Shifting from grassland to shrubland: New insights from recent experimental studies in the Chihuahuan Desert

Abstract:

The desert grasslands of the southwestern U.S. have undergone extensive woody shrub encroachment over the last 150 years. Although this process has been initiated and reinforced by a number of mechanisms, their relative importance has rarely been studied. The U.S. portion of the Chihuahuan Desert provides different stages of shrub encroachment from the south (middle-late) to the north (early), along with an intensity gradient for many biophysical processes including wind and fire. The objective of this study is to synthesize our recent studies in the south (Jornada LTER) and in the north (Sevilleta LTER), and to investigate the relative importance of abiotic factors, in particular wind and fire, in relating to the ecosystem dynamics in the Chihuahuan Desert. Our results demonstrated that aeolian processes play a vital role in the formation and reinforcement of shrub islands and associated soil resources heterogeneity in Jornada, whereas in Sevilleta, wind is deeply involved in the ecosystem dynamics only when the surface is disturbed, such as by fire. In the south (Jornada), the removal of grasses has resulted in a quick expansion of shrub islands owing to the increase in the distribution of sediments from grass interspaces. While in the north (Sevilleta), shrub islands have diminished and numerous small grass islands were formed after the removal of shrubs by fire. Collectively, we found that the soil resource islands are dynamic (e.g., can be destroyed and reformed) in Sevilleta due to the presence of fires while at Jornada they are static but continuously reinforced as fires are absent and the intensity of wind is strong.