Checklist of the Plants of the Guiana Shield - Department of Botany ...

of the region include Angel Falls (979 m) on
Auyán-tepui in Venezuela and Kaieteur Falls (226
m) on the Potaro River in Guyana’s Pakaraima
Mountains.
Granitic dome mountains occur in the southern
part of the three Guianas (Guyana, Surinam, French
Guiana), where they are known as “inselbergs,” as
well as in Venezuela where they are known as
“lajas” and extend in a broad arc all the way from
northeastern Bolivar state west and south along the
Orinoco and then further south as far as Brazil.
Deposits of low-nutrient white sands occur inland
of the coastal plain, in belts across the region and
in isolated pockets. Large areas of savanna are
found in the region, particularly the complex of
savannas that includes the Rupununi Savanna in
southwestern Guyana, the Gran Sabana in eastern
Bolívar, Venezuela, and the savannas of northern
Roraima, Brazil. In some parts of these areas there
is limited forest growth because sands overlay a
clay hardpan that is resistant to penetration by tree
roots and because they flood during the heavy rainy
season. Tertiary and Quaternary sediments
separate the southern edge of the Guiana Shield
from the Amazon River and the eastern edge of
the Shield from the Atlantic Ocean.
The boundaries of the Guiana Shield are
difficult to define precisely. In the south between
mountains on the southern boundary of the Shield
and the Amazon and Negro rivers there is a broad
band of outwash materials resulting from erosion.
In the Venezuelan state of Delta Amacuro much of
the area is covered by thick sediments deposited
primarily by the Orinoco River. In the west, there
are remnants of the Shield at least as far as
Colombia. In the east and south the hills give way
in some areas to lowland tropical rainforest more
similar to that found in the Amazon basin.
However, some remains of the Guiana Shield occur
in the southern section, and some portion of the
sediments are derived from outwash from the
highlands of the Shield. For more detailed
discussions of the geology of the area, readers
should refer to Gibbs and Barron (1993) and Huber
(1995a).
CLIMATE
Checklist of the Plants of the Guiana Shield
As a whole, the Guiana Shield region has a
tropical climate characterized by a relatively high
mean annual temperature exceeding 25º C at sea
level, an annual monthly maximum temperature
range of less than 5º C, and an average daily
temperature range of approximately 6º C. Because
of the Guiana Shield’s location just north of the
equator, its climate varies primarily according to
elevation and effects of the trade winds that
combine to affect rainfall patterns. The trade winds
blow consistently from the east and northeast, off
of the Atlantic Ocean onto northeastern South
America. Due to orographic effects, the
easternmost escarpments of the mountains of the
Guiana Shield are generally localities of increased
precipitation where these moisture-laden winds
meet the slopes (Clarke et al. 2001). Seasonal
oscillations of the Inter-tropical Convergence Zone
(ITCZ) also bring variations in rainfall as the
locations of low pressure zones near the equator
change (Snow 1976). Varying primarily by
latitude, one or two rainy seasons result from shifts
in the ITCZ. The heaviest rains usually occur
between May and August, whereas the rainy season
running from December to January is shorter and
less intense, with rains that do not penetrate as far
inland. In areas where there is only one rainy
season, the driest part of the year is January to
March; in areas where there are two rainy seasons
the driest months are March and October.
However, even during most dry seasons, frequent
storms provide adequate moisture to allow
evergreen tropical moist forests to persist in most
low elevation parts of the region.
DIVERSITY
The varied topology of the Guiana Shield
includes sandstone tepuis, granite inselbergs, white
sands, seasonally flooded tropical savannas,
lowlands with numerous rivers, isolated mountain
ranges, and coastal swamps, each supporting a
characteristic vegetation (Huber et al. 1995; Huber
1995b). This variety accounts for a great deal of
the high diversity and endemicity of the Shield’s
biota. The Shield has a flora and fauna with
numerous endemic species, ca. 40% according to
a recent analysis (Kelloff and Funk 2004). Some
tepui endemic species occur as low as 300 m
elevation, with increasing numbers by 1,500 to
1,800 m, and fully developed communities
occurring by 2,000 m (Funk and Berry, 2005).
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