Stiff Sunflower Population Dynamics on Summer-grazed Sandhills Rangeland

Abstract

Thousands of forb species are distributed among the diverse rangelands of North America. However, little is known about livestock grazing effects on the demographics and potential demise of palatable forbs in grassland ecosystems. A study was designed to quantify the cumulative effects of summer grazing on the demographics of stiff sunflower (Helianthus rigidus [Cass.] Desf. spp. subrhomboides [Rydb.] Heiser), a highly palatable, late-seral, perennial forb. Pastures were grazed for 5–7 days in mid-June or mid-July during 1995–1997 at 16, 32, or 48 animal unit days (AUD) per hectare. All grazing treatments reduced the plant height of stiff sunflower. However, population densities were maintained throughout the study at light stocking rates (16 AUD·ha−1). In contrast, a single year of heavy stocking (48 AUD·ha−1) in June reduced spring stiff sunflower densities 55%. Densities declined about 30% after 1 year at moderate stocking rates (32 AUD·ha−1) in either month or heavy stocking in July. After 3 years of short-duration grazing in June, moderate and heavy stocking rates eliminated some colonies and reduced mean pasture densities by about 90% compared with 40% and 70% reductions in moderately and heavily stocked July-grazed pastures, respectively. Reductions in spring densities corresponded to increases in premature senescence the previous year when more than 30% of the plants turned brown before mid-August. Critical levels of premature senescence were likely to occur when more than 60% of stiff sunflower plants within colonies were grazed. Light stocking rates are rare on privately owned Nebraska Sandhills rangeland (4.7 million ha); therefore, vigorous populations of stiff sunflower are most likely to occur in pastures used predominantly during the dormant season.