Switchgrass growth and development: Water, nitrogen, and plant density effects

Switchgrass (Panicum virgatum L.), an important component of the tallgrass prairie, is a productive warm-season forage grass. Interest in growing switchgrass for alternative uses has raised questions about resource use during production. The objective of our study was to examine how resource inputs affected interspecific plant competition in switchgrass. 'Alamo' switchgrass was established from seed in outdoor lysimeters in May 1993 and grown under 22 or 112 kg N ha(-1), and under field capacity or water-deficit conditions until August 1993. Plant spacing varied systematically from 10 to 70 cm. Plants were harvested in late summer each year and individual plant dry weight, tiller number, leaf area, and morphological development stage were measured. Soil moisture tensions below -45 kPa reduced switchgrass photosynthetic rates and xylem pressure potential. As plant spacing increased, tiller number, leaf area, plant dry weight, and morphological development stage increased. Plant dry weight and tiller number in the establishment year was not affected by N input. Established plants in 1993, however, responded to high N input at low plant densities with 50 to 100% greater leaf area and up to 3-fold greater plant dry weight compared to the low-N treatment. The increased plant dry weight at high N input resulted from increased individual tiller weight and not increased tiller number. Our data indicate that competitive responses of switchgrass plants at high plant densities were controlled by competition for aboveground resources, as plant yield and morphology at high densities were not affected by water or N inputs.