Effects of Natural and Human Disturbances on Floodplain Vegetation

<strong>Effects</strong> <strong>of</strong> <strong>Natural</strong> <strong>and</strong> <strong>Human</strong> <strong>Disturbances</strong>
on Floodplain Vegetation
Norbert Miiller
Dr., Priv.-Doz. for Vegetation Ecology at Technical University Berlin,
Office for Nature Conservation, Dr. Ziegenspeckweg 10, 86 16 1 Augsburg, Germany
Riparian l<strong>and</strong>scapes are dynamic ecosystems. Under natural conditions there is a high disturbance
<strong>of</strong> the floodplain vegetation by the dynamic <strong>of</strong> flooding <strong>and</strong> the change between erosion <strong>and</strong>
accumulation <strong>of</strong> bedloads. Today in most river ecosystems natural disturbances are altered or
replaced by human disturbances due to civil engineering measures. Flood controlling <strong>and</strong> reduction
<strong>of</strong> bedloads cause fundamental changes in structure <strong>and</strong> species combination <strong>of</strong> the floodplain
vegetation. The effects are obvious in most rivers <strong>of</strong> the industrial countries: there is a decrease or
extinction <strong>of</strong> typical plants <strong>and</strong> biocoenoses.
In this presentation the effects <strong>of</strong> natural <strong>and</strong> human disturbances on the floodplain vegetation is
shown by example <strong>of</strong> the vegetation on gravelbars <strong>of</strong> braided rivers. By comparing natural <strong>and</strong>
impacted rivers courses we can demonstrate how disturbances are important for:
- the structure <strong>and</strong> dynamic <strong>of</strong> vegetation
- the survival <strong>of</strong> plant populations
- the diffusion <strong>of</strong> plants
Under natural conditions the variety <strong>of</strong> disturbances supports the variation <strong>of</strong> species <strong>and</strong> plant
communities (high biodiversity) in river ecosystems. As result <strong>of</strong> human disturbances the high
biodiversity in river ecosystems is reduced <strong>and</strong> plants especially adapted to natural disturbances are
faced with extinction. In consequence nature conservation <strong>and</strong> restoration <strong>of</strong> riparian l<strong>and</strong>scapes
must focus on the preservation or restoration <strong>of</strong> natural disturbances in this ecosystem.
Keywords: biodiversity, braided rivers, civil engineering measures, gravelbar vegetation,
hydrodynat?iics, morphodynamics, plant diffusion, vegetation structure
l. Introduction
In ecology disturbance is defined as ,,any relatively discrete event in time that removes organisms <strong>and</strong> opens
space which can be colonised by individuals <strong>of</strong> the same or different species" (Begon et al. 1990, Sousa 1984).
<strong>Natural</strong> disturbances <strong>and</strong> stochastical events can be observed in many ecosystems, for example fire in forest
ecosystems, inundation in river ecosystems <strong>and</strong> avalanches in mountain ecosystems. <strong>Human</strong> disturbances are
the dominant ecological factor <strong>of</strong> all man-made ecosystems like plugging or cutting in agricultural ecosystems,
soil accumulation or dismantling in urban ecosystems. <strong>Human</strong> disturbances can influence or replace natural
disturbances too, for example flood controlling in river ecosystems.
The effects <strong>of</strong> natural disturbances on community structure <strong>and</strong> species diversity are well researched in forest
ecosystems (e. g. Denslow 1995) <strong>and</strong> natural grassl<strong>and</strong>s (e. g. Collines et al. 1995). An overview <strong>of</strong> the scales
<strong>and</strong> the effects <strong>of</strong> natural disturbances in different ecosystems is presented in White <strong>and</strong> Pickett (l 985). These
investigations led in principle to a better underst<strong>and</strong>ing <strong>of</strong> the dynamics as fundamental factors in
communities. While most research on natural disturbances is done in North America, in Europe ecological
science focuses on man made ecosystems. Under the aspect <strong>of</strong> vegetation dynamics the effect <strong>of</strong> cutting is
researched in different grassl<strong>and</strong>s e. g. semidry grassl<strong>and</strong>s <strong>and</strong> wetl<strong>and</strong>s. In recent times is there a growing
concern on the importance <strong>of</strong> natural disturbances <strong>and</strong> stochastical events under the aspect <strong>of</strong> nature
conservation (e. g. Plachter 1996).
