american samoa - University of Hawaii at Manoa

category recognized in the report on the Ta'u unit of
the National Park of Samoa (Whistler 1992b). Even
when the area is disturbed, the plants found on the
shore are often still littoral species, because non-
littoral species are not adapted to the harsh con-
ditions present.
Littoral vegetation typically has a zone of herba-
ceous species that covers the foreshore. A zone of
shrubby vegetation is often found inland from this,
and farther inland a zone of littoral forest is usually
present. These three plant zones or communities
recognized here-herbaceous strand, littoral shrub-
land, and littoral forest-are the same ones recogni-
zed in the Ta'u report (Whistler 1992b).
Littoral communities differ from other plant
communities in their narrow extent-they are
restricted seaward by the hightide mark of the ocean
and inland by lowland forest. Because of this, some
authors may regard the three types of vegetation
recognized here as zones of a littoral community
rather than communities themselves. This is
supported by the tendency of some of these vege-
tation types to be indistinct and to blend into each
other with no sign of a clear boundary. Also, littoral
vegetation sometimes lacks one or more of the zones
all together. However, because of distinct
differences in structure and flora, the three are
rccognited here as communities.
Littoral vegetation may occur on sandy, rocky,
talus, or coral rubble shores, but there are no sandy
shores within the park boundaries on Tutuila. Most
of the park's shoreline has a solid rock substrate in
the form of sea cliffs and windswept headlands. The
only coral rubble beaches are found in Amalau
Valley (Fig. 8), Vatia north of the village where the
park boundary begins (Fig. 9), and Tuafanua west of
Polauta Ridge (Fig. 10). There is also a very small
coral rubble beach at Tafeu Cove (Fig. ll), but
because of high waves, landing there was not
possible during the field study.
Environmental conditions found in areas of
littoral vegetation are harsher than in any other plant
community on Tutuila. Although the annual rainfall
in all areas of the island probably exceeds 2500 mm
(I00 in), the rocky surface retains little water for
growth, and often the water that is present is
brackish. This results in species on the exposed
coasts (particularly the herbaceous ones) being
xerophytic, i.e., adapted to dry conditions. The sub-
strate itself is also a limiting factor; sometimes the
plants present can survive only by clinging tena-
ciously to cracks in the rocks. Littoral species are
also adapted to bright light conditions, i.e., they are
heliophytes (light-loving species). Because of this,
littoral herbs and shrubs are generally absent from
the littoral forest floor. This may also explain why
littoral trees typically do not extend very far
inland-they may be shaded out by the taller inland
forest species that are, however, precluded from
dominating the shore itself by their intolerance to
the harsh environmental conditions. Temperature is
also an important factor for littoral species, parti-
cularly the herbaceous plants. While the mean
daytime temperature is 26.7"C (80°F) on most of the
island, the exposed rock heats up to higher
temperatures from the relentless sun.
The most critical environmental factor, however,
is the sea. Salty sea air, brackish ground water, and
occasional high waves make the habitat inhospita-
ble to most other species. Plants present must be
somewhat halophytic (salt tolerant) to survive.
Occasional high waves, some with destructive force,
may sweep the shore, ripping out poorly rooted
plants and leaving behind puddles of sea water.
Another characteristic shared by most littoral
plants is their method of dispersal-most have
seawater-dispersed seeds that can survive long
periods of seawater immersion before washing up
on a distant beach and germinating. Because of this
adaptation for long-distance dispersal, very few
littoral species are restricted in distribution; none of
the Samoan littoral species is endemic to the
archipelago. A few littoral species have seeds
dispersed by adhering to seabird feathers (e.g.,
Pisonia grandis), and a few have colorful fruits that
are eaten and transported internally by birds that
frequent the shore. The three littoral communities
are discussed below.
1. Herbaceous Strand
This community comprises the herbaceous
vegetation occurring on the immediate seashore, and
is the same community recognized on Ta'u (Whistler
1992b). It is sometimes further divided into "rock
strand" and "sand strand", but since the two habitats
are not always distinct (there are intermediate
types-coral rubble and talus) and the species that
dominate the two types often cross over to the other
habitat, this distinction is not retained here.