Environmental factors influencing semidesert grassland perennial grass demography

Long-term chart quadrat records from the Jornada Experimental Range in south-central New Mexico were photographed, the negatives analyzed with a flying-spot scanner, and the information converted for computer analyses. Some 35 quadrats with data from about 30 to 50 years duration were used giving records on the life-span of 12,437 perennial grass plants of seven species. Maximum life spans varied from 28 years for Bouteloua eripoda to only 7 years for Hilaria mutica. Mean life span for plants living at least 1 year varied from 3.8 to 3.0 years for these species. Average age-specific survival rates for seven perennial grasses increased from 0.39 at age 1 up to a maximum of 0.68 at age 7 and down to 0.48 at age 12. The level of grazing over the past years had little effect on plant survival, but cattle movement and trampling caused the death of some newly established grasses. A water-budget model produced results that could be readily interpreted in understanding plant survival. The relationship between the number of days of effective moisture in a given time period and the age-specific survival rates for each age class was pronounced. The period between the second and fourth age classes was most critical as regards availability of water. For newly established plants with shallow root systems, the precipitation of the immediate growing season was more important than precipitation for longer antecedent intervals.