Sporophyte frequency in British dioicous liverworts

The extent and causes of variation in sporophyte production among British dioicous liverworts are assessed. Reproductive attributes of dioicous species are taken from Paton’s <i>The Liverwort Flora of the British Isles</i> (1999). The extent to which sporophyte production is potentially restricted by low levels of sex expression, unbalanced phenotypic sex ratios, and probable spatial segregation of the sexes, is quantified. Patterns of variation in sexual reproduction are also characterised for species associated with different biogeographical zones in Europe. Of the 188 British liverwort species that are dioicous (62%), sporophytes are unknown in 65, rare in 58, occasional in 50 and frequent in 15. Some species are not or rarely sexually reproductive, but major causes of restricted sexual reproduction are sexual imbalance and probable isolation of the sexes. Among species with skewed sex ratios, female bias is substantially more common than male bias. Sporophyte frequency is relatively high among species widely distributed in temperate and boreal Europe, compared with taxa associated with other climatic zones, including the oceanic fringe. Restricted sporophyte production implies that asexual reproduction and clonal longevity are critical for the persistence of many liverworts. Further insights will require field surveys of patterns of sex expression and sporophyte distribution, together with information on reproductive plasticity, genotypic sex ratios, and levels of genetic diversity and structure.