San Pedro Valley Flora During the Pleistocene

[The main source of this essay is Spaulding, W.G, E.B. Leopold, & T.R. Van Devender, 'Late Wisconsin paleoecology of the American Southwest', pp. 259-93, in H.E. Wright, Jr., ed., Late-Quaternary Environments of the United States, Vol. 1. Minneapolis: Univ. of Minnesota Press. For useful descriptions of many of the plants referred to below, see references at the bottom of the page.]

Late Pleistocene Glacial Woodland

About 12-11,000 years ago Glacial Woodland was widespread in what is now mainly Sonoran Desert. Woodland  -- an open-structured arboreal vegetation -- dominated on rocky slopes from close to the basin floors upward until 4500-6,000 feet altitude, where Montane forest -- mixed montane conifers with sagebrush etc -- became dominant. East and uphill from what is now Cascabel, Arizona, Sierra Blanca for example would lie in this transitional zone. See the following link for a reconstructive map of North American vegetation: 11,000 ya -- and compare it with the changing picture of vegetation at 8,000 ya, as temperate woodland retreated and tropical semi-desert spread up the lower Colorado river drainage. Compare also San Pedro Valley Flora Today.

Characteristic plants of this Glacial Woodland

Pinyon pine (Pinus monophylla)-- Single-needle “pygmy conifer”. Today, this conifer is limited to 5,000-7500', and is not found on our Saguaro Juniper lands.

Junipers (Juniperus spp.) Today these grow on our lands from 3,000-11,000' -- but they are different species from those of Pleistocene times.

Live oaks” (Quercus turbinella). Today: 4,000-7500'; not found in our lands today.

"Succulents" (Agavaceae, Cactaceae):

Agaves -- Today, these range to 8,000' -- but different species now from those growing in the Pleistocene.

Yuccas -- During the Pleistocene, Joshua Trees (Yucca brevifolia) were widespread in the Arizona Uplands.   Today, Yucca baccata is the main variety found here, from 2,000-7,000 feet. It is very drought-tolerant and cold-hardy, and is found from Colorado/Utah to Mexico. Its leaves are thicker/stiffer than Yucca elata, and it has a lower profile. It is called “Banana yucca”, by virtue of its fruit shape. Yucca elata, found from 1,500 to 6,000', needs more water than Yucca baccata, but is also cold-hardy.  Its trunk grows higher than Yucca baccata, and its leaves are more bear-grass-like. In our uplands today, baccata is much more common than elata, presumably because of water limitations. We have seen Elata in our uplands only near Saguaro-Juniper Hill; otherwise it is found in some of the lower reaches of the washes coming out of Soza Mesa, and they are also present in the Red Tank Wash among the Junipers there. (These are better-watered areas.)

Beargrass -- Today, Nolina erupens (Nolina family) is only occasionally found on our lands. It is cold hardy to 15 degrees, and needs partial shade where slopes become very hot, otherwise it likes full sun.  It needs some water to get established.  N. microcarpa, also called sacahuista, may be the Arizona version of this plant. Its characteristic elevation is 3-6,000', but it is rare in our area today, found only on very well-protected north slopes of a few deep washes.

Location of these woodland communities in late Pleistocene times was more than 1800' below the limits of contemporary woodland on the nearby mountains.  Their presence then indicates increased annual precipitation (perhaps double today’s), cooler summers (with low rainfall) and mild winters (with much increased rainfall) -- very severe winters occurred mainly north of the Mogollon rim during the late Pleistocene.

At the Willcox Playa to the southeast of our area -- today an area of Chihuahuan desert scrub, at about 4,000 feet elevation -- the pollen sequences from the Pleistocene show very high Pinus pollen (90%, compared with today’s 20%).  Today, ponderosa pine forest in the vicinity begins only above 6300'.  In the late Wisconsin phase of the Pleistocene, pine woodland probably stood right at the edge of the lake. We also have no pines of any kind on or near our Saguaro Juniper lands today.

At Lehner Ranch (on the Upper San Pedro River, site of Clovis Hunter finds dated ca 11,000 years ago), today’s current pollen rain is mainly Chenopodiineae (“Goosefoot family”) -- which includes russian thistle, lambsquarters, and other coarse bushy annuals; Poaceae (“Grass family”)  -- the most abundant of flowering plants); and Asteraceae (“Aster family”). There is very little arboreal pollen (such arboreal trees now grow more than 7 miles away). Vegetation at the time of Clovis hunters there was probably grassland like that of the upper bajadas in the same area now; rainfall was somewhat higher than now (3-4" higher) and temperatures slightly cooler. Extensive grasslands in the San Pedro Valley are suggested for this time period.

Note that several of the warm-desert plants that are now common in our area -- columnar cacti (Cereus spp.), paloverdes (Cercidium and Parkinsonia spp.),ocotillo (Fouquieria), and bursage (Ambrosia spp.) and other shrubby Asteraceae  were absent from this region in the Pleistocene.  Apparently their northern ranges were then much more limited.

Cool Summers and mild winters during the late Pleistocene appear to have produced some associations of heat-intolerant “northern” (woodland) plants and cold-sensitive “southern” (desert-scrub) plants in some areas. The hypothesis is that arctic airmasses may have been trapped north of the ice-sheet. Summer rains were lower (due to lack of a strong “Bermuda High”, which today generates our Southwestern monsoons, and lack of heat for convective summer thunderstorms).  This would explain the lack of summer annuals and summer-rain-dependent perennials -- ocotillo for example, which is frost-hardy, but needs summer rains. Pacific storms today generate most of our winter storms -- these would have been displaced to the south (including into our area) in the late Pleistocene. The Pacific Northwest was more arid during the glacial period than it is today.

See Pleistocene Megafauna for descriptions of some of the animals associated with this floral complex.

Changes at the End of the Pleistocene

The Holocene shift from woodland to desert was sychronous in the Chihuahuan, Mohave, and Sonoran deserts, "and apparently occurred in response to a northward migration of the Aleution low and winter storm track which resulted in drastically reduced winter rainfall." (Brown, ed, 1994, p. 181. Plants moved in to fill abandoned niches -- e.g creosotebush rapidly evolved into 3 separate chromosome races in the three deserts. (ibid.)

Axelrod, D.I, 1979: "Age and origin of Sonoran Desert vegetation", California Academy of Sciences Occasional Papters 132:1-74; Desert vegetation, its age and origin, In Goodin JR and DK Northington, eds, Arid Land Plant Resources. Proceedings of the International Arid Lands Conference on Plant Resources, Texas Tec U, Lubbock. Van Devender, TR & Spaulding, WG, 1979, "Development of vegetation and climate in the southwestern United States," Science 204:701-10. See also see "Feis" refers to "Fire Effects Information System", which provides a database containing descriptions of almost 900 plant species, about 100 animal species, and 16 Kuchler plant communities found in North America. While these descriptions focus on the plant's relationship with fire, background descriptions and full bibliographies are also provided. A useful source for images is the Natural Resources Conservation Service Plants Database; see

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