SOME EFFECTS OF PRECIPITATION PATTERNS ON MESA DROPSEED PHENOLOGY

Phenology of mesa dropseed [Sporobolus flexuosus (Thurb.) Rydb.] was studied from 1979 to 1987 on the Jornada Experimental Range in southern New Mexico. Growing season (March through November) precipitation ranged from 99 to 308 mm during the 8-year period. Foliage height and number of leaves were recorded weekly for individually marked culms on 20 plants. New culms usually appeared during the first week in March and green leaf tissue often persisted until the end of November. Correlation analyses of accumulated weekly height increments and accumulated weekly precipitation showed that growth was highly dependent upon rainfall (r = 0.81 to 0.97). Leaf formation was also correlated with rainfall (r = 0.79 to 0.98). Even in relatively wet years there were 1 or 2 periods of no growth. In drier years, no growth periods totaled as much as 87 days. Periods of rapid growth occurred only after rainfall events > 13 mm. The first exsertion of seed heads occurred as early as the last week of July and as late as the second week of October. The temporal plasticity of mesa dropseed phenology indicates that it is well adapted to the arid environment.