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Bryophytes constitute a diverse plant group with important roles in ecosystem functioning, in particular in arctic and subarctic environments. As they are physiologically strongly dependent on climatic conditions bryophytes could serve as indicators of ongoing climatic change. Their spores are generally dispersed by wind, and because of contrasting phenologies among species, the composition of the spore cloud changes throughout the year. Unlike vascular plant pollen, airborne bryophyte spores have few specific morphological characteristics, and therefore spore dispersal phenology has, until now, relied on highly laborious in situ observations.

Here, we report on multi-decadal shifts in the phenology of spore dispersal in 16 bryophyte taxa using a unique 35-year time series of environmental DNA (eDNA) data collected in Kiruna, northern Sweden. We used shotgun sequencing data from air filters and matched reads to all major organism groups, of which a high proportion were bryophyte reads.

We found consistent shifts in bryophyte phenology, such that most bryophyte taxa advanced their (i) start of season with 4 weeks on average, and (ii) mid-season with 6 weeks, ranging between 4 and 7 weeks. Changes at the season end were less consistent across the 16 bryophyte taxa, although seven of them showed phenological delays over time. Rising temperatures during the third and fourth quarters of the year preceding spore release were correlated with phenological shifts, suggesting that bryophytes may enter hibernation at later stages of sporophyte development, with warmer conditions promoting more advanced sporophyte maturation by the onset of spring. As a consequence of the phenological shifts, seasons during which spores were observed became several weeks longer over the studied time period for most taxa.

Synthesis: We conclude that the phenological shifts in our study suggest strong perturbations in bryophyte phenology, consistent with ongoing climate change. Our results demonstrate that studying airborne particles using eDNA methodology is a valuable complement to other monitoring methods, not the least in bryophytes and other less well-surveyed taxa.