This paper demonstrates the importance <strong>of</strong> disturbances in riparian ecosystems by example <strong>of</strong> the floodplain
vegetation <strong>and</strong> with special regard to nature conservation.
kparian l<strong>and</strong>scapes are especially dynamic ecosystems. Under natural conditions there is a high disturbance
by the dynamic <strong>of</strong> flooding <strong>and</strong> the transportation <strong>of</strong> bedloads. These disturbances are especially strong in
braided rivers, when erosion <strong>and</strong> accumulation <strong>of</strong> bedloads are in balance. Typical <strong>of</strong> these rivers are

floodplains with a large littoral zone <strong>of</strong> gravel bars, without or scarcely covered with vegetation. This type <strong>of</strong>
river is predominant in alpine <strong>and</strong> pre-alpine regions e.g. the European Alps, the Rocky Mountains, the
fimalayas <strong>and</strong> also in the Japanese Alps.
Today in most river ecosystems natural disturbances are altered or replaced by human disturbances: dams
control flooding <strong>and</strong> hinder bedload transportation (e. g. Miiller 1995a).
Only a few years ago it was recognised that these natural disturbances play an important role for the
preservation <strong>of</strong> the typical vegetation <strong>and</strong> wildlife. Based on several examples <strong>of</strong> braided rivers in ths paper
the effects <strong>of</strong> natural <strong>and</strong> human disturbances on the gravelbar vegetation are shown, comparing natural <strong>and</strong>
impacted rivers courses. Questions are:
How do natural <strong>and</strong> human disturbances influence
- the structure <strong>and</strong> dynamic <strong>of</strong> vegetation?
- the survival <strong>of</strong> plant populations?
- the diffusion <strong>of</strong> plants?
Finally conclusions are made for nature conservation <strong>and</strong> the restoration <strong>of</strong> riparian l<strong>and</strong>scapes.
2. <strong>Disturbances</strong> in floodplain ecosystems
2.1 <strong>Natural</strong> disturbances
<strong>Natural</strong> disturbances are caused in floodplains by river dynamics, that means the hydrodynamics (change <strong>of</strong>
inundation <strong>and</strong> dessication) <strong>and</strong> morphodynamics (erosion <strong>and</strong> accumulation <strong>of</strong> bedloads).
The strongest physical disturbance for floodplain vegetation is caused by morphodynarnics. The main
transport <strong>of</strong> bedloads occurs during flood water events, that means gravel <strong>and</strong> s<strong>and</strong> is transported by the power
<strong>of</strong> the water. By this way plants on old gravelbars can be destroyed partially or totally, whereas on the other
h<strong>and</strong> new habitats for plants are created by the accumulation <strong>of</strong> bedloads.
Where morphodynamics works closely the main river course, hydrodynamics influence the whole floodplain
area. For plants <strong>and</strong> vegetation the change between drought <strong>and</strong> inundation is important. Inundation can be
connected with a destruction <strong>of</strong> plants or parts <strong>of</strong> them. Hydrodynamics influence the groundwater level too.
According the effects <strong>of</strong> these disturbances, which are different in time <strong>and</strong> space, natural floodplains show a
typical vegetation structure (compare fig. 1):
- gravelbars, either- bare or scarcely covered with pioneer vegetation (herbaceous vegetation <strong>and</strong> willow
shrubs) are mainly influenced by morphodynarnics,
- wetl<strong>and</strong> vegetation along old channels <strong>and</strong> floodplain forests which are influenced by hydrodynamics can be
summed up as flooding vegetation,
- fossil floodplain forests outside the actual river dynamics.
