Effect of Global Warming on Invasion of Alien Plants in Asia

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Effect of Global Warming on Invasion of Alien Plants in Asia

Rm.Kathiresan
Department of Agronomy, Faculty of Agriculture, Annamalai University
Annamalainagar, Tamilnadu, India – 608 002
rm.kathiresan@sify.com

Abstract:The world Meteorological Day celebrated on March, 2005 chose the theme "Weather, Climate, Water and Sustainable
development". Increasing "Green house gases" and Global warming is expected to trigger suppression of native bio-diversity by
invasive alien weeds in Asia. Alteration in the precipitation and evaporation pattern coupled with frequent inundation and drought,
increasing temperature regimes and sea-level rises that are regarded as consequences of global warming would alter the nature of
vegetation and agriculture in Asia. More number of wet years (annual rainfall excess by ten per cent or more resulting in frequent
inundation or flooding) between 1991 to 2000 compared to the preceding ten years resulted in invasion of rice fields of Cauvery
river delta in India by alien invasive weeds Leptochloa chinensis Nees and Marsilea quadrifolia (L.) by virtue of their amphibious
adaptation to alternating flooded and dry situations. Increasing temperature regimes are also observed to favor invasive potential
of alien weeds in monsoon Asia. Under upland conditions, increasing temperature above 35°C favored the germination and
establishment of Trianthema portulacastrum L, an invasive weed originated in Tropical Africa. Germination of noxious carrot
grass Parthenium hysterophorus L is observed to be triggered by a combination of higher temperature and moderate available soil
moisture. Mean monthly maximum temperatures above 30°C linked to available soil moisture between 40 and 60 per cent favored
the weed's germination and establishment. With increasing temperature and fluctuating precipitation, the weed may pose a severe
threat assisted by globalized trade and agriculture. Similarly, the rate of increase in root bio-mass of invasive alien weed Prosopis
julijlora Swartz (DC) under increasing temperatures is observed to be higher, increasing it's persistence potential and invasive
behavior. The interaction between the invasive potential of these alien weeds and climate change along with some of the
management aspects are discussed.

Introduction:
Unabated use of forest fuel and deforestation have contributed largely for global warming and the
ultimate climate change. The climatic change due to global warming in the last century has been greater
than at any other time during the last millennium. The concentration of carbon dioxide is 33 per cent
higher than it was before the industrial revolution. The sea level has been rising at the rate of 2mm a year
since the beginning of 20th century. Droughts and floods have become more common (The Hindu, 2005).
The year 1990 was the hottest in last century with all other five of the warmest years in the century falling
with in the last 22 years. Scientists agree that the planet’s temperature has risen by 0.5 degree Celsius
since 1900 and will continue to increase at an increasing rate. This sort of a climate change either directly
or indirectly induces changes in land use. Land use changes involve conversion from one type of land
cover to another along with changes in management practices. These include fertilization, type of land
preparation and changes in availability of surface water and river flow. Because of change in the land use
pattern, the terrestrial biosphere of the 21st century would probably be further impoverished in species
richness. The biosphere will be generally more weedier (Walker and Steffen, 1997).

Climate Change and green invaders:
Because of globalization of economies and subsequent movement of people and materials,
invasion by alien species is going to be the second important factor causing loss of biodiversity next to
land use pattern. Changes in atmospheric composition and climate are regarded as long term factors
influencing weed invasion, increasing in relative importance over time.

