Advanced
Weed Population Dynamics under Climatic Change
Weed Population Dynamics under Climatic Change
Weed & Turfgrass Science. 2014. Sep, 3(3): 174-182
Copyright © 2014, The Korean Society of Weed Science and The Turfgrass Society of Korea Archive
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License & #160; (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, & #160; and reproduction in any medium, provided the original work is properly cited.
  • Received : September 01, 2014
  • Accepted : September 11, 2014
  • Published : September 30, 2014
Download
PDF
e-PUB
PubReader
PPT
Export by style
Share
Article
Author
Metrics
Cited by
About the Authors
Md. Shahidul Haque Bir
Department of Crop Science, Chungnam National University, Daejeon 305-764, Korea
Min Yong Eom
Department of Crop Science, Chungnam National University, Daejeon 305-764, Korea
Md. Romij Uddin
Department of Agronomy, Bangladesh Agricultural University Mymensingh 2202, Bangladesh
Tae Seon Park
National Institute of Crop Science, RDA, Suwon 441-707, Korea
Hang Won Kang
National Institute of Crop Science, RDA, Suwon 441-707, Korea
Do Soon Kim
College of Agricultural and Life Sciences, Seoul National University, Seoul 151-921, Korea
Kee Woong Park
Department of Crop Science, Chungnam National University, Daejeon 305-764, Korea
parkkw@cnu.ac.kr

Abstract
This paper provides some of the scientific background on how projected environmental conditions could affect weeds and weed management in crops. Elevated CO 2 levels may have positive effects on crop competitiveness with C 4 weeds, but these are generally outnumbered by C 3 species in weed populations. Moreover, higher temperatures and drought will favor C 4 over C 3 plants. The implementation of climate change adaptation technologies, such as drought-tolerant germplasm and water-saving irrigation regimes, will have consequences for crop–weed competition. Rainfed production systems are thought to be most vulnerable to the direct effects of climate change and are likely to face increased competition from C 4 and parasitic weeds. Biotic stress-tolerant crop cultivars to be developed for these systems should encompass weed competitiveness and parasitic-weed resistance. In irrigated systems, indirect effects will be more important and weed management strategies should be diversified to lessen dependency on herbicides and mechanical control, and be targeted to perennial rhizomatous (C 3 ) weeds. Water-saving production methods that replace a weed-suppressive floodwater layer by intermittent or continuous periods of aerobic conditions necessitate additional weed management strategies to address the inherent increases in weed competition. Thus, climatic conditions have a great effect on weed population dynamics all over the world.
Keywords
Introduction
Global agricultural systems vary considerably in their sensitivity to climate and in their vulnerability to changes in the climatic regime. Intensive farming systems are generally considered to have low sensitivity to climate change because a given change in temperature or rainfall has a modest impact ( Chloupek et al., 2004 ) and because the farmers have resources to adapt and compensate by changing management. These systems may therefore respond favorably to a modest climatic warming ( Olesen and Bindi, 2002 ). On the other hand, some of the low input farming systems currently in marginal areas may be severely affected by climate change ( Reilly and Schimmelpfennig, 1999 ; Darwin and Kennedy, 2000 ). In particular, an increase in extreme events of both temperature and rainfall will affect the vulnerability of agroecosystems to climatic conditions.
Climate change is expected to affect agriculture very differently in different parts of the world ( Parry et al., 2004 ). The resulting effects depend on current climatic and soil conditions, the direction of change, and the availability of resources and infrastructure to cope with change. There is a large variation in climatic conditions, soils, land use, infrastructure, political and economic conditions ( Bouma et al., 1998 ). These differences are also expected to influence the responsiveness to climatic change ( Olesen and Bindi, 2002 ).
