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Aggressive Invasion of Eragrostis curvla in Gravelly Floodplainsof Japanese Rivers: Current Status, Ecological Effects andCountermeasures
Takashi MURANAKA and Izumi WASHITANI
Institute of Agricultural and Life Sciences, The University of Tokyo
Yayoi, Bunkyo-ku, Tokyo 113-8657, Japane-mail: aflood@mail.ecc.u-tokyo.ac.jp

Abstract
The floodplains on the middle reaches of rapid Japanese rivers are characterized by extensive gravelly areas with sparse vegetation cover. There are a number of plant species endemic to the gravelly
floodplain habitats, including two threatened species, Aster kantoensis and Ixeris tamagawaensiss. The
Kinu River is a typical rapid river in central Japan, and the river endemics have remained among thevegetation of the middle reaches until recently. These unique habitats, however, are rapidly being alteredby an aggressive invasion of the alien grass (Eragrostis curvula), which is supposed to have become
established through dispersal from upstream stands formed due to active introduction of the species forsoil erosion control. The substrate condition of invaded areas tends to be changed from gravelly tosandy/silty, because tussock of E. curvula trap sand and silt during flooding. Thus, the original condition
with sparse vegetation cover principally consisting of riparian endemics adapted to gravelly conditions
is being lost. E. curvula exerts a strong negative influence on the performance of riparian endemics. A
restoration project aimed at recovering the original condition of the floodplain by adaptive management
techniques was started in 2002. As a ‘first-aid’ minor restoration effort, E. curvula was mechanicallyremoved from a part of the floodplain (1,800 m2), fine sands were washed out by spray the ground witha jet of water to restore the original gravelly condition, and stocked seeds of A. kantoensis were sown. A
local population of A. kantoensis was successfully restored, but more extensive exclusion measures areneeded in the next stage of management to prevent further negative impact of the invasive species.
Key words: Eragrostis curvula, gravelly floodplain, invasive alien species, restoration, river endemics

1. Statuses of Vegetation and Ecosystems in
Gravelly Floodplains of Japanese Rivers
The Japanese archipelago has a unique combination of geomorphological and climatic features which
produces remarkable riparian conditions with active
sediment production and deposition, leading to rapid
geomorphological changes of its rivers (Tsukamoto,
1973; Takayama, 1986; Oguchi et al., 2001). Most
major rivers are much shorter and smaller than continental rivers, and have very steep profiles (e.g.,
Takahashi & Sakaguchi, 1976; Takayama, 1986). In
the middle reaches of such rivers, we can find uniqueoligotrophic habitats characterized by a channel shelfcovered by stones, gravel and sand (Umehara, 1996;
Okuma, 1998).
The riparian flora characterizing such unique typeof habitat include Anaphalis margaritacea (L.) Benth.
Global Environmental Research8(2)/2004: 155-162
printed in Japanet Hook. fil. subsp. yedoensis (Franch. et Savat.)
Kitam., Artemisia capillaris Thunb., Aster kantoensis
Kitam., Ixeris tamagawaensis (Makino) Kitam.,Potentilla chinensis Ser., Cassia mimosoides L. subsp.
nomame (Sieb.) Ohashi, Lespedeza stipulacea Maxim.,
Galium verum L. and Dianthus superbus L. var.
longicalycinus (Maxim.) Williams (Miyawaki, 1986).
Among these riparian species, A. kantoensis and I.
tamagawaensis have declined greatly in recent years
and are listed in the Japanese Plant Red Data Book
(Environmental Agency of Japan, 2000) as‘Endangered’ and ‘Vulnerable’, respectively. These
riparian plants are known to have adapted morphologically and eco-physiologically to the specific environmental conditions of gravelly floodplains withsparse vegetation and poor soil conditions. The shoots
are very short with rosette the leaves arranged in a leaf
and they show a high physiological tolerance ofintense light and accompanying heat under the un©2004 AIRIES156

T. MURANAKA and I. WASHITANI

shaded conditions of gravelly floodplain, as demonstrated for A. kantoensis (Matsumoto et al., 2000a,b).

