Biology Reference
In-Depth Information
river fl ow into the basins and, hence, precipitation. Cores from the Cariaco
Basin off northern Venezuela show low percentages of titanium during the
Medieval Warm Period, and exceptionally low amounts between 750 and
900 CE, revealing a series of severe droughts, each three to nine years in
duration. Extensive land use in a marginal environment, erosion, deteriora-
tion of habitats, a decline in resources, intensifying competition, warfare,
and climate change eventually combined to exceed the technology and the
will of the people to continue. For the Maya this occurred around 750 CE
and the civilization collapsed.
Modern Vegetation
The present plant communities of Mexico include about 20,000 species
of vascular plants arranged into eleven communities or plant formations
(fi g. 2.21). The desert (fi g. 2.22), and the shrubland/chaparral-woodland-
savanna (fi g. 2.23) cover about 40 percent of the country. The driest part
consists of Larrea , Agave , Yucca , and numerous cacti in the Sonoran and
Chihuahuan deserts. There is also Leucophyllum frutescens , the purple sage
of cowboy literature, and species of Cassia covered with masses of conspic-
uous yellow fl owers. The desert continues southward through the state of
San Luis Potosí, with outliers around Tehuacán in the state of Puebla.
The Sierra Madre Occidental is covered at the highest elevations by a co-
niferous forest (western montane association) that includes Abies , Pinus , and
the southernmost present-day distribution of Picea (fi g. 2.16). This plant
association, without Picea , is found again in the highlands of the Trans-
volcanic Belt. On the dry eastern slopes, in the Isthmus of Tehuantepec,
Balsas Basin, and the Yucatán Peninsula, there is a shrubland/chaparral-
woodland-savanna formation that includes Acacia , Prosopis , and the distinc-
tive red-barked Bursera (Mexican mahogany). On the eastern side of the
Sierra Madre Occidental and to the south, in more mesic habitats, is grass-
land, called pastizal at the mid to lower elevations and zacatonal at the high
elevations. Grassland constitutes about 10-12 percent of the vegetation of
Mexico.
Beginning at the coast and moving upslope, the vegetation zones are
mangrove ( Rhizophora , fi g. 2.24; Acrostichum aureum , mangrove fern; Hi-
biscus tiliaceus , II, fi gs. 3.19-22), beach/strand/dune ( Ipomoea , morning
glory; Uniola paniculata , sea oats; II, fi g. 3.23), freshwater herbaceous bog/
swamp/marsh ( Typha , cattail; Thalia geniculata , popal; II, fi g. 3.24), aquatic
( Ceratopteris and Salvinia , fl oating ferns; the invasive South American
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