Changes in atmospheric CO 2 levels, rainfall, temperature and other growing conditions will affect weed species distribution and their competitiveness within a weed population and within a rice crop. This may necessitate adaptations in crop management practices, which in turn will affect weed growth or the proliferation of certain species. Environmental conditions also have a large impact on the effectiveness of weed management operations such as chemical and mechanical control. The magnitude of these effects will largely depend on the extent to which environmental conditions change locally and regionally. Major global changes will comprise further increases in atmospheric greenhouse gases and likely changes in temperature (> 0.2℃ per decade), soil degradation, and competing claims for land and water ( IPCC, 1996 ). Thus, an experiment will be conducted considering the following objectives: (a) to monitor the effect of temperature on weed population growth; (b) to evaluate the direct effects of CO 2 on weed competition, abundance, and distribution; (c) to measure the effect of rainfall and humidity on the differential response of weeds.
- Direct effects on weed competition, abundance, and distribution
The CO 2 concentration in the atmosphere will increase. This will affect weed species in different ways, depending on their photosynthetic pathways. Under drought and high temperatures, plants with the C 4 carbon fixation pathway have a competitive advantage over plants possessing the more common C 3 pathway. This competitive advantage of C 4 weeds diminishes or even reverses under conditions of high nitrogen or CO 2 concentrations ( Bazzaz and Carlson, 1984 ). The C 4 - type species are most dominant in upland ecosystems (52%) and occur least frequently in the lowlands (23%). For a C 3 crop like rice, elevated CO 2 levels may have positive effects on crop competitiveness with C 4 weeds ( Fuhrer, 2003 ; Patterson et al., 1999 ), and tolerance to Striga infection ( Watling and Press, 2000 ). Yet, empirical evidence also shows that higher CO 2 levels stimulate biomass production of both C 3 and C 4 grasses: C 3 grass species had a greater increase in tillering while C 4 grass species had a greater increase in leaf area under conditions of elevated CO 2 concentrations ( Wand et al., 1999 ). Tillering and leaf canopy development are known important traits affecting inter-specific competition. Increased CO 2 levels are likely to be accompanied by higher temperatures favoring C 4 weeds over C 3 crops ( Fuhrer, 2003 ). The same outcome can be expected under increased or prolonged drought conditions ( Bjorkman, 1976 ). Although precise changes in rainfall are difficult to predict, precipitation will likely become more erratic with more frequent droughts and floods ( Giannini et al., 2008 ). Consequently, weeds adapted to these conditions might have a comparative advantage in rainfed rice. Apart from drought-tolerant C 4 weeds, parasitic weeds that thrive in erratic and low rainfall environments (e.g., Striga hermonthica ) or temporarily flooded conditions (e.g., Rhamphicarpa fistulosa ) could benefit from future climate extremes.
Temperature changes will affect the geographic distribution of weeds ( Patterson et al., 1999 ), with some species moving to higher latitudes ( Patterson, 1995 ) and altitudes ( Parmesan, 1996 ). For instance, Striga spp. might extend their geographical range because of climate change ( Mohamed et al., 2006 ). Ecological niche modeling suggests that the highly diverse Striga species might expand into moderate climate zones ( Mohamed et al., 2007 ). Striga asiatica is relatively insensitive to temperature ( Patterson et al., 1982 ) and its distribution may be more affected by changes in the geographical range of the host crop than directly by temperature ( Cochrane and Press, 1997 ). Phoenix and Press (2005) argue that this could be true for parasitic weeds in general.
- Indirect effects on crop management adaptations and weed management effectiveness
Water is becoming a scarcer resource in many parts of agricultural land ( Seckler et al., 1999 ) and rice varieties and cropping methods need to be adapted accordingly ( Ingram et al., 2008 ). For upland rice, drought tolerance will be important not only just to reduce losses due to moisture stress but also to maintain or improve the crop’s competitiveness against weeds ( Asch et al., 2005 ). In lowland rice, approaches to conserve irrigation water, such as aerobic rice and alternate wetting and drying, may be adopted, but will have consequences for weed management ( Krupnik et al., 2011 ), requiring more crop management skills and better access to production resources. Haden et al. (2007) observed a shift in weed populations, with an increased incidence of sedges under reduced flooding regimes. Where season-long flooding of lowland rice fields is replaced by only temporary flooding or aerobic conditions, increased weed infestations are observed ( Krupnik et al., 2011 ). Hand-weeding requirements may increase by up to 35% with temporary, rather than permanent flooding in lowland systems ( Latif et al., 2005 ). Maintaining a floodwater layer to suppress weeds is likely to become increasingly difficult in many areas as water becomes scarcer; consequently, farmers lacking the means for effective weeding are likely to suffer severe yield losses ( Barrett et al., 2004 ).
