The influence of temperature and water availability on seed germination can vary across the geographic range of species with a large distribution. Betula pendula is a widely spread European tree that has differentiated into two narrowly distributed taxa, endemic to Mediterranean mountains: Betula aetnensis in Sicily and B. fontqueri in Spain and Morocco. We tested the hypothesis that the regeneration niche, expressed by temperature and water potential thresholds, varies across these species and is influenced by the local climate. Seeds were collected from six populations of B. pendula, one of B. fontqueri and two of B. aetnensis. Germination tests were conducted between 5 and 30°C. The thermal thresholds were calculated before and after cold stratification. The osmotic potential tested ranged from 0 to −1.5 MPa. Time to reach 30 and 50% of germination was calculated by fitting non-linear models. Germination was promoted by high temperatures, but the response to stratification was heterogeneous. Tb and Ψb differed between and within species. Tb ranged between 2.22 and 8.94°C for unstratified seeds. Mediterranean species had higher drought tolerance, while B. pendula showed contrasting responses to low water potential. Ψb reached a minimum value of −1.15 MPa in B. fontqueri. High temperatures influenced the Tb of unstratified seeds negatively, while, after stratification, the Tb increased with precipitation in the driest month. The heterogeneity observed could reflect higher genetic variability in marginal populations of silver birch. Knowledge of their germination ecology may be useful to mitigate future impacts of climate change on core populations of B. pendula.

中文翻译：

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白桦与两种地中海特有桦木白桦和白桦萌发温度和水势阈值的比较研究

温度和水的可用性对种子萌发的影响可能因分布广泛的物种的地理范围而异。白桦是一种广泛分布的欧洲树木，已分化为两个分布狭窄的分类群，地中海山区特有：白桦在西西里和B.fontqueri在西班牙和摩洛哥。我们检验了由温度和水势阈值表示的再生生态位在这些物种之间变化并受当地气候影响的假设。从六个种群中收集种子B. 钟摆，其中之一B.fontqueri和两个B. aetnensis. 发芽测试在 5 到 30°C 之间进行。在冷分层之前和之后计算热阈值。测试的渗透势范围为 0 至 -1.5 MPa。通过拟合非线性模型计算达到 30% 和 50% 发芽的时间。高温促进了发芽，但对分层的反应是不均匀的。吨b和Ψb种间和种内不同。吨b未分层种子的温度范围在 2.22 和 8.94°C 之间。地中海物种具有更高的耐旱性，而B. 钟摆显示出对低水势的对比反应。Ψb达到-1.15 MPa的最小值B.fontqueri. 高温影响了吨b未分层的种子为负，而在分层后，吨b在最干燥的月份随着降水的增加而增加。观察到的异质性可能反映了白桦边缘种群中较高的遗传变异性。了解它们的萌发生态学可能有助于减轻气候变化对未来核心种群的影响B. 钟摆.