Max. tiigll Wa~cr
. .
-----
Present Floodplains
River Dynamics I
IIydrodynamics
I I I I l I
Fossile Floodl~lains
No Vegetarian Chondrillerurn Querco-Ulrnerum Erico-Pincru~n
l l
Salic. elaeagni
Typherurn min.
etum
Erica-Pinelunl Mesobrornion-Ass.
Molinion-Ass.
Pio~ieer vegetation 1 I'looclil~g Vegetation I Dry Woo(1.s
Fig. 1: Typical zonation <strong>of</strong> the floodplain vegetation under natural disturbances at an alpine river in the
European Alps (from Miiller 1995b)

2.2 <strong>Human</strong> disturbances
In industrialised countries there has been a long history <strong>of</strong> human impact, whch influences the intensity <strong>and</strong>
effects <strong>of</strong> natural disturbances. In Europe for example, as far back as the Stone Age the run-<strong>of</strong>f <strong>of</strong> the large
lowl<strong>and</strong> rivers, whch have their main catchment area in the Low Mountains, was influenced by the change <strong>of</strong>
l<strong>and</strong>scape in the catchment area (Litt 1992). A great change in the structure <strong>of</strong> lowl<strong>and</strong> rivers took place when
agriculture increased during the Roman Times. In contrast major intrusions on the natural water <strong>and</strong> bedload
regime <strong>of</strong> the alpine rivers started in Europe in the Middle Ages due to an increasing number <strong>of</strong> settlers in the
valleys <strong>and</strong> the clearing <strong>of</strong> forests in the catchment areas. Consequent civil engineering measures, that means
the regulation <strong>of</strong> rivers <strong>and</strong> the construction <strong>of</strong> hydroelectric power plants, finally started in most industrial
countries in the 19 th century <strong>and</strong> early 20 th century. Today almost 80 % <strong>of</strong> the total water discharge <strong>of</strong> the
139 largest river systems in North America, Europe <strong>and</strong> the former Soviet union is strongly or moderately
affected by fragmentation <strong>of</strong> the river channels by dams <strong>and</strong> by water regulation resulting from reservoir
operation, interbasin diversion <strong>and</strong> irrigation (Dynesius <strong>and</strong> Nilson 1994).
These human impacts lead in general to a decrease <strong>of</strong> the natural river dynamics especially the dynamic <strong>of</strong>
morphology <strong>and</strong> hydrology. Nowadays disturbances in floodplains are <strong>of</strong>ten influenced by man alone: power,
time <strong>and</strong> frequency <strong>of</strong> flooding <strong>and</strong> bedload transportation are controlled by hydroelectric paw$ plants
(Miiller 1995a). Regarding to hydrodynamics the peaks <strong>of</strong> flooding are lowered <strong>and</strong> the lowest values are
raised in order to use the waterflow most economicly. Where swell dams are used the water level changes
daily. Regarding morphodynamics, dams have the effect <strong>of</strong> completely retaining the bedload gravel <strong>and</strong> <strong>of</strong>
being effective traps for suspension material. Due to the lack <strong>of</strong> debris downstream <strong>of</strong> a dam, the river is
forced to compensate its capacity <strong>of</strong> transportation to the side <strong>and</strong> river bed erosion. The result is that in
impacted rivers the morphodynarnics get lost <strong>and</strong> reduced hydrodynamics occur. In impacted rivers the
floodplain vegetation is therefore changing. The pioneer vegetation declines <strong>and</strong> becomes extinct. Only
vegetation types adapted to flooding predominates. Due to the river bed erosion the area which is influenced by
flooding has decreased <strong>and</strong> fossil floodplain forests have increased (compare fig. 2).