The effectiveness of weed management is also hypothesized to change along with environmental conditions. Extreme weather may increase the risk of herbicides either causing crop damage or not being effective ( Patterson et al., 1999 ). Increased temperatures affect herbicide persistence in the soil and the ‘windows’ for herbicide effectiveness ( Bailey, 2004 ), while herbicides may be diluted and cease to be effective if rainfall becomes more frequent or intense ( Kanampiu et al., 2003 ). Herbicide use is expected to increase soon and with it, more resistant weed ecotypes are likely to emerge. Environmental changes can accelerate this. Raised CO 2 levels, for instance, have been shown to increase the tolerance of weeds to herbicides ( Ziska et al., 1999 ).
High CO 2 environments may also stimulate belowground root growth relative to aboveground shoot growth ( Ziska, 2003 ) and favor rhizome and tuber growth of (in particular, C 3 ) perennial weeds ( Oechel and Strain, 1985 ) rendering their control more difficult ( Patterson, 1995 ; Patterson et al., 1999 ). Increased tillage, for instance, could then lead to a multiplication of vegetative propagation material ( Ziska, 2008 ). For rice production in Africa, this could mean increasing problems with perennial lowland weeds like Oryza longistaminata, Leersia hexandra, Bolboschoenus maritimus, Sacciolepis africana , and Cyperus halpan . Other perennial weeds with difficult-tocontrol belowground structures (e.g., Imperata cylindrica and Cynodon dactylon in the uplands and Cyperus esculentus and C. rotundus on upland and hydromorphic soils) are all C 4 types.
- Differential response of weeds to elevated CO2
Over the past three decades, many experiments have tested the effects of higher atmospheric CO 2 on weeds with C 3 and C 4 photosynthetic pathways. Some examples from an early review by Patterson (1995) indicate significant variations in response to CO 2 , both within a species and between species, depending on experimental conditions, such as temperature, light, availability of water and nutrients. While the variability in plant responses is large, C 3 weeds generally increased their biomass and leaf area under higher CO 2 concentrations compared with C 4 weeds. In view of such results, it could be predicted that C 3 weeds, like Parthenium ( Parthenium hysterophorus L.) and Chromalaena [ Chromalaena odorata (L.) R. M. King & H. E. Robinson] will be much more competitive in an increased CO 2 environment, independently of temperature and rainfall effects.
Ziska and Bunce (1997) compared the effect of elevated CO 2 levels on the growth and biomass production of six C 4 weeds ( Amaranthus retroflexus L., Echinochloa crus-galli (L.) P. Beauv., Panicum dichotomiflorum Michaux, Setaria faberi Herrm., Setaria viridis (L) P. Beauv., Sorghum halapense (L) Pers.), and four C 4 crop species ( Amaranthus hypochondriacus L., Saccharum officinarum L., Sorghum bicolor (L) Moench, and Zea mays L.). Eight of the ten C 4 species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively
Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at higher CO 2 , which also resulted in significant increases in whole plant biomass for four C 4 weeds ( A. retroflexus, E. crusgalli, P. dichotomiflorum, S. viridis ) relative to the ambient CO 2 conditions. Leaf water potentials for three of the species ( A. retroflexus, A. hypochondriacus, Z. mays ) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. This study confirmed that C 4 plants may respond directly to increasing CO 2 in the atmosphere, and in the case of some C 4 weeds (e.g., A. retroflexus ), the photosynthetic increase is similar to those published for C 3 species.
Of the 15 crops, which supply 90% of the world’s calories, 12 have the C 3 photosynthetic pathway. In contrast, 14 of the 18 ‘world’s worst weeds’ are C 4 plants ( Patterson, 1985 ). The consensus of the above and other similar studies is that the greater majority of weeds in the world, which are C 3 plants, will benefit from increased CO 2 levels under climate change, while most tropical grasses, which are C 4 plants, are not likely to show greatly increased growth in higher CO 2 conditions However, because C 4 plants are generally more tolerant of heat and moisture stress, the simple notion that climate change will only benefit C 3 plants may not be accurate.
