Desert plant communities present challenges for the measurement of biodiversity and ecosystem function. Sparseness and heterogeneity in aboveground structure, and inter-annual variability driven by temperature and precipitation, make it difficult to compare across sites or over time. The Jornada Basin Long-Term Ecological Research Program established permanent plots in five Chihuahuan Desert ecosystem types in 1989 to examine spatial and temporal variation in aboveground net primary productivity. Temporal variation in plant species composition and relative abundance is also of interest, because annual plants can be diverse and productive in some but not all years, and historical information since the 1850s confirms dynamic changes in vegetation in certain locations. After 25 years of sampling, the Jornada dataset allows us to test hypotheses about plant community structure in these systems. Because the sampling period included dry years, wet years, and multi-year dry and wet periods, we can compare plant composition and relative abundance across environmental conditions. We ask whether there are any directional trends in species richness or composition over time, how species richness varies between dry and wet periods, whether subordinate and dominant species vary in responses to climate, and whether our spatial and temporal sampling has been sufficient to characterize the plant communities of the Jornada Basin.
Floristic richness varies considerably and consistently across ecosystem types; grasslands representing the historic dominant grass, Bouteloua eriopoda (black grama), have the highest richness, averaging nearly 60 plant species sampled per site per year, while shrub-dominated sites have less diversity (25-35 species per site per year). Species richness in all ecosystem types is low in extremely dry years (e.g., 1994). A wet series of years (2005 - 2010) resulted in high species richness in shrub-dominated systems (Larrea, Flourensia, and Prosopis) but not in Bouteloua grasslands or grassy playas. After this wet period, species richness returned to typical numbers in most ecosystems, but actually decreased below long-term averages in Bouteloua –dominated grasslands. No directional trends in species richness have been detected over the 25 years of sampling; there is no evidence that exclusion of livestock grazing at the start of sampling has led to the competitive exclusion of some species or the establishment and/or spread of others. Low species richness in dry periods in all ecosystem types, and high species richness in wet periods only in shrublands may reflect future patterns in diversity across the Jornada landscape under directional decreases or increases in precipitation, respectively.