Present Flood plains
. I w M w . - - -
River Dynamics
l
Fossile Floodplains
Flooding Vegetatio~l ( Dry Woods I Agricultural <strong>and</strong> Urban Arras
Fig. 2: Typical zonation <strong>of</strong> the floodplain vegetation under human disturbances (upstream dams) at an alpine
river in the European Alps (from Muller 1995b)
3. <strong>Effects</strong> <strong>of</strong> natural <strong>and</strong> human disturbances
3.1 On the structure <strong>and</strong> dynamic <strong>of</strong> the gravelbar vegetation
<strong>Natural</strong> disturbances caused by morphodynamics set the plant communities on gravelbars back to an earlier
stage <strong>of</strong> succession or even back to their starting points. Because <strong>of</strong> this, plants that are strong competitors are

hindered regularly on their way to domination, so that plants which are weak competitors also get the
opportunity to survive at the site. Large gravel bars in natural alpine floodplains, covered only partially by
hlgher plants, make these effects visible (fig. 1). Under natural conditions only herbaceous species with special
adaptations <strong>and</strong> pioneer shrubs grow in these places. Regarding phytosociological groups they can be
separated in 4 groups (compare fig. 4, left side):
a) species <strong>of</strong> natural floodplains, that means species which live only in this habitat <strong>and</strong> whch have special
adaptations to the natural disturbances in this habitat (e. g. Myricaria germanica Desv. <strong>and</strong> Calamagrostis
pseudophragmites Koeler comp. 3.2),
b) species from dry grassl<strong>and</strong> can grow on gravelbars because in summer time there are longer periods <strong>of</strong>
dryness.
Under natural conditions there are also, with lower frequency:
c) species <strong>of</strong> ruderal communities, that means common species <strong>of</strong> habitats with human disturbances for
example <strong>of</strong> urban areas or arable fields,
d) species <strong>of</strong> wetl<strong>and</strong>s meaning common species with a wide ecological amplitude which grow in different wet
habitats (e. g. Phalaris arundinacea L.).
How vegetation structure on gravelbars is changing after loss <strong>of</strong> natural disturbances <strong>and</strong> under influence <strong>of</strong>
human disturbances is shown by comparing the gravelbar vegetation at the Lech river in the European Alps
before <strong>and</strong> after the construction <strong>of</strong> upstream dams. Along an unregulated course .the gravelbar vegetation has
changed fundamentally within 30 years. Gravelbars without vegetation <strong>and</strong> with pioneer vegetation <strong>and</strong> open
willow shrubs are decreasing. On the contrary flooding, vegetation, dense willow shrubs <strong>and</strong> floodplain forests
are increasing (fig. 3). This development can also be observed in other braided rivers <strong>of</strong> the temperate zone
after the construction <strong>of</strong> dams (e. g. Bravard et al. 1986, Miiller 1995b, Okuda et al. 1995).
water
bare gravelbars <strong>and</strong>
pioneer vegetation
flooding vegetation
open willow shrubs
dense willow shrubs
floodplain forests
r-
1959
1991 1
Fig. 3: Alterations <strong>of</strong> vegetation structure on gravelbars after the construction <strong>of</strong> upstream dams at the Lech
river in the European Alps (after Miiller 1995a)

Regarding the species combination on the gravelbars, species <strong>of</strong> natural floodplains <strong>and</strong> <strong>of</strong> dry grassl<strong>and</strong>s
(pioneer vegetation) are decreasing, whereas species <strong>of</strong> ruderal communities <strong>and</strong> wetl<strong>and</strong>s (flooding
vegetation) are increasing. These also lead to a lowering <strong>of</strong> species diversity because only competitive species
are able to survive (fig. 4). Species highly specialised to natural disturbances become extinct (cornp. 3.2).
