Madro-Tertiary Flora

The woody flora at the start of Neogene time also included many genera that evidently originated and remained in North America and Mexico, being unable to penetrate high enough latitudes to use Tertiary land connections to Eurasia around either the North Atlantic or North Pacific. This Madro-Tertiary group includes nearly all the evergreen angiosperm genera listed in Table 5, which are still important members of the modern sclerophyll woodlands and chaparral in the region. The group also includes the deciduous angiosperms that survive in interior arid areas of the region (Table 5); these are members of modern thorn scrub and riparian desert vegetation. This flora also includes a few miscellaneous angiosperm genera no longer locally present but surviving in Mexico. Some of these genera are strictly confined to tropical lowlands today, such as Cedrela , a tall, compound-leaved deciduous tree of Mesoamerica.

During early Miocene time, some Madro-Tertiary species ranged much farther north than their surviving relatives. For example, in the Pacific Northwest's present Cascade Mountain region, the Arcto-Tertiary flora commonly had an admixture of Madro-Tertiary evergreen broadleaf trees and chaparral species, including Quercus (chrysolepis), Lithocarpus (densiflora), Arbutus (arizonica), Persea (borbonia), Heteromeles (arbutifolia), Ceanothus (cuneatus), Cerocarpus (betuloides) , and the broadleaf deciduous Cedrela (odorata) . During the Pliocene in central California, some Madro—Tertiary sclerophyll species still grew north of the ranges of their present relatives, Quercus (engelmannii, tomentella), Rhus (laurina, ovata), Ceanothus (spinosus) , and Lyonothamnus (floribundus) .

During early Miocene time, Madro-Tertiary sclerophyll woodland and chaparral species also ranged farther east than their surviving relatives. For example, in west-central Nevada, areas now dominated by desert shrubs and sagebrush had early Miocene floras including all the species noted in the preceding paragraph except Cedrela and Quercus (tomentella) . These fossil floras also included several other sclerophyll tree and shrub genera now confined to the west side of the Sierra Nevada.

In late Miocene time, the general level of the incipient Sierra Nevada was probably still 2,000 m lower than today. The 7-million-year-old Mount Reba flora, from a site now near timberline at 2,650 m, was dominated by Madro-Tertiary sclerophyll woodland species whose modern equivalents are present at about 600 m elevation in the adjacent foothills. Evidently, development of the intense Great Basin rainshadow and spread of desert flora postdate the Miocene. There is no Pliocene fossil record from Nevada.

Most of what is now southern California lay beneath the sea through the Neogene. The few Miocene floras known from the region all evidently grew at low or medium elevations and lack the montane conifers now present in the region. Along with the few deciduous riparian trees noted above, Miocene floras included a variety of chaparral, sclerophyll woodland, and desert shrub genera, mostly with close relatives surviving in the region (Table 6). Considering how much time and tectonic change has ensued, the Miocene floras were amazingly modern; by comparison the contemporary fossil fauna

included many strange, extinct genera. Also, Miocene floras found west of the San Andreas fault actually grew in much lower latitudes. For example, the Mint Canyon flora may have been displaced 200 km northward from its original location. This might be expected to explain the presence of xerophytic fossil plants north of the present limits of Mexican relatives (Table 6), but the same xerophytic species were present in Miocene floras found east of the fault and not displaced. Two species, Lyonothamnus (floribundus) and Prunus (lyonii) that were present on the southern California mainland during the Miocene have surviving relatives only on the offshore islands. When the retreat of the xerophytic and island species began is not known. Early, middle, and late Miocene floras from the region are very similar, and there is no Pliocene record from the region.