We investigated whether the European near-endemic moss Drepanocladus lycopodioides has declined in its core distribution area in the southern Baltic Sea region and explored potential explanations for this. First, we re-visited sites with documented records from 1854 to 1957 in the Stockholm archipelago and nearby mainland in southern Sweden. Second, we compared the numbers and distributions of Swedish occurrences, their habitats, and substrates up to 1950 (289 herbarium records), and from 1951 onwards (347) using different map layers. Third, we explored whether inter-annual variation in reproductive performance was related to moisture conditions at five sites on an island in the Stockholm archipelago during nine years. We estimated moisture conditions based on precipitation data and dry days during the time when fertilization occurs in the year prior to sporophyte maturation. Drepanocladus lycopodioides has strongly decreased in intensively managed agricultural landscapes but remains frequent in natural or semi-natural habitats on limestone. Sporophyte development varied between years and depended on the weather condition during the fertilization period. Climate forecasts for the study area predict longer dry spells during late spring to late summer, which coincides with the timing of sexual branch initiation and fertilization in the species. We expect that sporophyte production will likely decrease in the future, which will affect the species’ abilities to re-colonize spots where it vanishes during years with poor growth conditions.