Náttúrufræðingurinn - 1975, Síða 60

184
NÁTTÚRUFRÆÐINGURINN
SUMMARY
(Plant colonization on a drained lake bottom).
by
Hólmgeir Björnsson
Agricultural Research Institute, Reykjavik, Iceland.
A small (1 km2), shallow (1,5—3 m) lake, ca 21 m ovei' sea level, was partially
emptied in 1958, and the drainage improved in 1965. The invasion of higher
plants on the drained Itottom was observed and some soil characters studied.
The lake was a part of an extensive bog formation in the Borgarfjörður district
in Western Iceland. The bog is frequently interuptéd by SW—NE aligned
basaltic ridges, one of which hinders complete drainagc of the lake. It disappears
completely only in longer drv periods. The thickness of the peat around the lake
exceeds 4 m, and peat covers the whole lake bottom, indicating a recent origin
of the lake.
A log of a bircli tree stranded on tlie lake bottom (fig 1 and 2) was sampled
for C14 age determination. The result was 2720 ±110 years, indicating tliat the
tree grew near the end of a warm period when many Icelandic bogs were
invaded by birch forests. A layer of logs from this period is preserved at a depth
of 115 cm.
A profile of the peat (fig. 4) was sampled for determination of bulk weight
ancl loss on ignition. The results are given in fig. 3. They were recalculated on
a volume basis as percentage organic matter (lífræn efni) and mineral matter
(aska), appearing in tlie right hand part of the graph. The bulk weight is 0.24
to 0.30 g/cm3 in the main part of the profile, and the calculated pore volume
is 79—82%. The characteristics of the profile change at the log layer. The peat
is more compact just below it. Further down the profile loss on ignition in-
creases from ca 50—60% to ca 85%. Below ground-water level tlie bulk weight
is only 0.12—0.16 g/cm3.
The liigher plants that first inhabit the flat and wet lake bottom are Tri-
glochin palustris, Koenigia islandica, Juncus bufonius, J. alpinus and Sagina
nodosa. These plants do not form a vegetation cover, in 1964 the it average
density was 144 plant/m2 (n = 15). /. bufonius and T. palustris are found in
the wettest locations. When minor elevations are formed on the soil surface,
e. g. where /. alpinus grows to relatively large, tillered plants, or after fertiliza-
tion, other plants can grow and gradually form dense vegetation cover. F.rio-
phorum scheuchzeri is the first plant to form dense vegetation cover. The sparse-
ly vegetated areas are heavily subjected to wind erosion, especially in the
winter wlien the ground is frozen. The eroded material is partially captured
by the vegetion, wliich then becomes clevatecl above tlie surrounding flat peat
ground; in 1974 up to 40 cm high mounds were observed. These and otlier