american samoa - University of Hawaii at Manoa

Soils differ in origin and age of the parent
bedrock, and younger soils tend to be more fertile
than older clayey soils. However, most of the park
area Ins the same origin-the old Pago Pago volcano,
the caldera of which forms Pago Pago harbor. The
major differences in soil are between the hillside
soils and the alluvial soils of some of the valleys,
which leads to differences in the forests of the
alluvial valleys and the forests of the ridges. The
vast majority of the park area is classified as "Fagasa
family-Lithic Hapludolls-Rock outcrop association"
(Nakamura 1984), which comprises the ridges and
upper valleys. The only other significant type
present is "Aua very stony silty clay loam," which is
located on the lower slopes and lowlands around
Vatia, at Va'aogeoge Cove west of Vatia, and at
Amalau Bay east of Vatia.
Topography is related to soil, since the alluvial
valleys are flat to gently sloping and the hillsides are
steep. Steep slopes increase the rate of run-off, and
presumably create drier soil conditions. Conversely,
the soil of the alluvial valleys is flat, with slower
run-off and more water available from the streams
and rivulets from the slopes, and presumably the
trees found there thrive because of the moister soil
conditions.
Elevation is important for two reasons-tempera-
ture and rainfall. Rainfall generally increases with
elevation. In Western Samoa, Wright (1963)
estimated it increased at a rate of 22-30% with
every 100 m increase in elevation, at least in the
areas he identified as "foothills." Temperature
decreases with elevation, at a rate of about 0.7"C
(1.3"F) per 100 m. Elevation probably makes a
significant contribution to the variation of rainforest
within the park, since it varies from near sea level to
480 m elevation-a potential difference of 106 to
144% in rainfall, if Wright's figures can be applied
to Tutuila. Temperature is probably not that
significant a factor, since the difference between the
coastal areas and the summit of the park would be
less than 3.S°C (6.3"F).
Disturbance is a factor that is difficult to assess.
Vegetation is not static; it is always in a state of flux.
But when a forest becomes relatively stable in flora
and structure for long periods of time, it is termed
climax forest. But since there are continually
occurring disturbances-both man-made and
natural-that affect small and large areas of the
forest, the result is a mosaic of vegetation at
different ages of development. If the disturbance is
complete, i.e., the canopy trees are removed and the
disturbance is of short duration (a hurricane, or
several crops grown there before the land is
abandoned), the area will relatively quickly undergo
a series of predictable structural and floristic
changes, a process known as succession.
The longer the period between disturbance, the
more the mosaic parts tend to become uniform in
flora and structure. The larger the area of
disturbance, the larger the pieces in the mosaic.
There are certain species whose presence in forest is
an indication of past disturbance, particularly Rhus
taitensis, Alphitonia zizyphoides, Macaranga
stipulosa, and Hibiscus tiliaceus, which are
discussed in more detail in the section on secondary
forest.
The classification of the rainforest of the park into
smaller units (communities or associations) is a
difficult and often unrewarding task. While
categories can be constructed by looking at part of
the park, the classification becomes less and less
maintainable when more areas are sampled. That is
because such classifications are artificial-they are
constructed mainly for the convenience of those
studying the subject and may not represent any
meaningful reality in nature. Plant species each
distribute themselves in their own unique ways,
based on environmental factors, and to link these
species with variable distributions into concrete
vegetation units can only lead to inconsistencies.
Another problem encountered when trying to put
vegetation into categories is boundaries. If the
community to be defined is monodominant (i.e., it is
entirely dominated by a single species), a clear
boundary may be present. Mangroves, for example,
have a relatively clear boundary marked by the
upper limits of the influx of salt water. However,
communities with several dominant species are
more difficult to define with boundaries. This is
particularly a problem on ridges that extend from the
sea to high elevation with no abrupt environmental
boundaries (such as change in soil type). This
uniformity of environment and absence of abrupt
environmental boundaries is characteristic of the
park.