345
.s were made
Here vegetation starts when there is some stabilisation in the sand transport.
performed
. These field
The area below the high tension pylon shows an example of such a "flying" start
(compare figure 48 from ZONNEVELD 1959(60) extreme left). The rest of that
3S of photo-
figure shows a more general building up of sediments. From other permanent
irror-
sample areas we know places where the youngest stages have not changed very
plectus
much in vegetation or elevation in more than 15 years.
dinaoea 3
tifolia s
egetation
lotographed
d described
However, in the area under discussion we see a dynamic change. Directly after
the sandbar was more or less stabilized (around 1945) several Soirpus species
arrived (Soirpus maritimus 3 Soirpus triqueter 3 Soirpus laoustris glauous).
The first pylon photo (1950) shows an open pioneer vegetation with rounded
stands of Soirpus marztimus as a clear dominant, and with some smaller
submerged
patches of Soirpus triqueter 3 Soirpus laoustris glauous (tabernaemontani).
Also found were one example of Butomus umbellatus and one of Typha angusti-
ice in order
folia. The altitude of the top of the accretion was about 80 cm. below mean
high water, which is near the limit between the so called high and low backswamp,
and increasing
s visual
i. e. high and low levee (cf ZONNEVELD 1959(60) legend vegetation map). These
species combinations show that the whole area belongs to the Soirpetum trique-
tri et maritimi but that already the first "germs" were available for the
the various
Soirpeto phragmitetum-Sagittaria variant (a vegetation usually dominated by
aracterized
Soirpus laoustris and even (on the highest point) for the typical variant (a
riefly
g. 132).
ed until
community with usually Typha species and or Phragmites as dominant). During
the following years we see first a strong increment of Soirpus maritimus and a
decrease of Soirpus triqueter and Soirpus laoustrus glauous tabernamontani .
anthropogenic
Phalaris arundinaoea appears on the scene around 1953. This means that the
Soirpetum triquetri et maritimi typioum shifts over to the Soirpetum tri-
nde of about
quetri et maritimi phalaridetosum. An interesting feature from the geomorpho-
when the
logical point of view follows from the comparison of the photographs from 1950,
.dating a
1951, and 1952. In 1950 the accretion is almost level, gradually falling to the
large tidal gullies at both sides. In 1951 "suddenly" a great number of small
of the
gullies originate. Two of these appear to have developed already in 1952 into
was closed
clear meandering gullies which extend to the centre of the accretion, the others
remain smaller and disappear after 1953. The explanation is that prior to 1950,
! for this area
the vegetation is very sparse. Tidal waters have almost no hinderance and flow
to an amplitude
quickly on and off the accretion. In 1951, however, the vegetation is already
s.
rather dense and it is able to hold back the water a little, so that during the
3 distinguished.
a speed too
ebb-tide there is a small period of a "steady" stream just sufficient to induce
gully formation. In 1952 the vegetation is already dense, and mud accumulation