It is an annual, usually germinating in the autumn, overwintering as a seedling, then flowering in the spring or early summer. If winter rainfall is limiting and spring moisture is adequate, the seeds will germinate in the spring, and the plants will flower that summer. It typically reaches 40-90 cm tall, though plants as small as 2.5 cm may produce seed. It is cleistogamous and self-fertile, with no evident out-crossing. The seeds are dispersed by wind, small rodents, or attachment to animal fur, within a week of maturity. They are also moved as a contaminant in hay, grain, straw, and machinery. It is an abundant seed producer, with a potential in excess of 300 seeds per plant; seed production per plant is dependent on plant density. Under optimal conditions, it may produce 450 kg of seed per hectare with about 330,000 seeds/kg.
Drooping Brome has an extensive root system. The wide-spreading lateral roots are one of the keys to the survival of this plant. A study showed that it had the capability to reduce soil moisture to the permanent wilting point to a depth of 70 cm, reducing competition from other species.
The seeds maintain high viability in dry storage, lasting over 11 years. In the field, under buried conditions, seeds will lose their viability in 2-5 years. The primary limit to germination is inadequate moisture. Seeds can withstand high soil temperatures. Germination is best in the dark or in diffuse light. They germinate most quickly when covered with soil, but do not need to be in contact with bare soil; some leaf litter cover will generally improve germination and establishment of seedlings. Seedlings emerge rapidly from the top 2.5 cm of soil, and a few plants emerge from depths of 8 cm, but not from seeds 10 cm below the surface.
In the US, it grows on rangelands, pastures, prairies, fields, waste areas, eroded sites, and roadsides. It is much reviled by ranchers and land managers. On the other hand, Drooping Brome seeds are a critical portion of the diet of the Chukar and Grey Partridge which have been introduced to the US.
Drooping broom has demonstrated a quantitative and qualitative response to recent and near-term changes in the concentration of atmospheric carbon dioxide. Laboratory experiments have shown that above-ground biomass increased 1.5-2.7 gram per plant for every 10 part per million (ppm) increase above the 270 ppm pre-industrial baseline. On the qualitative side, rising carbon dioxide decreased the digestibility and potential decomposition of drooping brome. In addition to stimulation of biomass, rising carbon dioxide may also increase the above ground retention of drooping broom biomass by decreasing removal by animals or bacteria. Ongoing increases in atmospheric carbon dioxide may contribute significantly to drooping broom productivity and fuel load with subsequent effects on wildfire frequency and intensity.
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