Download Imperata cylindrica (cogon grass) Impacts Information

Imperata cylindrica (cogongrass)
Impacts Information
Prepared by the IUCN SSC Invasive Species Specialist Group
Contents
1.0 Introduction………………………………………………………………….... Page 1
2.0 Displacement………………………………………………………………….. Page 1
3.0 Agricultural…………………………………………………………………… Page 2
4.0 Habitat Alteration…………………………………………………………….. Page 2
5.0 Modification of Nutrient Regime…………………………………………….. Page 3
6.0 Modification of Fire Regime…………………………………………………. Page 3
7.0 Ecosystem Change……………………………………………………………. Page 3
8.0 Competition…………………………………………………………………… Page 3
9.0 Threat to Endangered Species……………………………………………….. Page 4
10.0 Inhibits Other Species……………………………………………………. Page 4
11.0 Other………………………………………………………………………. Page 4
12.0 References…………………………………………………………………. Page 4
1.0 Introduction
Imperata cylindrica is considered to be one of the top ten worst weeds in the world. Its
extensive rhizome system, adaptation to poor soils, drought tolerance, genetic plasticity
and fire adaptability make I. cylindrica a formidable invasive grass. Increases in I.
cylindrica concern ecologists and conservationists because of the fact that this species
displaces native plant and animal species and alters fire regimes (Lippincott 1997 2000,
in Brewer & Cralle 2003).
Displacement of native species is the greatest area of concern in the United States while
threats to agriculture are the major concerns in Africa and Asia. Dense swards of I.
cylindrica create an intense competitive environment for commercially important species
(Bryson and Carter 1993, Kuusipalo et al. 1995, Premalal et al. 1995, Dozier et al. 1998).
Fires used to clear vegetation in slash-and-burn agriculture perpetuate this weed by
stimulating rhizome sprouting and preventing the growth of secondary forests that would
otherwise shade it (Friday et al. 1999, in Chikoye & Ekeleme 2001).
Invasion of longleaf pine savannas by I. cylindrica in the southeastern USA represents an
ideal system in which to study the impacts of invasive species on communities (see
Brewer & Cralle 2003; Brewer 2008). Clones of this species can expand rapidly into
undisturbed pine savannas by rhizomes (Lippincott 1997, in Brewer & Cralle 2003).
2.0 Displacement
Invasion of longleaf pine communities by I. cylindrica will likely cause significant losses
of short habitat-specialists and reduce the distinctiveness of the native flora of these
threatened ecosystems (Brewer 2008). Brewer (2008) found that any plant shorter than I.
cylindrica is at risk of displacement, and most ground-cover herbs indicative to longleaf
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pine savannas are shorter than I. cylindrica. In contrast, the taller species (e.g. tall shrubs,
saplings, and climbing vines) that avoided displacement were not indicative of longleaf
pine savannas (Brewer 2008). These results strongly suggest that I. cylindrica displaced
most native species in this community, at least in part, by increasing shade (Brewer
2008).
3.0 Agricultural
The interference of I. cylindrica with the growth of both herbaceous and woody tropical
crop species is well documented (Brook 1989, in King & Grace 2000b). It is considered a
primary weedy species in tea (Camillia sinesis), rubber (Hevea spp.), pineapple (Ananas
comosus), coconut (Cocos nucifera), oil palm (Elaeis spp.) and other perennial plantation
crops in Asia. In Africa it causes the greatest damage in agronomic production (Ivens
1980, in MacDonald 2004). I. cylindrica is a major constraint in the establishment of
plantation crops such as rubber, pineapple, tea, banana (Musa spp.), citrus (Citrus spp.),
and coconut (Soerianegara 1980; Dela Cruz 1986; Ohta 1990, in MacDonald 2004). In
Asia, it has been shown to retard the growth of rubber by up to 96% within a period of 5
years (Soedarsan 1980, in Chikoye & Ekeleme 2001). I. cylindrica is also a serious weed
in slash and burn agriculture systems, particularly in Africa (Udensi et al. 1999, in
MacDonald 2004) and southeast Asia (Friday et al. 1999, in MacDonald 2004). It is
reported to be one of the top three weedy pests of cassava (Manihot spp.), cotton
(Gossypium spp.), maize (Zea mays), peanut (Arachis hypogaea), upland rice (Oryza
sativa), and sweet potatoes (Ipomoea batatas) (Holm et al. 1977, in MacDonald 2004).