Increasing temperature regimes and invasive behaviour of weeds:
Introduced from Central America as a drought tolerant species suitable for afforestation in arid
and semi arid zones of India in 1877, Prosopis juliflora has invaded nearly 5.55 million hectares of land
contributing for 1.8% of geographical area of the country. Remote sensing data has predicted the
expansion of the species in Gujarat State of India at the rate of 25km2 per year. Reports predict that by the
year 2020, more than 56 percent of the area in Banni with rich bio-diversity and grassland ecosystem
would be under Prosopis. The most potential invasive feature of the species is typical greater assimilate
partioning towards root, leading to extraordinary enlargement in the root mass with rich food reserves,
aiding rapid and robust regeneration after mechanical lopping or after revival of ecological stress
conditions such as drought or inundation. Studies at Annamalai University has shown that the root
enlargement in Prosopis species is greatly influenced by the temperature regime of the locality. The
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annual increase in root bio-mass is greater in areas where the mean annual temperature is higher than that
in areas of lesser mean annual temperature (Table.1)
Table.1. Temperature regimes and root biomass enlargement in Prosopis
Mean Annual
Mean Annual Increase in Mean Annual Increase in
Temperature °C
Root biomass (kg)
Shoot biomass(kg)
28 1.9 42
30 4.4 47

32 6.2 56

Increase in shoot biomass due to increasing temperature, though observed is not as significant as
the increase in root biomass. The increase in root biomass largely contributes for the weed’s ability to
tolerate climatic extremes such as peak summer associated with high temperature and water scarcity and
peak monsoon winter with water inundation and flooding. This adaptation favors the weed to predominate
over other native flora that are susceptible to any one of the two extremes.
Similar alien weed of wide occurrence in Asia is Parthenium hysterophours. This weed,
originated in Gulf of Mexico and Central South America has invaded India, Pakistan and Srilanka through
cereal and grass seed shipments from U.S.A during 1950s. The weed has been predicted to increase it’s
invasivity due to ecological niches provided by frequent flooding and higher CO2 resulting from global
warming (Adkins et al. 2005). These weeds are observed to posses periodicity of germination and
phenology to evade environmental stress conditions (Kathiresan et al.2005). Prevailing maximum
temperatures between 30-34°C linked to available soil moisture status of 40 to 60 per cent favour
germination and flowering, where as temperatures above 35°C (co -inciding with summer) or excessive
soil moisture (co-inciding with monsoon winter) above 80 per cent is detrimental. The weed has adapted
to complete two generations with in one year, programming it’s phenology between these climatic
extremes (Table.2)
Table 2: Phyto-eco-sociology of parthenium in veeranum ayacut region (Average values for 2000 &
2001)
Months Parthenium
Available soil
Mean Monthly
Important Value
moisture
Max Temperature
percentage
(Percentage)
(°C)
January
-
-
28.65
February 76
55
32.00
March 81
42
32.40
April 84
32
34.35
May 11
29
37.55
June -
25
36.25
July -
29
36.00
August -
40
34.65
September
48
42
33.75
October 51
58
31.65
November 32 81
29.80
December - 86
28.05
A major weed that has been invading irrigated upland agro eco-systems in several tropical Asian
countries is Trianthema portulacastrum. This weed is reported to have been originated from tropical
Africa and has invaded several continents viz., Australia, Africa and Asia (Rawson and Bath, 1984). A
survey conducted in different irrigated upland crops of Veeranum Ayacut in Tamilnadu, India indicate that
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T. portulacastrum predominates as the dominant species in all the three crops viz., sugarcane, sunflower
and gingelly with Important Value Index percentages of 28.73, 26.83 and 25.99, respectively. This weed
tops the list of 15 weed species recorded in all these crops in different locations. (Kathiresan, 2004). One
of the most important character responsible for it’s invasiveness is thermal induction of seed germination
with soil temperature around 35° C favouring synchronized and mass germination of seeds, covering the
soil as a green carpet. In a field study conducted at Annamalai University, it was observed that, increasing
soil temperature with the summer months of June and July triggered the mass germination of seeds of this
weed suppressing the native species (Table 3). The seeds of this weed undergo dormancy during winter
and thermo-induction to break the dormancy require soil temperatures above 35° C (Sundari and
Kathiresan 2001).
Table 3. Thermo - induction of seed germination in Trianthema portulacastrum
Seedling emergence m-2, 15 days
Mean Monthly Max
Month
after land preparation at the
Temperature(°C)
beginning of the month
January 8.5
28.5
February 11.6
30.4
March 16.7
33.1
April 23.4
34.2
May 32.9
36.9
June 85.6
37.3
July 102.4
36.2
August 126.8
35.0
September 20.5
33.9
October -
31.2
November -
28.7
December -
28.1

