Life-histories of annual plants in a heterogeneous salt marsh environment
1. Population differentiation was studied in the halophytic
annuals Salicornia and Suaeda maritima at Stiffkey,
2. The salt marsh at Stiffkey shows considerable spatial and
temporal heterogeneity. The gradient in height across the
marsh results in extreme differences in tidal regime and
edaphic conditions, while physiographic features such as
salt pans, creeks and their associated levees, impose
smaller-scale heterogeneity. Salicornia and Suaeda maritima
occur throughout the marsh and exhibit striking variations
in phenotype associated with particular microhabitats.
3. Monitoring of field populations of these annuals during
successive phases of the life cycle showed that
morphologically distinct populations differed in aspects of
their life-histories. These differences included the
control of dormancy and germination, the phenology of
growth and development, and fecundity.
4. The genetic and environmental components of these
differences were examined in reciprocal transplant
experiments of both seed and seedling material and in
uniform growth conditions in growth cabinets and in a
glasshouse. Survival and many aspects of both vegetative
and reproductive performance were measured throughout the
5. Much of the variation in natural populations was
attributable to differences in plant density. Plant size
and fecundity were extremely plastic in their response to
density. Phenological features were more stable and
structural characters were most stable. Survival was
6. Environmental differences between sites also had profound
effects on growth and survival of the plants. In
particular, some aspect of hypersaline edaphic conditions
and the presence of perennial vegetation reduced
considerably the survival and growth of annual plants.
7. When the density dependent components of performance were
removed, and plants from different parts of the marsh were
grown under the same environmental conditions, significant
differences remained between them in morphology and
life-history. This applied both to plants grown on the same
site in the field and to those grown under uniform
conditions. These differences between populations were
likely to be genetic.
8. Three sorts of explanation of the adaptive significance of
genetic differences between populations were examined.
First, the relative performance and survival of local and
foreign populations planted on the same site were examined
at all stages of the life cycle. These differences were
expressed as relative selection coefficients. The relative
success of populations was also measured over an entire
generation by comparing numbers of seeds sown with those
produced. This gives an estimate of the relative fitness of
the local and foreign populations at each site. Second,
correlations were examined between life history
characteristics of the populations and features of their
environment that were likely to act as selective agents.
Third, parallel variation was examined between Salicornia
and Suaeda maritima populations within the same zone of the
9. All three of these approaches were adopted to achieve a
concensus of information. Each approach suffers different
limitations and the interpretation of information obtained
from each is discussed critically.
10. Selection usually favoured the survival and growth of local
relative to that of foreign populations at all sites and at
all stages in the life cycle. The highest selection
coefficients were for survival of the lower marsh
1 t" ... upper morsn 1 .
popu a lon ln competltlon wlth the~perennla vegetatlon
during the growth phase (ca 0.7) and for seed production of
upper marsh Suaeda maritima on the lower marsh (ca 0.9).
11. Measures of the intensity of selection at specific stages
of the life cycle did not predict accurately the magnitude
of selection over an entire generation. All populations
planted as seeds on their native site were relatively
fitter than foreign populations transplanted to same site.
Relative fitness is likely to have been underestimated
because the local and foreign populations at each site were
not grown in competition with one another.
12. Chromosome numbers were counted in Salicornia plants
typical of the populations recognized by their morphologies
and life histories. Three tetraploid and three diploid
Salicornia populations and three diploid Suaeda maritima
populations were identified at stiffkey.
13. The breeding systems and possible genetic structures of
Salicornia and Suaeda maritima populations are discussed in
relation to population differentiation. salicornia
populations appear to be largely inbreeding and consist of
many predominantly homozygous lines. occasional outcrossing
provides sufficient variability to allow divergence of the
populations under selection. population differentiation in
Suaeda maritima apears to be maintained by restriction of
gene flow between populations by divergence in flowering
times, as well as by intense selection.