2. Invasion by Alien Pasture Grasses inFloodplains
Riparian habitats are considered to be particularly
prone to invasion by alien plants (e.g., Planty-Tabacchi
et al., 1996; Hood & Naiman, 2000). Plant invaderspose serious threats to natural ecosystems and the biodiversity of riparian habitats through alterations ofgeo-morphological processes (Buell et al., 1995), hydrology (Mueller-Dombois, 1973) and soil chemistryof the habitat (Vitousek & Walker, 1989). In combination with change of the physical environments, competition may lead to replacement of native vegetation
by single-species-stands of nonindigenous species(Witkowski, 1991; Holmes & Cawling, 1997; Mooney& Hobbs, 2000; Miyawaki & Washitani, 2004a).
During the age of mass construction works inJapan (1960s-1980s), alien grasses such as Festuca
arundinacea Schreb. (Tall fescue) F. pratensis Huds.
(Meadow fescue), Lolium multiflorum Lam. (Italianrye-grass), L. perenne L. (Perennial rye-grass),
Dactylis glomerata L. (Orchard-grass), Zoysia spp.
Cynodon dactylon (L.) Pers. (Bermuda grass) and
Eragrostis curvula (Schrad.) Nees (Weeping-lovegrass) were introduced in enormous amounts into construction sites for erosion control and/or re-vegetation(Muranaka & Washitani, 2002; Washitani, 2002a,
2004). Unfortunately, many of these alien grasses
have become major invaders and are now exerting asignificant negative impact on certain types of ecosystems including riparian areas (Committee for Investigating the Effects of and Countermeasures against
Riparian Nonindigenous Species, 2001, 2003;
Washitani 2002a).
E. curvula (Fig.1) is a representative invasive perennial grass, which was first introduced into Japan for

Fig. 1 Eragrostis curvula.
Photo taken in 1999 along the
Kinu River in central Japan.

erosion control. In recent years, vast quantities of theseeds of the species (30,689 kg/year; 1991-2000) havebeen imported mainly from North America (Plant Protection Station, The Ministry of Agriculture, Forestry
and Fisheries of Japan, 1991-2000). The species dispersed from the anthropogenically established stands
across the basin and spread over the river floodplains(Committee for Investigating the Effects of and Countermeasures against Riparian Nonindigenous Species
2001, 2003; Miyawaki et al., 2004b). The species established monospecific long-lasting stands in middle
reaches of 105 out of 123 Japanese rivers under thejurisdiction of The Ministry of Land, Infrastructure
and Transport Government of Japan (Committee forInvestigating the Effects of and Countermeasuresagainst Riparian Nonindigenous Species, 2001). Up to1999, the total area occupied by vegetation dominatedby the species was 379 ha (the total area investigated
was approximately 82,000 ha) (Miyawaki & Washitani,2004).
Nakatsubo (1997) reported that in the Ota River ofHiroshima Prefecture sand accumulation in andaround tussocks of E. curvula alters the micro-topography of gravelly floodplains. Altered substrate conditions which are totally absent from the original gravelly floodplains facilitate the invasion and population
growth of ruderal plants including itself (Umehara,
1996). Thus, the original condition and vegetation ofgravelly floodplains, i.e. sparse vegetation cover consisting principally of riparian endemics growing ingravelly-sandy conditions, is lost.

3. Invasion of Eragrostis curvula into the
Riparian Floodplains of Kinu River
3.1 Vegetation status and changes after flooding of
the Kinu River
Aggressive invasion of E. curvula into the gravelly
floodplain was revealed by a floristic and vegetational
study in the middle reaches of the Kinu River (Fig. 2),
which is a rapid-flow river in central Japan and where

Fig. 2 Location of the Kinu River incentral Japan.
Distances from the confluence
point with Tone River are
shown.

