The Biology of Coastal Sand Dunes M. Anwar Maun - Inecol

54 BIOLOGY OF COASTAL SAND DUNES
of the dune system. Seed size is an important
factor that is related to whether a species
would possess a transient or a persistent
seed bank. Usually species with a persistent
seed bank have small seeds (Thompson and
Grime 1979).
4.2.1 Transient seed bank
Many species of the foredunes possess a transient
seed bank and can only survive in habitats
with recurrent sand movement, high wind
velocities and full sunlight. They are opportunistic,
are dispersed by wind and water,
may possess both vegetative and reproductive
means of regeneration and require regular disturbance
for survival. The energy required to
develop adaptations for persistence of the seed
bank will be too costly and will not increase
their fitness. Based on observations over the
last 30 years on the Pinery sand dunes, the persistence
of fugitive annuals and monocarpic
perennials on the strand is dependent on prolific
and reliable seed production and effective
dispersal. Seed production in these species
has fluctuated over time but has never failed
in these years. These plants are resilient and
capable of high phenotypic plasticity in size
which varies depending on the microhabitat
of their occurrence on the shoreline. On beach
sections used heavily for recreation by people,
most of the seedlings are killed by trampling,
but in early spring or next summer, the driftline
is covered yet again with a large number
of seedlings from seeds that have immigrated
from nearby undisturbed beaches.
Planisek and Pippen (1984) examined sand
samples (7.5 cm diameter × 11 cm deep) from
the beach, foredunes, slack and dune forest in
November 1981 along the eastern shore of
Lake Michigan. The sifting of soil samples
yielded no seeds from the beach and foredune
which is rather surprising because Cakile edentula
(a common plant on the beach) must have
dispersed its fruits by November. The slack
and forest contained 0.74 and 3.2 seeds per
sample, respectively, but none of these seeds
was viable. It was concluded that coastal foredunes
do not have a seed bank primarily
because of substrate instability. Similarly,
Looney and Gibson (1995) showed that in four
habitats, the frequently disturbed beaches
(extensive sand movement, salt spray), unvegetated
sandy areas, early successional dredge
spoils and the strand of a coastal barrier island
of Florida, the seed banks of Cakile constricta,
Oenothera humifusa, Iva imbricata, Hydrocotyle
bonariensis and Heterotheca subaxillaris were
transient and poorly developed. The dune
annuals, Cerastium atrovirens (Mack 1976),
Vulpia fasciculata and V. ciliata (Carey and
Watkinson 1993) at Aberffraw, North Wales,
also did not have any carry-over of seeds from
one year to the next. This is rather surprising
because annuals or biennials are more likely
to have persistent seed banks in habitats with
recurrent disturbance (Thompson et al. 1998).
Baptista and Shumway (1998) determined
the seed bank composition of sand dunes with
different disturbance histories at Cape Cod
National Seashore and found that both the
density of viable seeds in sand samples and
species diversity were very low (Table 4.1). The
seeds exhibited a clumped distribution, probably
because of (i) accumulation of seeds in
depressions following wind dispersal, and
(ii) burial of complete infructescences of plants
before the release of their seeds. A similar
seed bank pattern was also observed in desert
dunes by Kemp (1989), who concluded that the
frequency of seed distribution was kurtotic,
with a large number of seeds in some samples
and a few or none in most and that the abundance
of a species was not a good predictor of
the abundance of its seeds in the seed bank.
Seed bank dynamics of Cirsium pitcheri, an
endangered species in Canada and federally
threatened species in the United States of
America, was examined by Rowland and
Maun (2001). They removed 700 cores of sand
each 2.5–5.0 cm diameter × 35 cm deep located
on transects perpendicular to the beach and
extending to the leeward side of the second
dune ridge. They then sifted the sand to