Potential Consequences of Repeated Severe Drought for Shortgrass Steppe Species

Future climate projections indicate temperatures in the shortgrass steppe region are likely to increase up to 3°C by midcentury, with a corresponding reduction in soil moisture even without precipitation deficit. Although periodic drought is a natural disturbance in shortgrass rangeland, negative effects on characteristic shortgrass species are possible as the frequency and severity of drought events increase in comparison with recent historic norms. As part of a study intended to detect vegetation changes at a shortgrass steppe site on Colorado's eastern plains, frequency and canopy cover percentage were measured in 37 permanently marked plots over a period of 17 yr. The study period included the two lowest total annual precipitation yr (2002 and 2012) in the period of record for regional weather stations, exceeding even the driest years of the extended 1930s drought. Growing season mean temperatures during those drought years were 1°C and 1.6°C above the 1971–2000 average, respectively. Three of the six perennial grass species monitored showed a decline over the period of the study. Blue grama (Bouteloua gracilis), a dominant and important forage species in the shortgrass steppe, declined in both cover and frequency, while alkali sacaton (Sporobolus airoides), not an important forage species, slightly increased. In addition to changes in graminoid dominance, we observed an increase in cholla (Cylindropuntia imbricata) and a decrease in sandsage (Artemisia filifolia) densities between 1999 and 2015. Even if total productivity of the shortgrass steppe ismaintained under warming and drying conditions, changes in species composition have implications for rangeland quality with regard to its use for livestock grazing and use bywildlife such as small mammals and songbirds.