Key message

Lowered pollen and seed quality of B. humilis compared to its widespread tree relatives may threaten its populations at the southwestern edge of the species range.

Abstract

Inability to produce vital propagules is the main threat to the persistence of rare plants at the edges of species distribution. In the present investigation, factors responsible for the low germinability of shrub birch (Betula humilis) seeds in the peripheral populations were analysed. As shrub birch populations are found in phosphorus (P)-limited peatlands with high groundwater levels, the impact of groundwater PO₄³⁻ ion contents and groundwater table for B. humilis flowering was also studied. Qualities of the B. humilis seeds and pollen were compared to those of tree congeners silver birch (B. pendula) and downy birch (B. pubescens), which usually represent a high reproductive performance. We revealed that the poor germination ability of B. humilis seeds was caused by the high proportions of empty seeds (44.8%) and seeds with partially developed embryos (47.3%). The insufficient numbers of viable seeds may be a consequence of low-quality shrub birch pollen, as only 1.6% of pollen grains sprouted. The germinability of shrub birch pollen was significantly lower than that of Betula tree pollen; thus, there is a risk of pollination of B. humilis female flowers by pollen grains of tree birches. Positive relationships between the P concentrations in seeds and seed germinability were revealed in tree birches, but not in B. humilis. The availability of PO₄³⁻ ions in the groundwater did not affect B. humilis flowering, but male inflorescences were more numerous in stands with higher groundwater levels. Consequently, the maintenance of peatlands is crucial for the efficient sexual reproduction of shrub birches.

中文翻译：

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桦木濒危种群的开花和种子和花粉质量

关键信息

与广泛分布的树亲缘相比，B. humilis降低的花粉和种子质量可能威胁到其在物种范围西南边缘的种群。

抽象的

无法产生重要的繁殖体是稀有植物在物种分布边缘的持久性的主要威胁。在目前的调查中，分析了导致外围种群中灌木桦 ( Betula humilis ) 种子发芽率低的因素。由于在地下水位高的磷 (P) 限制泥炭地中发现了灌木桦树种群，因此还研究了地下水 PO ₄ ³⁻离子含量和地下水位对B. humilis开花的影响。将B. humilis种子和花粉的品质与树木同系物银桦 ( B. pendula ) 和绒毛桦 ( B. pubescens ) 的品质进行比较。)，这通常代表高繁殖性能。我们发现B. humilis种子发芽能力差是由于空种子 (44.8%) 和胚胎部分发育的种子 (47.3%) 比例高造成的。有活力的种子数量不足可能是低质量的灌木桦树花粉的结果，因为只有 1.6% 的花粉粒发芽了。灌木桦树花粉的发芽率显着低于桦树花粉；因此，存在通过桦树的花粉粒为B. humilis雌花授粉的风险。在桦树中发现了种子中 P 浓度与种子发芽性之间的正相关关系，但在B. humilis 中则没有。PO的可用性地下水中的₄ ³⁻离子不影响B. humilis开花，但在地下水位较高的林分中雄花序较多。因此，泥炭地的维护对于灌木桦树的有效有性繁殖至关重要。