THE IMPORTANCE OF SOIL-WATER IN THE RECRUITMENT OF BOUTELOUA-GRACILIS IN THE SHORTGRASS STEPPE

In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving < 380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long-term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30-50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures evaluated. The differences in silt content and available water holding capacity accounted for the difference among soil textures in the probability of occurrence of recruitment events. Therefore, soil texture variability may explain the spatial pattern of recruitment and of population recovery after disturbance that occur at the soil type and microsite scales. Annual precipitation explained a large fraction of the temporal variability in recruitment. On average, recruitment occurred in years when precipitation was above the mean. The occurrence of recruitment events in some dry years (precipitation < mean), and their absence during some wet years (precipitation > mean), emphasizes the importance of the intraseasonal distribution of precipitation. The sensitivity of recruitment to soil water availability suggests that climate change, particularly changes that increase or decrease the amount or the effectiveness of soil water, could have important effects on the future of populations of B. gracilis.