(1050 m), Laguna Poco Sol (760 m) in the lower
Perms Blancas valley, and Laguna Cocoritos (500 m)
at the mouth of the same valley are crater lakes cov-
ering several hectares (Alvarado 1989). A smaller (1
ha) lake is perched at 1000 m elevation on the north
side of the Penas Blancas valley, and a similar-sized
lake lies in the upper Rio Palmital valley at 1250 m.
Laguna Arancibia lies at 1250 m in the upper Rio
Aranjuez valley across the Continental Divide. Nu-
merous smaller permanent pools lie hidden under
the forest canopy. Pollen records from an altitudi-
nal transect across these lakes would help interpret
the paleoecology of the Cordillera de Tilaran.2.5. Soils of MonteverdeLittle quantitative research has been conducted on the
soils of Monteverde. We discuss soils in a general
manner and rely on data collected from other tropi-
cal montane forest sites for characteristics that are
undoubtedly similar to soils in Monteverde.2.5.1. Soil Formation and Classification
Soil formation represents a complex set of interdepen-
dent biotic and abiotic factors. For example, geologic
factors (primarily rock type and structural features)
largely determine the relief and geomorphology of the
landscape, which affect microclimate, which in turn
strongly influences the flora and fauna. Over time, an
unconsolidated mass of rock is altered by physical and
chemical processes such that it becomes measurably
different from its original parent material. Organic mat-
ter derived from vegetation, microbial biomass, and
animals is added. The resulting soil develops layers at
varying depths below the surface; a vertical section
through these layers is referred to as a profile. Soils are
classified based on characteristics of their profiles,
which result from a unique combination of factors (Jenny
1980, Troeh and Thompson 1993, Ritter et al. 1995).
The upper part of a soil profile is referred to as the
A horizon, and it is typically darker in color than the
lower horizons, due to the accumulation of organic
matter. The middle part of the profile is the B hori-
zon, or subsoil. The lower part of the profile is the
C horizon, which extends to unweathered bedrock.
Soils that have developed under vegetation typically
have accumulations of organic material above the A
horizon. Organic matter accumulations are referred
to as the Oa horizon, which is composed of identifi-
able organic debris, and the Oe horizon, which con-
tains partially decomposed organic debris referred
to as humus. These two layers comprise the forest
floor. A large accumulation of amorphous organicmatter throughout the O, A, and occasionally the B
horizons is referred to as a histic epipedon (zone).
Although forests at higher elevations in Monteverde
are typically less productive and have lower rates
of litterfall compared to those at lower elevations
(Vitousek and Sanford 1986, Bruijnzeel and Proctor
1993; see Chap. 9, Ecosystem Ecology), rates of litter
decomposition are also lower, which leads to large ac-
cumulations of litter and humus in the O horizon and
of organic matter in the A and B horizons. This is one
of the most striking differences in the soils of tropi-
cal montane forests compared to tropical lowland for-
ests (Grubb 1977, Marrs et al. 1988, Grieve et al. 1990,
Kitayama 1993, Bruijnzeel et al. 1993).
The majority of soils at Monteverde are formed on
slightly to moderately weathered volcanic parent
materials of the Aquacate and Monteverde Formations
and are classified as Andisols (USDA soil classifica-
tion system [1992]). Andisols at Monteverde are fur-
ther classified as Udands because they have formed
under udic, or wet, moisture regimes. Inceptisols,
which are poorly weathered soils at an early stage of
development derived from landslides or alluvium,
occur on steeper slopes and along streams. Both
Andisols and Inceptisols are characterized by poorly
to moderately differentiated soil horizons. Histosols,
highly organic "soils" derived primarily from host
tree and epiphyte litter on tree stems and branches,
occur at higher elevations in forests exposed to high
cloud water and precipitation inputs (see Clark and
Nadkarni, "Epiphytic Histosols," pp. 34-35).2.5.2. Physical and Chemical Properties
Volcanic parent materials of the Aguacate and Monte-
verde Formations were formed by rapid cooling,
which resulted in the imperfect crystallization of alu-
minosilicate minerals. When these and other amor-
phous minerals are hydrated and the initial weather-
ing reactions occur, the rapid weathering of feldspars
and leaching of silica lead to the formation of amor-
phous clays (e.g., allophane, aluminum, and iron
sesquioxides). Organic matter contributes to the
weathering process by producing organic acids that
exchange hydrogen ions for silica, exchangeable bases
(e.g., calcium, magnesium, potassium, sodium), and
other ions. Organic matter also interacts with clay
particles and promotes their aggregation (floccula-
tion). Because of the relatively young geologic age of
the Aguacate and Monteverde Formations, mineral
weathering reactions have not occurred to the extent
that they have in highly weathered Oxisols and
Ultisols in some lowland tropical regions. Those soils
have relatively high clay contents and low concen-
trations of exchangeable bases. Mass wasting and the27 The Physical Environment