Biology Reference
In-Depth Information
demic disease, the scars of which mark the year of the event. Information
on wood anatomy and dendrochronology can be found on Henri Grissino-
Mayer's Ultimate Tree-Ring Web Pages (http://web.utk.edu/~grissino/)
and on North Carolina State University's Inside Wood site (http://inside
wood.lib.ncsu.edu/).
To the west of the Rocky Mountains the land has been stretched, thinned,
and weakened by uplift of the mountains. Some parts have collapsed to
form down-faulted grabens (basins) and others crumpled upward to form
horsts (ranges) constituting the Basin and Range Province (McPhee 1981).
It is a winter-cold desert covered by
Artemisia tridentata
(sagebrush). The
current trend is to recognize higher elevations that were reached earlier
in time for the western cordilleras and that, if verifi ed (and when quanti-
fi ed), will be important in refi ning our concepts of Paleocene and Eocene
paleoclimates and biotas under the eastern rain shadow of the mountains.
If the proto-Rocky Mountains had reached substantial heights by that time,
winds off the Gulf of Mexico would have risen along these steep Paleogene
slopes to create locally moist areas and a more complex humid/dry altitudi-
nal mosaic of habitats than earlier envisioned.
Farther west is the Sierra Nevada. Radiometric dating of early volcanics
suggested an origin about 33 Ma, but recent estimates are that at 50-40 Ma
locally they may have been about 2200 m in elevation compared to the
present 2780 m (Mulch et al. 2006). Sequoia National Park with the mag-
nifi cent
Sequoiadendron giganteum
(big tree, sequoia) is located in these
mountains. A related species,
Sequoia sempervirens
occurs along the coast
in northern California and southern Oregon. The Coast Ranges and the
Cascade Ranges, extending into southern Canada, are the westernmost and
youngest of the cordilleras.
Deserts
There are four arid regions in North America, including the comparatively
high-altitude, winter-cold Basin and Range Desert. The others are the
warm Mojave, Sonoran (fi g. 2.6), and Chihuahuan deserts. By defi nition
a desert receives about 250 mm (10 in) per year or less of rainfall, has high
evapotranspiration, often strong winds, and high radiation levels. Adaptive
features of the vegetation include low stature, heavily cutinized microphyl-
lous leaves, or leaves replaced by spines, sunken leaf stomata with overarch-
ing epidermal cells to reduce water loss, pleated stems that expand and
contract like an accordion to accommodate the sudden changes in water
availability (e.g., many cacti), deep taproot or extensive shallow fi brous