Aggressive Invasion of Eragrostis curvla in Gravelly Floodplains of Japanese Rivers

157

typical gravelly floodplain vegetation has beenpreserved until recently (Muranaka & Washitani,
2001a,b). In the 1990s, river endemics such as Asterkantoensis and Ixeris tamagawaensis still dominatedthe vegetation, but E. curvula began to invade certain
parts of the floodplain (Fig. 3).
After a heavy flood caused by a typhoon in August
1998, the river channel pattern had changed greatly

with sand and silt deposited over a large tract of thechannel shelf (Fig. 4). E. curvula began to spread rapidly and became one of most dominant alien grasses in
the riparian habitat by 1998 (Muranaka & Washitani,
2001b). A similar rapid increases in E. curvula andsand accumulation have also been observed after
floods in 2001 and 2002. In contrast, a declining trend
in endangered river endemics has accelerated since

(a)

(d)

(b)

(e)

(c)

(f)
Fig. 3 Physiognomy and representative species of a gravelly floodplains dominated by river endemics.
(a-b) Floodplain vegetation in 1996. Photo taken 104 km from the confluent point to the River Tone.
(c-f) River endemics and endangered plants. (c) Aster kantoensis, (d) Pulsatilla cernua, (e) Anaphalis margaritacea subsp.
yedoensis, (f) Ixeris tamagawaensis.

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T. MURANAKA and I. WASHITANI1998. The number of A. kantoensis plants decreased
rapidly from 100,000 in 1996 to 110 in 2001
(Muranaka & Washitani, 2001b), when an area of 50
ha was already dominated by E. curvula (Fig. 5).

Fig. 4 Sand accumulation in and around tussocks of E.
curvula altering the microtopography of the floodplainby the formation of sandy mounds after flooding.
Photo taken along the Kinu River in 2001.

Fig. 5 Gravelly floodplain area invaded by E. curvula.
Photo taken 91 - 92 km from the confluent point to theRiver Tone in 2004.

3.2 Influence of the Eragrostis curvula invasion on
performances of river endemics
Vegetation survey data suggest that river endemics
can hardly coexist with E. curvula in the same microsite (0.25 m2) (Muranaka & Washitani, 2001a).
Ecophysiological investigation demonstrated thatshading by the grass has a significant negative impact
on seedling growth of Aster kantoensis (Matsumoto
et al., 2000a,b). No seedlings survived to the stage ofestablished rosettes, even under light availability of
20% - 60% relative PPFD in mid-summer (Matsumoto
et al., 2000a). In a field experiment, it was also
demonstrated that seedling survival and growth weregenerally low at low light availability with relative
PPFD of less than 30% (Matsumoto et al., 2000b).
Another study (Takenaka et al., 1996) suggested
competitive exclusion of A. kantoensis by alien
grasses. Being less than 10 cm in height in the vegetative stage, A. kantoensis cannot compete for light infertile habitats with intense competition among plants.
Thus, invasion by E. curvula tussocks exerts a strong
negative influence on the colonization, establishment
and growth of the river endemics, whose substratepreference of which is partially shared by E. curvula.
In an in situ experiment, seedling survival, growth,
flowering, and seed production of A. kantoensis werecompared under four types of semi-natural conditions,
i.e., combinations of existence or absence of E.
curvula combined with gravelly or sandy substrates(Muranaka & Washitani, submitted). Under conditions
without E. curvula with gravelly substrate, the highest
percent survival, growth, and fitness (= number of
productive seeds / number of sown seeds) wereobserved (39.7), but under the three other conditions,the fitness percentage was less than 2.4.
Recent environmental changes due to catchment
area development, intensive flood control management and recreational uses of the floodplains as golfcourses and athletic grounds have reduced both occurrence and persistence of potential safe sites for thecolonization of A. kantoensis (Washitani et al., 1997).
Reduction of safe sites and seed sources leads to afurther decline of A. kantoensis, which seems to be
trapped in an extinction spiral, in which the reduction
in size and number of subpopulations diminishes thepotential for new colonies (Washitani et al., 1997). In
the Kinu River, aggressive invasion of E. curvula was
the principal cause of the loss of safe sites for colonization of the species.
3.3 Prediction of the expansion of Eragrostis
curvula in the floodplain using a simulation
model
The range expansion of E. curvula was predictedusing model simulation based on its life history anddemographic traits, including seed dispersal patterns.
We applied a lattice model to this simulation. Population processes in each cell (0.5 m × 0.5 m) were expressed by a stage-structured vector and a projection

Aggressive Invasion of Eragrostis curvla in Gravelly Floodplains of Japanese Rivers

matrix according to the plant’s demographic traits,
dormancy/germination, growth, flowering, seed production, and seed dispersal (Muranaka & Washitani,2003). The seed dispersal pattern was estimated fromthe distribution of adult plants and seedlings in a survey grid (5,000 m2). The simulation predicted thatafter 10 years 50% of the floodplain area will be occupied, and after 12 years the whole area will be covered(Muranaka & Washitani, 2003; Fig. 6).
The simulation revealed the effectiveness of mechanical removal of E. curvula. It was suggested thatthe suitable safe sites for regeneration of A. kantoensis
could be maintained if mechanical removal of the invasive species is performed every five to seven years
in the season immediately before seed production of E.
curvula (Fig. 7).

