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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. 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 PO 4  3−  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 PO 4  3−  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.

Flowering and quality of seeds and pollen in endangered populations of Betula humilis