Species no. <strong>of</strong> natural floodplains
wetl<strong>and</strong>s
Fig. 1: Alterations <strong>of</strong> species richness <strong>and</strong> -combination on gravelbars after the construction <strong>of</strong> upstream dams
at the Lech river in the European Alps (according 25 relevees from Muller 1995b)
ln\.estigations into nutrients have shown that, in river courses with natural disturbances the nutrients
3~ allable to plants on gravelbars are low in contrast to river courses impacted by dams. Under the influence <strong>of</strong>
&IS bedload transportation is stopped <strong>and</strong> the extremes <strong>of</strong> hydrodynamics are lowered. Plant succession <strong>and</strong>
911 development on gravelbars are as a consequence no longer interrupted regularly <strong>and</strong> therefore nutrients
xrcase in the soil (cornp. fig. 5).
Nitrogen
n
Herbaceous Shrubbery Herbaceous
gravel bar
gravel bar
gravelbar
vegetation
vegetaton
vegetation
1 natural floodplains H impacted floodplains 1
10
Phosphorous
Shrubbery
gravelbar
vegetation
Fig. S: Plant available nutrients on gravelbars <strong>of</strong> natural <strong>and</strong> impacted (by dams) floodplains at the Lech river
is the European Alps (according Muller 1995a)

With the change <strong>of</strong> the hsturbance regime recolonisation processes are also altered, that means the way in
whch new gravelbars are colonised after a physical disturbance (compare fig. 6). In natural river courses the
seed rain <strong>of</strong> the gravel bar vegetation is dominated by plumed seeds (trichometeorochores), which can be
transported by wind over great distances. Seeds dispersed by gravity (barochores) dominate in impacted
courses. No persistent seed bank could be detected in the gravelbar vegetation <strong>of</strong> natural floodplains, whereas
in impacted floodplains a persistent seed bank is well developed (fig. 6). In each case, optimal regeneration
strategies are realised according to the habitat conditions. In natural floodplains, newly created gravel bars can
be colonised quickly by species with far-ranging dispersal mechanisms. They are less competitive <strong>and</strong> develop
no persistent seed bank as it may be destroyed at any time in this dynamic habitat. In impacted floodplains,
where river dynamic is reduced, competitive species which develop a persistent seed bank dominate (Miiller
<strong>and</strong> Scharm 1998).
80
60
40
20
0
<strong>Natural</strong>
floodplains
Dispersal mechanism
Wind dispersal: trichometeorochores
Wind dispersal: other anemochores
Gravity dispersal: barochores
seed rain
Impact.
floodplains
<strong>Natural</strong>
floodplains
Impact.
flood plains
Fig. 6: For recolonisation important mechanisms (seed rain <strong>and</strong> seed bank) on gravelbars <strong>of</strong> natural <strong>and</strong>
impacted floodplains at the Lech river in the European Alps (according Miiller <strong>and</strong> Scharm 1998)
3.2 On the survival <strong>of</strong> populations
<strong>Natural</strong> disturbances are an important factor for .the survival <strong>of</strong> typical floodplain species (species <strong>of</strong> the
pioneer vegetation). Different investigations into population biology in recent years made this obvious (Bill et
al. 1997, Miiller <strong>and</strong> Scharm 1998). In one way typical floodplain species have special adaptations to
disturbances in their life form <strong>and</strong> cycle. In another way they become extinct if the frequency <strong>of</strong> natural
disturbances is changed under the human impact.
A well researched species is Myricaria germanica Desv. a typical pioneer shrub on gravelbars in braided
alpine rivers in the temperate zone <strong>of</strong> Europe <strong>and</strong> Asia (Bill et al. 1997). Myricaria germanica is adapted to
the lack <strong>of</strong> water by a small leaf surface (protection from evaporation). Each plant has developed a long root
system which allows it to reach the ground-water or the pressure-water even in the case <strong>of</strong> low water level.