Dominated Area (m2)

4000
3000

2000
1000
0

0

5

10

15

Year
Fig. 6 The results of a model simulation
predicting a trend of increasing area
dominated by E. curvula.
Modified from Muranaka &Washitani (2003).

(a)

159

4. Countermeasures in the Floodplains of
the Kinu RiverAt present, there are large seed sources of E.
curvula distributed across the basin. Moreover, recent
geomorphologic alteration of the river such as the formation of compound cross-sections (e.g., Shimatani,2000) and sand accumulation (e.g., Nakatsubo, 1997)
are likely to result in further decline in populations ofriver endemics through the loss of suitable habitat due
to invasion of alien grasses. Therefore, long-term
countermeasures should be considered at the catchment level. However, to prevent the extinction of river
endemics such as A. kantoensis, immediate short-term
restoration is urgently needed.
Such a pilot restoration was started on the floodplains of the Kinu River in 2002. As a ‘first-aid’ management effort, invasive grasses were mechanicallyremoved from a part of the floodplain (1,800 m2), finesand and silt were washed out using a high-pressure
water jet to restore the original gravelly substrate conditions (Fig.8). Subsequently 10,000 seeds (56seeds/m2) of A. kantoensis collected from plants remaining were sown (Fig.9). As a result of this successful restoration of a local population, most of thesurviving individuals flowered during 2002 - 2003.
Almost 710,000 seeds were produced, and fitness of
71 was achieved (Fig.10). An experiment to restore
indigenous riparian vegetation in a more extensive
area (almost 1.0 ha; 54,000 seeds of A. kantoensissown) was started in April 2003, and more extensive
exclusion measures against E. curvula will be implemented within the course of fiscal year 2004. Data
gathering from newly established populations fromseeds sown in the pilot restoration project will contribute to the planning of long-term conservation management strategies for floodplain vegetation against theinvasion of alien grasses.

(b)Fig. 7 Model simulation of the effects of mechanical removal of E. curvula from part of the floodplain
dominated by the species.
Cases of mechanical removal (a) immediately after (July) and (b) before (March) the seed production of
E. curvula are shown. (Drawn according to data from Muranaka & Washitani 2001b, 2003, and Muranaka
et al. submitted ).
(a) □: plot with no removal, ○: plot with removal at five year intervals, ●: plot with removal at six
year intervals. (b) □: plot with no removal, ○: plot with removal at six year intervals, ●: plot withremoval at seven year intervals.

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T. MURANAKA and I. WASHITANI

(a)

(e)

(b)

(f)

(c)

(g)

Fig. 8 Exclusion of invasive grasses and restoration of asuitable habitat for endemic river species on afloodplain of the middle reaches of the Kinu River.
E. curvula was mechanically removed from two
30 m × 30 m sites on the floodplain, and fine sandand silt were washed out by high-pressure water jet
to restore the original gravelly substrate conditions.
Photo taken in March 2002 along the Kinu River.
(a-b): an area dominated by E. curvula; (c-d): after
the mechanical removal of the E. curvula; (e-g):
after washing out to resume gravelly substrateconditions.

(d)Aggressive Invasion of Eragrostis curvla in Gravelly Floodplains of Japanese Rivers

161

Fig. 9 Human-aided seed dispersal along the Kinu River in April 2002, in which 10,000 seeds of A. kantoensis were sown at twosites (1,800 m2).

Fig. 10

Diagrammatic representation of the life table of A. kantoensis at each restored site (Drawn according to data from
Muranaka & Washitani, submitted).

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(Received 30 September 2004, Accepted 9 December 2004)