Geographic patterns of seed dormancy strategies along latitudinal and climatic gradients, Japanese East Asian islands

Aim: Seed traits and climatic covariates are hypothesised to be potential drivers determining the large-scale patterns of plant diversity; seed dormancy strategies function to escape unfavourable seasons. The associations of dormancy with seed mass and dispersal mode might facilitate for better understanding of plant biogeography in relation to trait-based community assembly. Here, we explore the geographical patterns of dormancy/non-dormancy strategies to infer seed trait-dependent filtering/sorting processes associated with climate factors. Location: Japan. Taxon: Woody plants. Methods: We compiled data on seed mass, dispersal and dormancy for 696, 1196 and 662 species respectively. We quantified the relationships between dormancy, seed mass and dispersal mode, and their geographical patterns related to minimum temperature of the coldest month and the length of the unfavourable period for plant growth. Results: Seed masses of plants with anemochorous, zoochorous and hydrochorous dispersal decreased towards higher latitudes, but those of barochorous seeds increased. The proportions of physiologically dormant species were comparatively high at higher latitudes with increasing climate harshness, whereas non-dormant, large-seeded species occurred at both climatically milder, lower latitudes, and harsher, higher latitudes. These geographic patterns demonstrate that plant regeneration responses are the outcome of multi-dimensional interactions between seed dormancy, seed mass and dispersal mode. Main conclusions: Seed trait assembly patterns were shaped by correlative relationships between seed mass, dispersal modes and dormancy types in response to the minimum temperature and the unfavourable period. Physiologically dormant species with smaller seeds are predominant in higher latitudes with colder unfavourable seasons, suggesting a risk spreading strategy to delay germination timing until the optimal seasons for growth. The high proportion of non-dormant species with larger seeds at higher latitudes indicates a stress tolerance strategy for seedling survival after germination. Seed dormancy is a critical factor adapted to abiotic/biotic niche filtering, and consequently contributes to shaping biogeographical patterns along climatic gradients.