Due to the fact that the seeds are distributed a long way by the wind, the plant is well adapted to space
isolation <strong>and</strong> therefore able to inhabit newly created habitats after a highwater event (Miiller <strong>and</strong> Scharm
1998), which also can render local populations extinct. There is no such thing as a long living seedbank, but
due to a long life expectancy (up to 70 years) the species is well adapted to temporal isolation. If covered by
debris, individuals are able to spread vegetatively by developing polycormones (Miiller 1995a).
Besides these adaptations to natural disturbances in floodplains this species also makes special dem<strong>and</strong>s on
the habitat, which can only be hlfilled by a river with natural dynamics. The establishment <strong>of</strong> seeds depends
on the period when the flood water is flowing away. The germination <strong>of</strong> the seeds only takes place in moist
s<strong>and</strong>y substrate on bare gravelbars <strong>and</strong> has to happen rather quickly. Investigations on germination have

proven that the first seeds start to germinate after four hours <strong>and</strong> that 94 % after twenty hours have
germinated. Therefore the success <strong>of</strong> recruitment depends on the fact that new habitats with bare soils are
available for establishment regularly. <strong>Natural</strong> disturbances especially the morphodynamics fulfils these
conditions. If the frequency <strong>and</strong> the power <strong>of</strong> disturbances are lowered under human disturbances (e. g. dams),
plant succession on gravelbars continues <strong>and</strong> is not interrupted through the flooding. Therefore no bare
gravelbars are available after the flooding <strong>and</strong> Myricaria germanica is not able to recruit <strong>and</strong> finally become
extinct under the competition <strong>of</strong> willows (compare fig. 7). As in most alpine rivers in Europe the hsturbance
frequency is lowered by dams, the populations <strong>of</strong> Myricaria germanica have declined <strong>and</strong> become extinct (Bill
et al. 1997, Miiller 1995b).
-.--.-
Percentaee <strong>of</strong> recruitment
0
in local populations
Rivers
0%
El -15 %
Border 16-35 %
36-55 %
Settlements 56-100 %
Kochelsee 6
Disturbance regime:
1 = <strong>Natural</strong> morpho- <strong>and</strong> hydrodynamics
2 = <strong>Natural</strong> morphodynamics, hydrodynamics
lowered by upstream dams
3 = no morphodynamics <strong>and</strong> hydrodynamics
lowered by upstream dams
Fig. 7: Recruitment <strong>of</strong> Myricaria gerrnanica Desv. - a typical pioneer shrub on gravelbars - at three sites <strong>of</strong> the
upper Isar (Northern European Alps) with different disturbance regime (after Plachter 1996)
Similar strategies in the life cycle were observed from other species <strong>of</strong> braided rivers such as Myricaria
germanica. Aster kantonensis Kitam., a perennial herb <strong>of</strong> the Compositae is a rare endemic plant in braided
rivers in Japan. The species has the capability for wide seed dispersal <strong>and</strong> fast germination on bare gravelbars.
After highwater events local populations can become extinct, but also newly created gravelbars can be
colonised quickly by wide wind dispersal. Due to the loss <strong>of</strong> the morphodynamics as a result <strong>of</strong> dam
construction at most Japanese rivers the species Aster kantonensis has no suitable establishment conditions
<strong>and</strong> is therefore highly endangered today (Takenaka et al. 1996, Washitani 1997).
These examples make the fact that natural disturbances can destroy local plant populations obvious but are
important for the recruitment <strong>of</strong> specially adapted species <strong>and</strong> therefore play a central role in the survival <strong>of</strong>
meta populations (Reich <strong>and</strong> Grirnrn 1996).

3.3 On the diffusion <strong>of</strong> plants
IClparian l<strong>and</strong>scapes are famous as being important corridors for plant diffusion (e. g. Johansson et al. 1996,
van der Pijl 1982). Braided rivers with origin in the alpine zone are under natural conditions especially famous
as corridors for alpine species. Light preferring plants whch have their main habitat on disturbed sites in the
mountains (e. g. screes slopes), also find ecological niches on gravelbars in the fore l<strong>and</strong>, due to the hgh
natural disturbances on these sites. Ths is especially proven for rivers in the Carpathian Mountains (Walas
1938) <strong>and</strong> the European Alps (Miiller 1990).