- How will ‘colonizing species’ (weeds) react to changing climate?
Weeds are opportunistic colonizing species or pioneers of secondary succession that are well adapted to grow in locations where disturbances, caused either by humans or by natural causes, have opened up space. Species can become weeds, because they are competitive, adaptable, highly fecund, and are able to tolerate a wide range of environmental conditions, including those in agricultural fields, or disturbed habitats. A set of common biological characteristics ( Baker, 1965 ) allows weeds to colonize disturbed habitats, to form extensive populations and, sometimes, to dominate disturbed landscapes.
However, a species may become an invader of landscapes only if a chance combination of circumstances makes its attributes particularly advantageous to its growth and survival. In many cases, this opportunity arises because of a lack of specific parasites or herbivores i.e., ‘natural enemies,’ which gives them an advantage over crops or native flora ( Naylor and Lutman, 2002 ). In terms of evolutionary success i.e., continuation of a genetic line over time, most weeds are highly successful, because of their high reproductive capacity and the range of habitat they can occupy. Thus, in terms of the Darwinian concept of ‘struggle for existence,’ weeds, as a class, are the most successful plants that have evolved on our planet ( Auld, 2004 ). Weeds are likely to possess many preadaptations at the molecular, biochemical or whole plant level to respond more positively to climatic change, including elevated CO 2 and increased temperature, than other plants, as discussed below.
- Weed-crop competition under climatic change
The differential responses of CO 3 and CO 4 plants to increasing CO 2 are especially relevant to weed-crop competition in agroecosystems. However, studies on competition outcomes between C 3 crops and C 4 weeds, or vice versa, are limited in the literature. In general, elevated CO 2 levels would stimulate the growth of major C 3 crops of the world; the same effect is also likely to increase the growth of both C 3 and C 4 weeds. In all probability, this would lead to increased weed-crop competition, negating some of the otherwise beneficial effects of CO 2 ‘fertilization’ of the C 3 crops and their yields. Some examples of relevant crop/weed competition studies are discussed below:
Carter and Peterson (1983) found that Festuca elatior L., a C 3 grass, out-competed Sorghum halepense (L.) Pers., a C 4 grass, in mixed cultures, under both ambient CO 2 levels and elevated CO 2 , even under temperatures unfavorable to C 3 photosynthesis (between 25 and 40℃). The authors predicted that global CO 2 enrichment would alter the competitive balance between C 3 and C 4 plants and this may affect seasonal niche separation, species distribution patterns, and net primary production within mixed communities.
Ziska (2000) evaluated the outcome of competition between ‘Round-up Ready’ Soybean ( Glycine max L.) and a C 3 weed (lambsquarters, Chenopodium album L.) and a C 4 weed (redroot pigweed, Amaranthus retroflexus ), grown at ambient and enhanced CO 2 (ambient+250 μL L -1 ). In a weed-free environment, elevated CO 2 resulted in increased soybean growth and yield, compared to the ambient CO 2 conditions. However, soybean growth and yield were significantly reduced by both weed species at both levels of CO 2 . With lambsquarters, at elevated CO 2 , the reduction in soybean seed yield relative to the weed-free control increased from 28 to 39%. Concomitantly, the dry weight of lambsquarters increased by 65%.
Conversely, for pigweed, soybean seed yield losses diminished with increasing CO 2 from 45 to 30%, with no change in weed dry weight. This study suggests that rising CO 2 could alter yield losses due to competition from weeds, and that weed control will be crucial in realizing any potential increase in the yield of crops, such as soybean, as climate change occurs.