Rainfall pattern and weed invasion

Global warming directly reflects on rising sea levels due to melting of ice caps and natural
expansion of sea water as it becomes warmer. Consequently, areas adjoining the coast and wetlands could
be frequently flooded and the distribution pattern of monsoon rains gets altered with more intense
downpours, storms and hurricanes. The meteorological data available at the Annamalai University, in tail
end of Cauvery river delta region of Tamilnadu state, India, shows that the average annual rainfall during
the period of 1991 to 2000 has increased by 129 mm compared to that during 1981 to 1990. The record
also reveals that the annual evaporation has reduced by 255 mm from the period between 1981 to 1990 &
1991 to 2000. Further, wet years (years with excess average annual rainfall by more than ten per cent) are
also more frequent between 1991 to 2000 than 1980 to 1990. (Table 4). Phytosociological survey of
floristic composition of weeds in this region reveals the recent invasion of these rice fields by alien
invasive weeds Leptochloa chinensis and Marsilea quadrifolia (Table 5). These two weed species
dominated over the native weeds such as Echinochloa sp. and others by virtue of their amphibious
adaptation to alternating flooded and residual soil moisture conditions prevalent during recent years in this
region (Yaduraju and Kathiresan, 2003; Kathiresan, 2005).
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Table 4. Rainfall and evaporation pattern in the Cauvery river delta region of India
Average annual
Annual
Rainfall defecit /
Year
rainfall (mm)
evaporation (mm)
excess percentage
1981 1192.0 2701.0 -14.86
1982 926.8 2482.0 -33.8
1983 1148.6 2336.0 -18.0
1984 1479.6 1861.5 5.69
1985 1985.2 1971.0 41.8
1986 1257.5 1971.0 -10.18
1987 1438.7 2080.5 2.76
1988 1132.6 2007.5 -19.10
1989 1448.3 2153.5 3.45
1990 1543.0 1971.0 10.21
Average 1355.2 2153.5

1991 1196.4 1934.5 -14.54
1992 1257.3 2007.5 -10.19
1993 2024.1 1934.5 44.58
1994 1349.8 1971.0 -3.59
1995 1124.8 1861.5 -19.66
1996 1701.5 1825.0 21.54
1997 1636.0 1861.5 16.85
1998 1560.1 1861.5 11.44
1999 1434.0 1861.5 2.43
2000 1555.0 1861.5 11.07
Average 1483.9 1898.0

2001 1343.2 1788.5 -4.06
2002 1281.0 2007.5 -8.50
2003 938.7 1825.0
-32.95
2004 1699.5 1745.1 21.26
2005 1745.1 1733.3 24.51
Average 1401.5 1819.88


Leptochloa chinensis owes it’s invasive behaviour to longer life span that extends in to the relay
crop of mung bean after transplanted rice. These two crops differ widely in the soil conditions with
transplanted rice surviving in inundated water, where as mung bean thrives in residual soil moisture below
30 per cent. Leptochloa shows adaptation to both the extremes of climate, with in the same generation.
Marsilea quadrifolia is tolerant to most of the grass killer herbicides used like butachlor. Further, frequent
floods do favour it’s perpetuation.
Wetlands are more prone to weed invasions in part, because they serve as landscape sinks that
accumulate materials resulting from both terrestrial and wetland disturbances (excess water, debris,
nutrients, sediments, salts and other contaminants). Opportunities that make wetland more prone to weed
invasions are that the riparian habitats are subjected to flood pulses and inflows from surface water.
Habitats fed by surface water are low in species richness and the inhabitants are low in quality with co-
efficient of conservatism scoring less than five. (Kercher and Zedler, 2004). Studies in wisconsin show
that wetlands with a history of hydrological disturbance show more widespread invasions (Zedler and
Kercher, 2004). Despite the emphasis on increased influxes as causing disturbance, it has also been
observed that some invasives are abundant where the regions have reduced flood flows. Both increased
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and decreased run off will alter wetland water regimes and the floristic composition could be invaded by
the floating weeds, like Eichhornia crassipes under inundation and Ipomoea aquatica under semi or
complete dry situations of water sheds in Southern India.
Table 5. Floristic composition of weeds in rice fields irrigated by channels in Cauvery river delta
(IVI %), India
Channel I
Channel II
Channel III
Weed species
1992 2002 1992 2002 1992 2002
Echinochloa colonum 25.56 5.45
28.48 4.42 27.52 5.34
Leptochloa chinensis 22.74 28.81 24.81 29.79 23.64 29.50
Cyperus rotundus
17.23 9.13
22.28 7.61 17.01 3.60
Echinochloa crusgalli 8.33 3.44 6.00 2.96 5.80 3.00
Sphenoclea zeylanica 2.02 4.60 0.68 4.47 1.68 5.08
Marsilea quadrifolia
1.46 19.10 0.63 20.98 0.46 21.97

