Abstract
Key messageLowered 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 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.
Highlights
Human-driven climate change, habitat fragmentation and degradation resulting from the human demography and economy, overexploitation of organisms, pollution, and Communicated by DesRochers.1 3 Vol.:(0123456789) TreesP-poor habitats the number of species is limited; less competitive plants have a chance to develop (Wassen et al 2005; Fujita et al 2014)
We investigated the reproductive performance of B. humilis in peatland stands to indicate reasons for low reproductive efficiency at the end of the species range
The shrub birch seed quality analysis showed that the direct cause of the reduced seed germinability reported in previous studies (Chrzanowska et al 2016; Bona et al 2019) was a small share of seeds containing fully developed embryos
Summary
Human-driven climate change, habitat fragmentation and degradation resulting from the human demography and economy, overexploitation of organisms, pollution, and Communicated by DesRochers.1 3 Vol.:(0123456789) TreesP-poor habitats the number of species is limited; less competitive plants have a chance to develop (Wassen et al 2005; Fujita et al 2014). It is likely that lack of conspecific, genetically different mates in small, isolated populations will increase the risk of hybridisation with a common congener (Rhymer and Simberloff 1996). All these phenomena can drive small, isolated populations to extinction (Rasmussen and Kollmann 2004). Tree birches always coexist with endangered shrub birch, they are selectively superior to this species in human- or climate-impacted habitats. They represent a wide ecological tolerance and produce pollen grains in large quantities, and their seeds exhibit high germination capacity (Atkinson 1992; Franiel 2012)
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