Ecology of Germination and Flowering in the Weedy Winter Annual Grass Bromus japonicus

Abstract

From the time of seed maturity in late June and early July until December when temperatures drop to near freezing, habitat temperatures are within the range of those required for germination of seeds of the winter annual Bromus juponicus. However, a large proportion of the seeds in a given seed crop fail to germinate in the autumn of the year in which they are produced because they are not dispersed until winter. A high percentage of of the winterdispersed seeds is induced into dormancy and must undergo a period of afterripening the following summer before germination can occur the next autumn. Thus, many of the plants that become established at a population site in autumn are from the previous year’s seed crop. Plants overwinter in the field as “rosettes” and require long days for flowering. Nonvernalized plants exposed to natural short photoperiods of late autumn and winter flower under long days in spring, but plants flower much sooner if they are subjected to both low temperatures (vernalization) and short photoperiods during winter. Bromus japonicus Thunb. (Japanese chess) is a native Eurasian winter annual that has been introduced into the United States where it ranges from Vermont to the state of Washington south to North Carolina and California (Hitchcock 1935, Fernald 1950). As is true for some other introduced bromegrasses in the United States, B. japonicus is a problem in wheat fields, in grass and alfalfa seed fields, and in pastures, meadows, and overgrazed range lands (Slife et al. 1960, Finnerty and Klingman 1962, Steyermark 1963). To better understand the ecological life cycle of this weedy winter annual, we have studied (1) the role of temperature in controlling the timing of germination in nature and (2) the influence of vernalization and photoperiod on the timing of flowering. In northcentral Kentucky where we have observed its phenology, B. japonicus begins to flower in early May, and caryopses (hereafter referred to as seeds) are ripe by late June or early July. Plants senesce as the seeds mature, and there are no living plants at a population site during summer. The seeds are retained on the dead erect shoots until autumn and winter. Seed dispersal begins in early October, and continues until the following March, with a high percentage of the seeds being dispersed during November, December, and January. Seed germination occurs in early autumn, and thousands of seedlings may appear at a population site at any time from early September to mid October, depending upon the time when soil moisture conditions become suitable for germination. Only an occasional seedling is found in the field in spring. Since germination occurs before very many of the seeds are shed, population establishment each year results primarily from germination of a reserve of seeds present in and/or on the soil surface. Following germination, each plant produces a “rosette” of 10-l 5