PREDICTING WEED INVASION IN RESPONSE TO GLOBAL WARMING
Range extension and invasion of plant community by both an exotic invader and a native species
though differs in several aspects, have one attribute in common. In both the cases plants invade new
territory through efficient reproduction and dispersal. The long-haul mechanisms of dispersal including
dispersal by wind and vertebrates deserve special attention in predicting large-scale migration of invasive
species. For some, these natural agents are likely to be superseded by human activities. Water hyacinth
(Eichhornia crassipes) native of Brazil, was first traced in history, as to have been gifted as a compliment
to visitors of cotton Expo in New Orleans state of USA in late 1870s. At present, the weed has invaded all
continents except temperate Europe. However, dispersal is only the first phase ensuring entry of an
invader but to pass through successive phases of establishment, expansion and explosion, the species need
to successfully germinate, grow and reproduce. The possibility of a new species establishing itself is
resisted by the existing vegetation and favoured by the type and frequency of disturbances in that
particular habitat. In general, the disturbances of established vegetation by fire, flood and grazing
mammals render it more vulnerable for invasion. This indicates that climate changes in future that would
suppress native vegetation would increase the probability for open niches in the ecosystem favouring
invasion by alien weeds. An alien species getting naturalized involves escaping from cultivation and
establishing self sustaining populations in the wild. This critical step before the species becoming invasive
was studied for different introduced species in Australia and New Zealand by Williams et al. 2004. Results
indicated the difference in naturalization rates for the same species in these two countries. This
emphasizes the importance of role of climate and climate matching in predicting invasive species. Though,
weed invasion has been observed to be influenced by the cultural practices at the field scale, in the larger
context, regional weed invasions have been observed to be mainly influenced by climatic factors. Global
warming with the causal and consequent effect of elevated carbon dioxide levels, increased temperature
regimes, changes in precipitation pattern might influence the invasive potential of different weed species
in various parts of the globe under differing situations. Accordingly, any programme for prediction and
prevention of weed invasion should consider the climatic factor as one of the vital component.
Several systems have been developed to help predicting invasion for the purpose of plant
quarantine. All these systems involve applying a series of questions to be answered from the view point of
a suspected weed species. Based on the answer, either the weed is eliminated from the list of suspected
invader or is kept under observation, monitoring and/or preventive programme. In certain cases the
invasive weeds are also classified as “highly invasive”, “moderately invasive” and “possibly/potentially
invasive”. These systems satisfactorily predict whether an individual species will/will not become invasive.
In addition to the Invasive potential A (rhizomatous, climbing, small and readily dispersed seed) and
Damage potential B (competitive, toxic etc), Geography potential C (the magnitude of range of geography
or ecologies to be invaded) and Entry potential D (mode of entry either deliberate or accidental) are also
considered, to arrive at a product of A X B X C X D, for finally ranking the suspected weed’s invasive
nature (Parker 2001), In this new approach, inclusion of Geographic potential and Entry potential based on
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the major predictive character of weediness elsewhere imparts better scope for predicting the invasive
alien species. It would be more appropriate to include climate matching and adaptive features of weed
species along with Geographic potential to confer more precision to the predictive models.

