Paleoecological studies using packrat middens

One of the first realizations of the paleoecological importance of midden deposits came from P. Wells and D. Jorgensen in 1961, at Aysees Peak in the Mojave Desert, which is the highest mountain in the extremely arid Frenchman Flat area (Wells, 1976). Wells and Jorgensen were at 1906 meters elevation, which was presumably high enough to support a relict stand of pinyon-juniper woodland, but none was visible on the western side of the mountain, and desert shrubs ascended to the summit from all sides. An ancient midden was found, and contained abundant leafy twigs of the woodland conifer Juniperus osteosperma. They knew that this was ecologically significant since the nearest wooded mountain was 30 kilometers from Aysees Peak, and in this vast area of sparse, creosote-bush scrub, they had uncovered dramatic evidence for vegetational and climate change in the Mojave Desert. Radiocarbon dates for this midden were around 9320 +/- 300 BP (Wells, 1976).

In the arid interior of North America, about 2,500 midden deposits have been dated to the limit of the radiocarbon method (the last 50,000 years), and analyzed for plant and animal remains. The preservation of plant remains is excellent, allowing identification of species and diverse geochemical, morphological, and genetic analyses. This extensive archive of identified, sorted, and dated material represents the richest, and best documented source of plant remains in the world, with hundreds of species identified and available for study (Allen et. al., http://biology.usgs.gov/luhna/chap9.html).

(Courtesy of Allen, et. al., USGS)

Maps showing comparison of modern versus Pleistocene vegetation in the Southwest. Landscapes have changed dramatically since the end of the last ice age, 12,000 years ago. During the last ice age, desert vegetation was restricted to lower elevations (less than 300 meters) in Death Valley and the mouth of the Colorado River. Giant saguaro cactus (Carnegiea gigantea) and the palo verde (Cercidium sp.), were displaced far south into Mexico, and creosote bush (Larrea tridentata), the dominant shrub of the Chihuahuan, Mojave, and Sonoran deserts, had its northernmost populations along the Arizona-Sonora border. Extensive pinyon-juniper-oak woodlands, now restricted to the highlands, covered what are now desert elevations (300-1700 meters)(Allen et. al., http://biology.usgs.gov/luhna/chap9.html).

Sierra Bacha, Sonora, Mexico

Midden analyses were done by Van Devender, et. al., (1994) at the Sierra Bacha on the coast of the Gulf of California near Puerto Libertad, Sonora, Mexico. 93 plant taxa were identified from 11 packrat middens, and indurated samples have radiocarbon dates ranging from 9970 to 320 years B.P. In the Sierra Bacha, plants are mostly restricted to rocky northward-facing or oceanward slopes that have less dessication.

One of the most widely studied plants is the Fouquieria columnaris, a climatically sensitive species which is limited by summer maximum temperatures, the amount and dependability of cool season precipitation, and maximum and minimum winter temperatures. F. columnaris found in middens from Sierra Bacha are dated to the early Holocene (8900 - 11,000 years B.P.), and it was found on xeric slopes that face away from the Gulf. This possibly reflects environmental conditions that are more like those of modern peninsular F. columnaris communities. It has been suggested that changes in local and regional distributions of plants in the early Holocene are strong evidence of cooler summers, wetter winters, and persistent upwelling of cold water (Van Devender et al., 1994).

In the middle Holocene (4000 - 8900 years B.P.) the vegetation of Sierra Bacha shifted to a more modern desertscrub, and aridity increased and summers became hotter. Specimens such as F. columnaris, Cercidium floridum (blue paloverde), and others disappear from middens during this time (see scale below).

Relative abundance scale for Sierra Bacha, Sonora, Mexico. Scale ranges from 0, which indicates no occurrence of plant specimen, 1 for a single specimen found, and 5 being the most common specimen found for a specific time. Seven plant species on left of scale are present during Early Holocene, but are absent after the Middle Holocene (Van Devender et al., 1994).


Northern Mojave Desert

Much work has been done in the northern Mojave Desert by G. Spaulding and P. Wigand, who were researching past climatic changes over the last 10,000 years at the proposed Yucca Mountain nuclear waste repository. This work was done in order to understand how climate change can affect groundwater characteristics, and past vegetational history was determined from packrat midden analysis (Grayson, 1993).

The results of this work showed that between around 19,000 and 12,000 years ago, woodlands composed of Utah juniper (Juniperus osteosperma) or juniper and singleleaf piñon (Pinus monophylla) were widespread throughout the Mojave Desert from elevations as low as 2,000 feet to as high as 6,000 feet.(Grayson, 1993). Also found in this xeric woodland were plants now common at higher elevations or in the Great Basin to the north, such as Mormon tea (Ephedra viridis), rabbitbrush (Chrysothamnus spp.), and shadscale (Atriplex confertifolia). What is amazing is that abundant plants that today thrive in the modern Mojave, such as creosote bush (Larrea tridentata) and white bursage (Ambrosia dumosa), were extremely rare or absent between 19,000 and 12,000 years ago (Grayson, 1993).

It was also determined that subalpine conifers such as limber pine (Pinus flexilis) descended to very low elevations. Limber pine was found at an elevation of 5,500 feet in the Pahranagat Mountains between 18,000 and 19,000 years ago, and this elevation is 3,000 feet beneath the lower limit of limber pine today. Work by P. Mehringer and C. Ferguson (1969) showed that at 23,600 years ago, bristlecone (Pinus longaeva) and limber pines, along with Utah juniper and white fir (Abies concolor), grew on the south side of Clark Mountain in southeastern California, 50 miles southwest of Las Vegas. The packrat midden that was analyzed sat at an elevation of 6,270 feet. The significance of this midden is that bristlecone pine does not occur on Clark Mountain today, but occurs in the massive Spring Range to the north, at an elevation 5,600 feet higher than the elevation with which it was growing during the late Pleistocene. Prior to 19,000 years ago, sub-alpine conifers, such as white fir and bristlecone pine extended to remarkably low elevations, sometimes a mile beneath their current lower elevational limits in nearby settings, and were found on ranges were they are now extinct (Grayson, 1993).