As the natural disturbances have been reduced due to the construction <strong>of</strong> dams, the ecological niches for light
preferring species from the alpine region were destroyed in riparian l<strong>and</strong>scapes. Additionally dams function as
barriers for seed diffusion. Therefore at impacted rivers decline <strong>and</strong> extinction <strong>of</strong> alpine species is generally
observed together with the decline <strong>of</strong> typical floodplain species (Miiller 1990,1995b).
On the other h<strong>and</strong>, a fast diffusion <strong>of</strong> species <strong>of</strong> ruderal sites can be observed today at river courses with
human disturbances (comp. 3.1). When regarding their origin - beside native species from arable fields <strong>and</strong>
urban areas - we also can find alien plants. These species were introduced by man from foreign countries <strong>and</strong>
are naturalised meanwhile mainly in urban areas (Drake et a1 1989). Comparing the different types <strong>of</strong>
floodplain vegetation the main concentration <strong>of</strong> alien plants is in the gravelbar vegetation (Miiller <strong>and</strong> Okuda
1998). This is in contrast to floodplain forests, where there are fewer invaders. Regarding the frequency <strong>and</strong>
dominance <strong>of</strong> aliens in floodplains it becomes clear that there is a strong connection between the human impact
<strong>and</strong> the fast expansion <strong>of</strong> alien plants. Investigations into braided rivers in Europe <strong>and</strong> Japan have shown, that
after the construction <strong>of</strong> upstream dams competitive invaders are able to disperse in a fast way (Miiller 1995b.
Washitani 1997). At the Isar river in the European Alps for example it has been documented that 30 years
after the construction <strong>of</strong> an upstream power plant dam alien species mainly Erigeron annuus, Oenothera
biennis, Solidago canadensis from North America <strong>and</strong> Impatiens gl<strong>and</strong>ulfera <strong>and</strong> Impatiens pawrflora from
Asia, have found a new habitat on gravelbars.
Well documented is also the dispersal <strong>of</strong> the North American tree Robinia pseudoacacia L. on dry gravel
bars at the Tama river by Tokyo (compare fig. 8).
m Quercus acutissima forest
Annual communities
a Herbs dom. perennial com.
m Grass dom. perennial corn.
m Artificial bare grounds, trails
Fig. 8: Spreading <strong>of</strong> the North American tree Robiniapseudoacacia L. on gravelbars <strong>of</strong> the Tama river by
Tokyo after the loss <strong>of</strong> natural disturbances (construction <strong>of</strong> upstream dams) between 1976 <strong>and</strong> 1995 (from
Miiller <strong>and</strong> Okuda 1998)

With the decreasing <strong>of</strong> natural river dynamics, due to the construction <strong>of</strong> dams in the upper course, Robinia
pseudoacacia has had an enormous expansion in the last 20 years This is connected with a change <strong>of</strong> the
habitat as described in chapter 3.1. Bare gravelbars <strong>and</strong> gravelbars scarcely covered with vegetation (annual
plant communities, herbs dominated perennial conununities <strong>and</strong> willow shrubs) are decreasing <strong>and</strong> forests are
increasing. But instead <strong>of</strong> native species such as Quercus acz4rissima the fast reproducing Robinia
pseudoacacia is in the floopplain forests the dominating tree today.
4. Conclusions for nature conservation <strong>and</strong> river restoration
<strong>Natural</strong> disturbances play a central role for the survival <strong>of</strong> typical floodplain plants <strong>and</strong> the preservation <strong>of</strong>
the typical floodplain vegetation as the examples have shown. But also for the preservation <strong>of</strong> typical wildlife
in riparian l<strong>and</strong>scapes the importance <strong>of</strong> natural disturbances is proven (e. g. Plachter 1996, Reich 199 1).
