Environmental Engineering Reference
In-Depth Information
Table 5.2
Effectiveness of wind dispersal of some wetland species (R. van Diggelen, unpublished observations). Results were
obtained with the simulation model of Jongejans and Schippers (1999) in a spatial context. Seed production was measured in
the fi eld, terminal velocity was measured in the laboratory under standard conditions (Askew
et al
. 1997) and height of
release was taken from standard fl ora. The last column shows the size of the receptor area as a fraction of the source area of
576 m
2
. A value of 1.00 means that the receiving area is as large as the source area.
Species
Seed production
(seed m
−
2
)
Terminal
velocity (m s
−
1
)
Height of release
Source/area
Receptor area
Agrostis stolonifera
25
1.64
57
0.2921
Anthoxanthum odoratum
2258
1.52
32
0.3357
Holcus lanatus
7383
1.68
60
2.2452
Lemna minor
—
—
—
0
Mentha aquatica
16 296
2.18
55
0.5099
Phalaris arundinacea
2072
1.39
150
5.6288
Phragmites australis
18 000
0.21
200
44.4788
Plantago lanceolata
1222
3.8
32
0.2032
Ranunculus acris
262
3.14
65
0.3862
Ranunculus repens
224
2.7
37
0.2599
Typha angustifolia
2 600 000
0.14
150
1443
Tackenberg
et al
. (2003) modelled seed dispersal of rare
plant species that grew on small porphyritic hardrock
outcrops in an otherwise intensively used agricultural
landscape, illustrating that heat-induced air turbu-
lence enables seed exchange between sites with dis-
tances of up to some hundreds of metres for many
species despite a small size and a low number of seeds.
found a sharp decrease in similarity only when the
river merged with major side branches, suggesting that
at such points 'propagule fl ows' of two or more areas
with different
species pools
merge.
By a nimals and h umans
With the exception of waterfowl (Vivian-Smith &
Stiles 1994 ; Charalambidou & Santamar ı a 2002 ) and
fruit-eating birds (Handel 1997), animals are assumed
to be of little importance for seed dispersal in most
modern landscapes. This is in contrast to the past
when
transhumance
was widely practised whereby
livestock were moved overland from winter to summer
areas, and vice versa, over distances hundreds of kilo-
metres (Poschlod
et al
. 1996 ; Seifert & Fisher 2010 ).
Also the activities of most wild animals are more
restricted by roads and fences in the landscapes of
today than in the past. Consequently, gene exchange
of seed-bearing plants via animal vectors between
(now isolated) nature reserves has decreased steeply
(Mouissie 2004 ).
Humans have always played a role as a 'moving cor-
ridor ' (Poschlod
et al
. 1996). Plants lack all morphologi-
cal adaptations to dispersal by human beings, yet people
have probably been the most effective long-distance dis-
persers since the Middle Ages (Sukopp 1972; Poschlod
et al
. 1996 ). Modern traffi c networks facilitates rapid,
long-distance dispersal, for example along motorways
and railway lines (Tikka
et al
. 2001 ; Garnier
et al
. 2008 ).
At the same time, the decreasing degree of agricultural
By w ater
Field studies showed that running waters contain large
numbers of seeds and this suggests that they are a
major dispersal vector for wetland plant species (Mid-
dleton 2000; Boedeltje
et al
. 2004 ; Soomers
et al
.
2011). Unlike dispersal by wind, water dispersal does
not occur across whole landscapes but rather is only
possible in relatively narrow corridors where it tends to
be highly unidirectional (i.e. downstream). Laboratory
measurements on seed buoyancy (Danvid & Nilsson
1997; van den Broek
et al
. 2005) showed that most
seeds are capable of fl oating for many weeks, including
seeds without clear morphological adaptations. More-
over, even seeds with a restricted fl oating capacity can
travel large distances in running waters. Nilsson
et al
.
(1991) calculated that only 2.5 days would be suffi -
cient for seeds to cover a 230-km long river stretch in
northern Sweden during the spring fl oods. Studies of
riparian vegetation have indeed shown a large fl oristic
similarity between upstream and downstream areas,
which decreased slowly with distance (Tabacchi
et al
.
1990 ; Johansson
et al
. 1996 ). Tabacchi
et al
. (1990)
