A larger network of collectors along the Continental
Divide is necessary to estimate maximum amounts of
cloud water and precipitation inputs to the region.
Micrometeorological techniques to measure cloud
water inputs to forests are also promising, but they
require complex instrumentation (Gallagher et al.
1992, Vong and Kowalski 1995).
2.1.4. Weather Systems
Weather systems that affect Monteverde are regional
to global in scale and can be classified into three cat-
egories: (1) temporales del norte, (2) temporales del
Pacifico, and (3) hurricanes. Temporales del norte are
the result of outbreaks of cold, dry, polar air that origi-
nate in the North Pacific, occurring most frequently
from December to February. As these strong cold
fronts pass over the Gulf of Mexico and the Carib-
bean Sea, warm, moist air masses are forced to as-
cend above cooler, denser air. Adiabatic cooling forms
stratus and stratocumulus clouds, and the trade winds
force these air masses over the Cordilleras. Orographic
uplift and further adiabatic cooling result in intense
wind-driven precipitation and mist in Monteverde.
Temporales del norte typically have the longest
duration of the three types of weather systems at
Monteverde, lasting up to 14 days with continuous
precipitation (J. Campbell, pers. comm.).
Temporales del Pacifico are the result of tropical
low-pressure systems in the Caribbean basin, occur
frequently from August to October, and correspond
with the hurricane season in the Caribbean. They can
reverse surface winds such that warm moist air is
drawn over Monteverde from the Pacific Ocean. Oro-
graphic uplift and adiabatic cooling of these air
masses, combined with high rates of sensible and
latent heat exchange with land surfaces, produce
stratus and stratocumulus clouds that result in cloud
immersion and precipitation throughout the day
and night. Although temporales del Pacifico are typi-
cally shorter in duration than temporales del norte,
they may result in high precipitation. The maximum
daily precipitation depth recorded during a tempo-
ral del Pacifico was 160 mm (J. Campbell, pers.
comm.).
Hurricanes are relatively rare in Monteverde. In
the last century, only one hurricane hit Costa Rica
directly (Hurricane Martha, 21-25 November 1969).
However, high rainfall reported for September and
October reflects the indirect effects of tropical de-
pressions, some of which form hurricanes as they
travel northward. For example, precipitation depth
from Hurricane Gilbert (October 1988) totaled 240
mm in 30 hr (J. Campbell, pers. comm.).
2.2. Geology and Geologic History
of MonteverdeIn recent decades, the earth sciences have been revo-
lutionized by the theory of plate tectonics. The outer
layer of the earth is now known to be composed of a
series of large rigid plates that move in response to
the earth's internal heat flow. Over millions of years,
the plates have shifted to create ocean basins, conti-
nents, and mountain ranges. The present assemblages
of plants and animals in Central America have been
strongly influenced by the recent plate movements
that connected North and South America.
The geology and geologic history of southern Cen-
tral America are distinct from those of land masses
to the north and south (Dengo 1962, 1985, Escalante
1990). Costa Rica, along with southern Nicaragua,
Panama, and a portion of northern Colombia, is part
of the Caribbean continental plate (Fig. 2.7). North-
ern Central America is part of the North American
continental plate; the rest of the South American con-
tinent is part of the South American plate. As plates
move, they can either converge, slide past one an-
other, or separate. Costa Rica is in a zone of plate
convergence that occurs along the west coast at the
western edge of the Caribbean plate, referred to as the
Middle America Trench (Fig. 2.7). At present, two
small oceanic plates, the Cocos and Nazca plates,
are converging with the continental Caribbean plate.
Because the material that makes up oceanic plates
(primarily mafic rocks of basaltic composition) is
generally denser than the material that makes up
continental plates (primarily rocks of intermediate to
andesitic composition), the Cocos and Nazca plates
are being driven down (subducted) below the Carib-
bean plate. The present crustal deformation (which
includes tectonic uplift), seismic activity, and active
vulcanism in southern Central America result from
plate motion along this subduction zone (Fig. 2.7).
Subduction of oceanic crust beneath the Caribbean
plate began in the Cretaceous Period, about 60-70
million years ago (Ma; Minster and Jordan 1978,
Lundberg 1982, 1991, DeMets et al. 1990, Mclntosh
et al. 1993).
The tectonic history of southern Central America
over the last 10 million years has been complex. The
most important events influencing the modern con-
figuration of Costa Rica and the Central American
land bridge were (a) the rotational and translational
movement of the Chortis block, a continental frag-
ment associated with rocks currently found in west-
ern Mexico, which now composes central and east-
ern Honduras and Nicaragua; (b) the northeasterly
movement of the southern Central American volca-20 The Physical Environment