Prehistoric San Diego County, Part 1

By Tom Oberbauer, CNPS-SD President

 

Introduction

Much evidence exists that during the Pleistocene the vegetation in the Southwest of North America was composed of trees and shrubs that require much more precipitation than currently falls here. Some of the evidence is based on the patterns of existing vegetation including isolated and disjunct populations and consideration of prehistoric distribution of plants influenced them. Other evidence is based on fossil woodrat middens which contain plant parts that can be identified as representing the vegetation from thousands of years ago. Fossil evidence also exists for times ranging from the past few million years when the climate was wet enough to provide for native members of the genus Persea (Avocado) with Pinus torreyana (Torrey pine) and Pinus radiata (Monterey pine) and Palms growing with them (Axelrod and Deméré 1984) to closer to 10,000 years ago when forests existed in coastal Southern California (Axelrod and Govean 1996). Fossil evidence also exists that indicates an extremely diverse fauna of large animals that fed on the vegetation supported by greater levels of precipitation.

Leptosyne gigantea, John Rusk from Berkeley, CA, United States of America, J20170314-0037—Leptosyne gigantea—RPBG—DxO (33553514456)CC BY 2.0

There are local indications that in the past, the vegetation in San Diego County specifically must have represented wetter conditions. In the Sierra de San Pedro Mártir, well south of the Border with Mexico, an entire community of subalpine vegetation exists complete with Pinus contorta (Lodgepole Pine) and Populus tremuloides (Quaking Aspen). This subalpine vegetation is far away and disjunct from its typical distribution. For example, the Lodgepole Pines are typically found in the Sierra Nevada and the San Jacinto and San Bernardino Mountains more than 200 miles away from the Sierra de San Pedro Mártir. Except for a small grove in the San Bernardino Mountains 250 miles away, the Aspen is also found far north in the Sierra Nevada of California but also the Spring Mountains in Nevada. Not only that, a form of Douglas tree squirrel (Tamiascurus mearnsii) a close relative to species typically found in the Sierra Nevada and Coast Ranges of California north of San Francisco, occurs in the Sierra de San Pedro Mártir, nearly 400 miles south of its closest relative. Since the aspen is not a good long distance disperser because its seeds are only viable for a very short time, and tree squirrels don’t travel far,they are indicators that the vegetation between the current populations to the north and south of San Diego County must have been connected. These species along with Sarcodes sanguinea (Snowplant), another subalpine disjunct species indicate an entire subalpine ecosystem must have extended between the Sierra Nevada and the Sierra de San Pedro Mártir. The path between their current distribution and the Sierra de San Pedro Mártir passes through the San Diego and low elevation areas near Valle de Trinidad of northern Baja California. Therefore, they must have occurred in San Diego County during the prehistoric times that conditions were favorable (Oberbauer 1986). Another indication of different vegetation patterns is the presence of ironstone concretions, rust nodules, small round balls, in the soil that are thought to have formed under the acidic conditions of decaying conifer needles. These nodules are found in a variety of locations in San Diego County. Folded in with these pieces of evidence is the presence of fossils in La Brea of between 10,000 and 40,000 years ago. The La Brea flora includes Sequoia sempervirens (Redwood trees) and an assortment of pines and cypress demonstrating a wetter climate (Stock et al 1992). Evidence of wetter vegetation in the past also exists in the form of isolated populations of a redwood associated shrub Vaccinium ovatum (California huckleberry), in places in San Diego County (Oberbauer 2013). Arbutus menziesii (Madrone) in isolated locations in northern San Diego County Mountains and Acer macrophyllum (Big leaf maple) on Mount Laguna and Castro Canyon on the Agua Tibia Mountains are other examples of hold over species which are generally found in Central and Northern California in modern times (Oberbauer 2013b and 2013c). Also, Pinus muricata (Bishop pine), and Pinus attenuata (Knobcone pine) in small locations in northern Baja California indicate that in the past, there must have been more continuous distributions of those two species that are currently absent from San Diego County, but also occur to the north. The vegetation of the offshore islands, Leptosyne gigantea (Giant coreopsis) and Lyonothamnus floribundus (Island ironwood) may also provide a source of species from the Pliestocene woodland. All of these factors together provide evidence that at times during the Pleistocene and previous periods, conditions in San Diego County favored vegetation that was representative of greater moisture than currently occurs here. One other point is the seasonality of rainfall. It is likely that it would have generally been Mediterranean with winter precipitation. It is also very probable that summer rain enhanced the winter rain to keep streams and rivers flowing even through the summers but still not wet enough to prevent the natural cycle of vernal pools and seasonal ponds to become desiccated during part of the summer.

The last glacial period or pluvial period, the Wisconsin pluvial period, when referring to San Diego County ended only 10,000 years ago with the maximum cooling only 21,000 years ago after occurring the pluvial period lasted for 75,000 years. During the peak period, the sea level was more than 400 feet lower than present due to the fact that so much of the earth’s water was frozen in the northern and southern latitudes (USGS 2012).

Generally the landforms, Mountains and Valleys, would not have been much different than today since geologically, that was not very long ago. However, during the time of the sea level retreat, examination of sea floor elevations indicate that the coast line was quite different. The southern three Coronado Islands were connected to the mainland with a great coastal plain. South of the La Jolla canyon and in the area west of Camp Pendleton, the coast line was up to 6 miles west of its current location, though in between it was as narrow as only two miles west in the Carlsbad area. San Diego Bay was a shallow, broad valley. The coast consisted of a long series of cliffs and bluffs except for the area of the La Jolla Canyon that was a deep inlet into the Pleistocene coastline. The offshore islands of San Clemente, Santa Catalina, San Nicolas and Santa Barbara Islands would have been larger but farther out, Tanner and Cortez Banks provided additional island exposed by the lower sea level.

There were opportunities for waves of habitat moving up and down the west coast during the past several million years and during the Pleistocene. However, the end of the Pliestocene would have maintained conditions that could provide for the almost inconceivable assembly of species of the La Brea collection of animals while the geographic landforms were the same as they are now.

With all of these climate and geographic differences, we can imagine, with this information as a guide, what the environment looked like in San Diego County in the late Pleistocene, between 10,000 and 15,000 years ago. At that time, nearly all of the species of plants and animals that we have now already existed. However, the animals and plants that occurred here at the time of European contact was only a subset of the real diversity of animals and plants that existed in the Pleistocene.

Similar to the discussion about the environment when Europeans arrived (CNPS-SD Newsletter February 2018) we will again start with the Coast in this discussion and work our way through the Valleys, Foothills, Mountains and Deserts.

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