The importance <strong>of</strong> floodplains as corridors in the l<strong>and</strong>scape is changing by the alteration <strong>of</strong> the disturbance
regime. Under natural disturbances floodplains are important for the difision <strong>of</strong> native plant species <strong>and</strong> the
connection <strong>of</strong> habitats. Under human disturbances the invasion <strong>of</strong> alien plants is supported, due to the
changing habitat conditions - riparian l<strong>and</strong>scapes function as corridors for the diffusion <strong>of</strong> aliens in the
l<strong>and</strong>scape. From 13 most frequent invaders in the natural vegetation <strong>of</strong> Europe 12 species can be found in
floodplains (Lohmeyer <strong>and</strong> Sukopp 1992).
The loss <strong>of</strong> natural disturbances in riparian ecosystems due to civil engineering measures must be regarded as
one <strong>of</strong> the inain reasons for the decline <strong>and</strong> extinction <strong>of</strong> typical floodplain species as well as the loss <strong>of</strong>
biodiversity in this ecosystem. Nature conservation <strong>and</strong> restoration in riparian l<strong>and</strong>scapes must focus therefore
on the disturbance regime.
The major aim <strong>of</strong> nature conservation must be the preservation <strong>of</strong> the last remaining natural river courses
because natural disturbances are replaced or altered by human disturbances in the most big rivers. The
protection <strong>of</strong> natural river dynamics is the requirement for the protection <strong>of</strong> typical species <strong>and</strong> habitats <strong>of</strong>
floodplains here.
In strongly impacted river courses where the typical pioneer species has declined or disappeared the complete
restoration <strong>of</strong> floodplains, especially <strong>of</strong> the gravelbar vegetation, seems difficult, even if the river dynamics are
restored completely. This is:
-Populations <strong>of</strong> typical floodplain species must be in direct contact with new gravelbars if a succesful
recolonisation by the seed rain should happen (the probability <strong>of</strong> a dispersed seed to bridge a certain distance
diminishes exponentially with distance). hver courses where there are still populations <strong>of</strong> the typical
floodplains species, must therefore be restored immediately, before the typical floodplain species are replaced
by common species.
-Most typical floodplain plants do not establish a persistent seed bank. Regeneration efforts from the seed
bank, where possible for the restoration <strong>of</strong> other habitats e. g. wetl<strong>and</strong>s, will not work in this situation.
Although there is not the possibility to restore river dynamics completely in many riparian l<strong>and</strong>scapes in the
\vhole river course, there are different possibilities to improve the situation for plants <strong>and</strong> wildlife by short
term measures which also improve other essential functions <strong>of</strong> river ecosystems such as the retention <strong>of</strong>
flooding <strong>and</strong> the groundwater renewal:
-Dismantling <strong>of</strong> longitudinal <strong>and</strong> diagonal constructions (dams) make at least a minimum <strong>of</strong> morphodynarnics
possible <strong>and</strong> extend the area influenced by hydrodynamics,
-In floodplains separated by dyke buildings, old river channels could be reconnected with the main channel <strong>and</strong>
in this way hydrodynanlics can be improved in larger areas.
In addition there is a particular need <strong>of</strong> long term concepts which examine the possibilities <strong>of</strong> restoration
under the viewpoint <strong>of</strong> all essential functions <strong>of</strong> floodplain ecosystems <strong>and</strong> which extend from the catchment
areas to the estuaries.
Due to actual situation <strong>of</strong> riparian l<strong>and</strong>scapes <strong>and</strong> the difficulties in restoring river ecosystems there is a
particular need for an international framework on river restoration. This framework must include physical
aspects as well as biological ones. In general it can lead to a better underst<strong>and</strong>ing <strong>of</strong> the importance <strong>of</strong>
disturbances <strong>and</strong> how they are correlated to biodiversity in ecosystems.

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