Environmental Engineering Reference
In-Depth Information
stimulated, whereas other types of organism are ham-
pered. This trend is especially pronounced in easily
cultivable - that is, generally flat - areas. Moun-
tainous regions are much more difficult to cultivate
and geomorphology-based differences between land-
scapes are much more preserved.
of the species. Under more normal conditions the height
at which seeds are found and the distances covered
are much more restricted. Studies in wind tunnels (van
Dorp
et al.
1996, Hammill
et al.
1998, Strykstra
et al.
1998) and in the field (Verkaar 1990, Coulson
et al.
2001, Jongejans & Telenius 2001) found that hardly
any seeds reached distances of more than 10 m from
the parent plants. This general pattern was confirmed
by modelling studies. Jongejans and Schippers (1999)
developed a model for seed dispersal by wind based
on the parameters terminal velocity, seed-release
height, number of seeds per surface unit and wind
speed. Simulations showed that the majority of
species covered small distances of up to a few metres.
Only species that produced large numbers of very light
seeds high above the surface of the ground, for ex-
ample large marsh plants such as
Phragmites australis
and
Typha angustifolia
, were capable of reaching
greater distances from the parent plant (Table 3.2). This
is even more true for species with extremely small seeds
such as orchids or spores, as in the case of mosses
and ferns. Van Zanten (1993) showed that moss spores
could survive in the troposphere and suggested that
in this case wind transport could take place over
3.3.2 Movement of organisms
Dispersal by airflows
In flat, homogeneous landscapes wind transport of
organisms is diffuse and little affected by existing
corridors. The study of this phenomenon has been
intensified during the last few decades both in the field
and theoretically, the latter especially with the help
of computer models. Despite a general assumption that
wind is an important long-range dispersal vector for
plants this has only been measured under exceptional
conditions. Whelan (1986) showed that diaspores
could be found at heights of 3 km or more in the ther-
mal updrafts generated by large fires. Consequently
the distances covered can be quite large, depending
on the characteristic speed of fall (
terminal velocity
)
Table 3.2
Effectiveness of wind dispersal of some wetland species (van Diggelen, unpublished observations).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 field, terminal velocity was measured in the laboratory under standard conditions (Askew
et al.
1997) and height of release was taken from standard flora. 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
Terminal velocity
Height of release
Source area/
(seed m
−
2
)
(m s
−
1
)
(cm)
receptor area
Agrostis stolonifera
25
1.64
57
0.2921
Anthoxanthum odoratum
2,258
1.52
32
0.3357
Holcus lanatus
7,383
1.68
60
2.2452
Lemna minor
-
-
-
0
Mentha aquatica
16,296
2.18
55
0.5099
Phalaris arundinacea
2,072
1.39
150
5.6288
Phragmites australis
18,000
0.21
200
44.4788
Plantago lanceolata
1,222
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
1,443
