Differentiation of seed germination traits in relation to the natural habitats of threeUlmusspecies in Japan

Abstract

Germination traits of three Japanese Ulmus species, including two summer-fruiting species, U. davidiana var. japonica (UD) and U. laciniata (UL), and an autumn-fruiting species, U. parvifolia (UP), were investigated. Germination tests that controlled light (16 h dark-8 h light/24 h dark) and temperature (alternating 20-30°C/ constant 23°C) conditions were conducted just after seed dissemination. Under the preferred (light and alternating temperature) conditions, UD germinated quickly and percentage germination was high (99%), whereas UL and UP germinated moderately to slowly (65.2 and 23.8%, respectively) and the remaining seeds were primarily dormant. When the seeds were exposed to unsuitable conditions for germination, secondary dormancy was induced in all three Ulmus species. Because primary and secondary dormancy of these species was not strict, the existing dormancy could be released by moist conditions and low temperature during the winter. Thus, all three Japanese Ulmus can germinate either just after seed dissemination or in the following spring. Nevertheless, the predominant germination of summer-fruiting UD just after seed dissemination seems to be advantageous for finding safe sites that might become available after a large-scale fluvial disturbance. Similarly, the delayed germination of autumnfruiting UP until the following spring seems to be advantageous for preventing germination at the end of the growing season. Summer-fruiting UL had a more negative response to favorable light conditions just after seed dissemination, which seems to be advantageous for establishment in canopy gaps. These germination traits seem to be adaptive responses to the dominant disturbance regimes of their natural habitats.