In West Africa, it is a serious weed of intensive agriculture particularly in areas prone to
recurrent burning in the coastal/derived savanna (also called forest/savanna transition
zone) (Chikoye et al. 1999, in Chikoye et al. 2000). In these areas it devastates cereals,
legumes, root and tuber crops and plantations (Chikoye et al. 2007). I. cylindrica
infestation causes severe crop yield losses, land abandonment, deforestation and land
degradation (Chikoye et al. 2007). Crop yield reduction attributable to competition from
I. cylindrica has been estimated at 76 to 80% in cassava, 78% in yam and 50% in maize
(Koch et al. 1990, in Chikoye et al. 2000). Complete crop failure usually occurs when
crops are grown in plots slashed during land preparation, without additional weeding
during the growth cycle (Udensi et al. 1999, in Chikoye et al. 2005).
In addition to yield loss, I. cylindrica increases the cost of crop production, reduces the
market value of tuber- and root-damaged crops, and increases the risk of fire. Recurrent
bush fires during the dry season cause can result in loss of organic matter and soil
degradation. Mechanical injuries to the skin caused by rhizome ramets reduces the
efficiency of planting, fertilizer application, staking, weeding and harvesting, resulting in
increased labour demand (Chikoye 2003; Chikoye et al. 2000).
4.0 Habitat Alteration
I. cylindrica invasion of an emerging pine forest may be an example of a grass converting
woodland with high understory diversity into a grassland with low diversity. However,
unlike other examples where fire is the major driver of this change (Mack & D’Antonio
2003, in Daneshgar & Shibu 2009), I. cylindrica presents a case where its ability to
deprive competitors of N may lead to the conversion of the ecosystem. With fewer
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canopy species and decreased understory diversity, the change of pine forests to I.
cylindrica dominant savannas seems probable (Daneshgar & Shibu 2009).
5.0 Modification of Nutrient Regime
The changes in nutrient cycling caused by exotic grasses can endanger young tree
seedlings in a regenerating forest (Daneshgar & Shibu 2009). Also, because I. cylindrica
allocates significant carbon below-ground, it is able to recover quickly after fire, which is
why Lippincott (2000, in Daneshgar & Shibu 2009) suggested that frequent intense fires
could convert a pine savanna into an I. cylindrica-dominated grassland. Daneshgar and
colleagues (2008) showed evidence of I. cylindrica lowering soil water availability and
foliar nitrogen which resulted in a reduction in the photosynthetic capacity of pine
seedlings. Detailed examination of I. cylindrica impacts on belowground resources is
essential to fully understand the mechanisms responsible for the observed reduction in
pine seedling growth (Daneshgar et al. 2008).
6.0 Modification to Fire Regime
Pyrogenic pine savanna ecosystems occur on well-drained sands in the southeastern
coastal plains of the USA (Lippincott 2000). Such sandhill sites are managed with
relatively frequent low-intensity fires fueled by short caespitose grasses and pine needles
(Lippincott 2000). Lippincott (2000) found that sandhill invaded by I. cylindrica had
significantly greater fine-fuel loads that resulted in fires that had higher maximum
temperatures at greater heights. Fire-induced mortality of juvenile Pinus palustris
(longleaf pine) was higher for pines growing in invaded sandhill. Platt and Gottschalk
(2001) also found that the invasion of south Florida pine savannas by I. cylindrica and
Neyraudia reynaudiana resulted in increased potential fine fuel loads, expressed as
above-ground plant biomass. Both exotic grass species almost doubled the litter biomass,
increasing potential fine fuel loads at ground level (Lippincott 1997, in Platt &
Gottschalk 2001).
