Factors affecting variation in seed production in the heterostylous herb Primula modesta

Spatial and temporal variation in the seed production of a species and the various factors affecting seed production provide essential information for understanding biotic and abiotic controls on reproduction and for assessing population trends. We conducted a 4‐year study combining field observations and pollination experiments to evaluate the major factors causing variation in seed production of Primula modesta at two sites (fen and grassy sites) on Mt Asama in central Japan. Primula modesta switched from a vegetative to reproductive life phase at a stage that depended on its accumulation of biomass, and the number of flowers produced by each plant was also highly correlated with plant size. Plants at the grassy site, where snow melts 2–3 weeks earlier than at the fen site, can accumulate more biomass and, thus, bear more flowers and ovules than those at the fen site. Although P. modesta is distylous with both self and intramorph incompatibility, pollen availability did not limit seed production at either site. Annual seed production was relatively constant at both sites over the four seasons except 2002, but it was influenced by many environmental factors, including stochastic events, and by antagonistic interactions with herbivores and pathogens. In 2002, many flowers failed to bloom at the fen site. This was likely caused by frost damage to flower buds because of an unusually early snowmelt in that year. Unusual climatic events may be a crucial factor causing spatial and temporal variation in seed production at our study sites. Longer‐term observations are needed to evaluate the effect of unusual climatic events on reproduction.