调了对气候变化导致降水模式变化的脆弱性，尤其是在更北部的中性物种中。