PORCUPINE MARINE NATURAL HISTORY SOCIETY NEWSLETTER

30
are apparently suitable salt marshes. In the Menai
Strait L. humile is abundant just inside the southwest
entrance, but apart from an isolated colony
4.5 Km in, does not extend far up the strait. The
lack of spread in this channel is probably due to
the timing of tidal currents. There are differences
in tidal amplitudes and times of high water at the
two ends of the strait so the flow begins to run
south-westwards more than an hour before local
high tide (Campbell et al, 1998). Thus, at the times
when sea water covers the marshes drifting seeds
would not go further up the strait. Furthermore there
is a substantial net residual flow south-westwards.
Elsewhere in the inlets of the south-west Anglesey
coast, highest water levels, often coinciding with
onshore winds would favour more seed retention.
Tidal movements were shown by Boorman (1971) to
play a key role in the export of Sea Lavender seeds
from salt marshes.
Figure B. Distribution of Limonium vulgare around
Anglesey
In contrast to Limonium humile, L. vulgare (figure
B) is found on both sides of the island. Anomalously
however it mainly occupies quite different habitats
on the two coasts. On the north-east side, as would
be expected, it occurs almost exclusively in salt
marshes, but it is really abundant only in one small
estuary, Traeth Dulas. This is a semi- enclosed tidal
inlet with a narrow entrance, so once established,
conditions should have favoured the long-term
maintenance of the population. This contrasts with
the smaller and more widely separated colonies in
Red Wharf Bay, where conditions would have been
more conducive to seed export, and loss from some
monads seems to have occurred in recent decades. In
some west coast marshes there are just a few earlier
records of L. vulgare. Unusually however, on part
of the south-west coast it grows exclusively in the
crevices of splash zone rocks and amongst boulders.
The particular section of coast where this occurs has
a frontage of only about 5.5 km, within which there
PMNHS Newsletter No.24 Summer 2008
are about 6 discrete colonies. Given the atypical
habitat, the very restricted spread and the particular
vigour of these rock habitat plants, questions arise
as to whether they might be genetically distinct from
the salt marsh variety. If this is so from where, when
and how did they get here?
Figure C. Distribution of Limonium binervosum agg around
Anglesey
Forms of Limonium binervosum agg, (figure C) occur
in both rocky coast and high level sandy salt marsh
situations, and on both sides of Anglesey, but they
are absent from both the predominantly rocky north
coast and from the Menai Strait. The hottest spot for
them is the west coast of Holyhead Island, particularly
the section between Trearddur Bay and South Stack.
Several morphologically distinguishable variants can
be found. In places more than one of these variants
occurs adjacent to each other, a feature that may
be relevant to the history of colonisation. Rising
sea levels and ecological roll back during the postglacial
transgression could have a bearing on colony
isolation. Not yet explicable is the occurrence of
flourishing colonies in the sandy salt marshes at
the mouth of the Cefni Estuary and on the tip of
Llanddwyn Island, but an absence in apparently
suitable habitats around Traeth Abermenai.
None of the Sea Lavenders currently have distributions
that extend further north than the extreme south
west of Scotland and the north coast of Ireland
(Preston et al, 2002). It must be assumed that
climatic conditions did not ameliorate enough for
them to colonise North Wales until some time after
about 10,000 BP and probably after rising sea levels
had separated Britain from the continent. Present
day distributions often in discrete colonies with
plenty of seedlings in them, suggest that successful
medium and especially long range dispersal has
been relatively infrequent. Nevertheless there must
at times have been some longer distance spread
of propagules to found the present colonies. With
a few exceptions the flowering plants of the salt