The University of Arizona Desert Laboratory

For almost one hundred years, the Desert Laboratory has been dedicated to studying plants, animals, and the environment of North American deserts. Since deserts are particularly sensitive to human disturbance, the need to understand desert life has become increasingly important as the population of arid regions has grown.

Research Highlight of the Month

Inferences about summer rains and winter temperatures from bioclimatic modeling and the late Quaternary record of C4 perennial grasses and C3 desert shrub distributions in the USA-Mexico borderlands

Camille A. Holmgren, California State University, Long Beach
Julio L. Betancourt, Desert Laboratory, U.S. Geological Survey

Despite steady and focused research that now spans half a century, questions linger about the changing seasonality of southwestern U.S.A. paleoclimates during the late Quaternary. There is general consensus that late Pleistocene (30,000-13,000 yr B.P.; all ages are reported in calendar yrs B.P.) winters were wetter and temperatures were generally cooler than today. The resulting increases in effective moisture permitted large pluvial lakes to grow and persist in what are now dry playas along the Arizona-New Mexico near the international border (USA-Borderlands). Increases in effective moisture also supported mixed-conifer forests and open woodlands where there are now pinyon-juniper woodlands, and pinyon-juniper woodlands where there are now deserts. Uncertainties remain, however, about the history of summer rains and winter temperatures as could be inferred primarily from the packrat midden record and modern bioclimatic distributions of select species of C4 perennial grasses and C3 desert shrubs that characterize the northern Chihuahuan Desert in the U.S.A.-Mexico borderlands.

Figure 1. Occurrence of C4 perennial grasses and C3 desert shrubs in fossil packrat middens from the Peloncillo Mountains near San Simon, AZ. This same pattern repeats itself across midden sites across southeastern Arizona and southern New Mexico.

The presence and abundance of select C4 herbs and perennial grasses and C3 desert shrubs in middens from the USA-Mexico borderlands demonstrate interestingly disparate histories (see example from Peloncillo Mountains near San Simon in Fig. 2). C4 herbs and grasses persisted from ~30,000 to the present, whereas many C3 desert shrubs did not occupy the northern extent of their ranges until the last 4000-5000 years. To evaluate the climatic significance of these patterns, we focused on several C4 grasses (Botriochloa barbinodis, Digitaria californica, Leptochola dubia, Setaria vulpiseta) and C3 shrubs (Acacia constricta, Acacia greggii, Flourensia cernua, Fouquieria splendens, and Larrea tridentata) common in midden records from the region and with easily obtainable distributional data from herbarium collections. We used distributional data to construct a multi-dimensional climatic niche for each species, and distances from the niche were calculated and mapped on a 4-km grid for each of ten climatic variables. The maps and distributional histories from the midden records are used to evaluate hypotheses about the late Quaternary history of summer rains and winter temperatures in the southwestern United States.

Figure 2. Example of bioclimatic modeling of modern species distributions used in this study. Niche space for Setaria vulpiseta (shown in black) with the distance from the niche space for 10 climatic variables. The area in gray indicates the area outside of the niche space when all variables are considered, but within the potential niche space for a particular variable.

Conclusions from the study:

C4-dominated desert grasslands apparently represent one of the most stable biomes in North America. At their northern extent, these grasslands persisted in more or less the same geographic location through MIS 1, 2 and 3. This persistence is most easily explained by the glacial-interglacial permanence of warm winters and summer rainfall in the region. It should follow then that the core of the monsoon in the U.S.A.-Mexico borderlands and northern Chihuahuan Desert must have remained relatively intact throughout the last glacial-interglacial cycle. In its present form, the North American monsoon is primarily a Neogene phenomenon associated with formation of the Gulf of California (Lonsdale, 1989), which substituted cold, upwelling Pacific waters with the warm Sea of Cortez immediately west of the Sierra Madre Occidental.

The persistence of the core of the monsoon contrasts with conventional wisdom, mostly derived from coarsely-resolved general circulation models, that southward displacement of the westerlies in glacial summers suppressed monsoonal circulation and rainfall over the southwestern U.S.A. Mesoscale climate models have been able to resolve the present-day role of the Sea of Cortez and Sierra Madre Occidental in the multistage transport of moisture in the North American monsoon (Fawcett et al. 2002). We submit that mesoscale models will eventually show they played a similar role during the Last Glacial Maximum and throughout the Quaternary. This permanence of summer rains and desert grassland, and the inconstancy of adjacent desert and woodland biomes, surely have left clear imprints on the biogeographic and evolutionary history of the region, akin to those outlined by Hafner and Riddle (2005).

Finally, the location of Pleistocene refugia for desert shrubs, presumably to the south in Mexico, remains elusive and warrants future study. Their abrupt arrival and expansion in the northern Chihuahuan Desert did not occur until ~5000-4000 cal yr B.P., concurrent with increasing winter insolation and the onset of modern ENSO variability. It remains to be sorted out whether the warming that forced this expansion happened gradually or abruptly as a series of steps that may be ongoing. Ongoing warming in all seasons and African grass invasions, such as Lehman's lovegrass (Eragrostis lehmanni) and buffelgrass (Pennisetum ciliare) are now producing unprecedented changes in both desert grasslands and shrublands of the USA-Mexico borderlands.

Fawcett, P.J., Stalker, J.R., Gutzler, D.S. 2002. Multistage moisture transport into the interior of northern Mexico during the North American summer monsoon. Geophysical Research Letters 29: 1-4.

Hafner, D. J., and B. R. Riddle. 2005. Mammalian phylogeography and evolutionary history of northern Mexico's deserts. pp. 225-245. in Biodiversity, ecosystems, and conservation in northern Mexico. (J-L. Cartron, G. Ceballos, and R.S. Felger, eds.). Oxford University Press, New York.

Holmgren, C., Norris, J., Betancourt, J. L. 2007. Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert. Journal of Quaternary Science 22, 141-161.

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