Management

Managing the alien invaders include several key components, such as Weed Risk Analysis
involving pre-entry as well as post entry assessment, modeling for matching climate change and invasive
behaviour of weeds, prevention by raising awareness etc. But a simple but vital clue is to continue to
practice traditional farming leads and techniques. Raising a green manuring legume in the summer in rice
fields that would be ploughed in situ at the time of preparing fields for rice has significant impact on
suppressing Leptochloa in rice. Similarly allowing pigs to burrow in the puddled field at the time of land
prepation for rice helps suppressing perennials such as nut sedge and Marsilea. Plantation of Avenue
Trees Such as Tamarind (Tamarindus indica L) largely keeps the road sides and social sites free from
Parthenium infestation
.
References:
1. Adkins S.W., S.C.Navie and K.Dhileepan. 2005. Parthenium Weed in Australia; Research
Progress and prospects. In. Proc. Of second International Conference on Parthenium
Management Bangalore, India. P.11 – 27.
2. Kathiresan, RM. 2005. Case study on Habitat management and rehabilitation for the control
of alien invasive weed (Prosopis juliflora), Report submitted to water Resource
Organization, Public works Department, Tamilnadu, India. (Unpublished)
3. Kathiresan,R.M., I.Gnanavel, R.Anbhazhagan, S.P.Padmapriya, N.K.Vijayalakshmi and
M.P.Arulchezhian.,2005. Ecology and control of Parthenium invasion in Command Area.In:
Proceedings of Second International Conference on Parthenium Management, Bangalore,
India. P.77-80.
4. Kathiresan RM. 2004. Invasive weeds in agro-ecosystems of South India. In: Abstracts of
national workshop on Invasive alien species and biodiversity in India, Banaras Hindu
University, Varanasi, India,p.149.
5. Kercher S.M. and Zedler J.B. 2004. Multiple disturbances accelerate invasion of reed
canary grass (Phalaris arundinaceae L.) in a mesocosm study. Oecologia 138:455-464.
6. Parker C.2001. Prediction and prevention of plant invasions – what is feasible. BCPC Symposium
proceedings no. 77. The world’s worst weeds, 91-106.
7. Rawson, J.E. and S.J. Bath. 1984. Chemical control of giant pigweed, Sesbania pea and
fierce thorn apple in sorghum. Queensland Journal of Agricultural & Animal Science,
38(1): 13-19.
8. Sundari A. and Kathiresan RM. 2001. Weed biology – Periodicity of germination of Trianthema
portulacastrum. In: Abstracts of Biennial Conference on Eco –friendly weed management
options for sustainable Agriculture, Indian Society of Weed Science, Bangalore, India. pp.61.
9. The Hindu 2005. Report from The Director and meteorologist, Regional Meteorological Centre,
Chennai, India, In: ‘The Hindu” popular daily dated 26th March 2005.
10. Walker B. and Steffen W. 1997. An overview of the implications of global change for natural and
managed terrestrial ecosystems. Conservation Ecology (online) 1 (2) : 2.
URL:http/www.consecol.org/voll/iss2/art2
11. Williams P.A., Duncan R.P. and Randall R.P. 2004. Plant naturalization rates as a risk assessment
tool. In: Abstract of 4th International Weeds Science Congress, Durban, South Africa, June 20 -
24, p.66
12. Yaduraju N.T. and Kathiresan RM. 2003. Invasive Weeds in the Tropics. In Proceedings:
19th Asian Pacific Weed Science Society Conference, Manila, Philippines, vol. I. p. 59-
68.
13. Zedler J.B. and Kercher S. 2004. Causes and opportunities of Invasive Plants in Wetlands:
Opportunities, Opportunists, and Outcomes. Critical Reviews in plant sciences, 23(5): 431-452.

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