7.0 Ecosystem Change
A study by Holly and colleagues (2008) supports the growing consensus that invasive
plant species alter normal ecological processes and highlights a possible mechanism
(alteration of microbial assemblages) by which I. cylindrica may alter an ecosystem
process (decomposition). The authors’ paper focuses on the consequences that a
dominant invasive grass species may have on decomposition in an invaded herbaceousdominated ecosystem. Decomposition of plant material is an important component of
natural systems as it returns nutrients that are bound in organic biomass back to the soil
(Holly et al. 2008). Decomposition thus serves as a bridge linking above-ground
processes to those of the integrated below-ground communities, including effects on soil
structure, physicochemical environment and biotic assemblages (Holly et al. 2008).
8.0 Competition
Competition by cogongrass impedes and perhaps even halts the natural regeneration of
vegetation (Chikoye 2003; MacDonald 2004; Jussi et al. 1995; Peet et al. 1999). The
results of a study by Brewer and Cralle (2003) suggest that I. cylindrica is a better
competitor for phosphorus than are native pine-savanna plants, especially legumes. The
competitive effects of this species on plant diversity may be of more immediate
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conservation concern relative to the effects of this species on fire regimes in longleaf pine
ecosystems (Brewer 2008).
9.0 Threat to Endangered Species
Longleaf pine savannas of the southeastern USA contain extraordinarily species-rich
plant communities and are home to numerous threatened endemic plant and animal
species (Walker & Peet 1983, Bridges & Orzell 1989, Brockway & Lewis 1997, in
Brewer & Cralle 2003). Frequent fires and wet soils of low pH and relatively low
nutrients provide conditions favorable to many unique endemic species including
carnivorous pitcher plants (Sarracenia spp.) and sundews (Drosera spp.), and the
endangered Mississippi sandhill crane (Grus canadensis pulla) (King & Grace 2000a).
Also, the federally-listed threatened gopher tortoise (Gopherus polyphemus) depends
upon the abundant herbaceous groundcover found in remnants of longleaf pine-bluestem
forests (Yager 2007). I. cylindrica threatens the habitat quality of these pine-bluestem
forests because it provides lower quality forage than most of the vegetation that it
displaces (Yager 2007). I. cylindrica may also threaten the Camp Shelby burrowing
crayfish (Fallicambarus gordonii) based on its negative effects on habitat (W.
McDearman Pers. Comm., in Yager & Smith 2009).
Cutting and burning practices associated with Imperata grasslands in Nepal threaten less
mobile species and those less tolerant to disturbance such as the IUCN Critically
Endangered (CR) pygmy hog (Porcula salvania) and the Endangered (EN) hispid hare
(Caprolagus hispidus). Evidence suggests increases in regular fires associated with I.
cylindrica can reduce the abundance of small mammals, herpetofauna and invertebrates
(Chikoye 2003; Van Loan Meeker & Minno 2002; Johnson & Shilling 2009; Peet et al.
1999; Holly & Irvin 2006).
10. Inhibits other species
The extensive rhizome network of I. cylindrica not only allows rapid regeneration of
foliage, but also produces allelopathic root exudates that can inhibit germination and
growth of other plants, including some pines (Hussain et al., 1994, in Ramsey et al.
2003).
11. Other
I. cylindrica can negatively affect forests in a variety of ways. The density of the belowground rhizome network makes I. cylindrica a mechanical hindrance to growth of roots
of native species. The rhizome tips are sharp: they may even penetrate the roots of native
species, leading to damage or mortality by infection (Eussen & Soerjani 1975, in
Daneshgar et al. 2008).
12.0 References
For references please see the GISD Species Profile for Imperata cylindrica (References
Section).
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