Long-term forb dynamics in semi-arid grasslands of the northern Chihuahuan Desert, New Mexico, USA.

Perennial grasses and shrubs have been well-studied in semi-arid grasslands, but relatively little is known about the forb community.  We analyzed a twenty-two-year dataset (1989-2010) from the Sevilleta LTER, located in northern Chihuahuan Desert, to quantify forb dynamics in a grassland environment.  While forbs were sparse, seasonal richness was high and the species pool robust.  Most taxa were ephemeral and could be considered “satellite” species while a few were persistent “core” taxa.  Trends in vegetation variables over time were not significant and the community shifted constantly.  Precipitation was significantly correlated with vegetation variables in the spring but not following the fall monsoon.  This is likely attributable to increased competition in wetter periods between forbs and more robust perennial grasses.  There was no relationship between diversity and periodicity of rainfall events; diversity was generally moderate throughout the study. 

In the drier, hotter climate predicted for the American Southwest, forbs may benefit from competitive release in the fall if monsoon precipitation becomes less pronounced.  On the other hand, warm-climate grasses and shrubs may expand northward, outcompeting endemic forbs unable to adapt to the shifting environment.  Continued long-term research is needed to more fully predict potential climate-induced forb dynamics in semi-arid grasslands.