Alberto et al. (1996) , studied competition outcomes between rice and Echinochloa glabrescens L., which is a C 4 weed, using replacement series mixtures at two different CO 2 concentrations (393 and 594 μL L-1) under day/night temperatures of 27/21℃ and 37/29℃. Increasing the CO 2 concentration, at 27/21℃ resulted in a significant increase in above ground biomass (+47%) and seed yield (+55%) of rice, averaged over all mixtures. For the C 4 weed, higher CO 2 concentration did not produce a significant effect on biomass or yield. When grown in mixture, the proportion of rice biomass increased significantly relative to that of the C 4 weed in all mixtures at elevated CO 2 indicating the increased competitiveness of rice. However, under elevated CO 2 levels and the higher temperature regime, competitiveness and reproductive stimulation of rice was reduced compared to the lower growth temperature, suggesting that while a C 3 crop like rice may compete better against a C 4 weed at elevated CO 2 alone, simultaneous increases in CO 2 and temperature could still favor a C 4 species.
- Weed distribution and abundance in the range under climatic change
Bunce (2000) indicates that elevated CO 2 levels are likely to increase the ability of plants to tolerate both high and low temperatures. However, the responses are linked with moisture availability through modified rainfall patterns, and possibly other factors like nitrogen deposition. Most colonizing species have wide ecological amplitudes i.e., the capacity of a species to establish in various habitats along an environmental gradient, and are already adapted to a broad range of conditions under which they can thrive and perpetuate. This innate ability to tolerate varying and extreme conditions will enable weeds to benefit under climate change, at the expense of less ‘weedy’ species. Boese et al. (1997) established the increased tolerance for low temperatures under elevated CO 2 for several chillingsensitive plants of tropical or sub-tropical origin. The possible reasons were improved plant-water balance, less severe wilting, and less leaf damage under elevated CO 2 compared with ambient levels.
Temperature is recognized as a primary factor influencing the distribution of weeds across the globe, particularly at higher latitudes. Increased temperature and precipitation in some parts of the earth may provide suitable conditions for stronger growth of some species, which are currently limited by low temperatures. The distribution of some tropical and subtropical C 4 species could shift northwards. This would expose temperate zone agriculture to previously unknown, aggressive tropical colonizers ( Parry, 1998 ), particularly C 4 grasses.
Similar range shifts are predicted in the southern hemisphere, due to climate change. For instance, in Australia, climate predictions for the next 30+ years are for a general increase in mean temperatures with a larger increase in mean minimum temperatures, as well as a reduction in frost days ( CRC, 2008 ). In the tropical north of Australia, an increase in rainfall is expected, especially in the northwest. Reduced rainfall is predicted for southwestern Western Australia, and generally, across eastern and southeastern Australia. In all areas, an increase in extreme events, including droughts, floods, severe storms, and extended wet seasons is expected. With such climate predictions, models indicate a southward range shift of major invasive plants, with tropical and subtropical species moving south, and temperate species being displaced southward. An example is a modelling study on current and projected distribution of Prickly Acacia (Acacia nilotica (L.) Willd. ex Delile), a woody legume, previously introduced for landscape improvement, now spreading in Australia. The modelling by Kriticos et al. (2003b) indicated the potential for significant (a) southward shift of Prickly Acacia, favored by increasing temperature; and (b) spread further inland, favored by increased water use efficiency (WUE), under elevated CO 2 .
These and other studies ( Kriticos et al., 2003a , b ; 2006 ) have indicated significant and increased risks of spread and invasion of new areas by well-known aggressive colonizers. In Australia, species currently restricted to the lowlands, such as Lantana ( Lantana camara L.) are expected to move into higher altitude areas. Frost-intolerant species such as rubber vine ( Cryptostegia grandiflora R. Br.) and Chromalaena odorata could also shift their ranges significantly further south ( Kriticos et al., 2003a ; CRC, 2008 ). However, the actual spread of weeds may lag behind the predicted spread, depending on factors such as the dispersal potential of individual species and any management efforts that are taken to slow their spread.
Increased rainfall may also cause range shifts in the distribution of some weeds that are currently limited to higher rainfall zones. Reduced rainfall will also reduce the growth of pastures and crops, increasing bare ground and reducing canopy cover, which favors weed invasion. Increased extremes, e.g., long drought periods interspersed with occasional very wet years, will worsen weed invasion, because established vegetation, both native and crops, will be weakened, leaving areas for invasion. For example, mass germination and spread of Prickly acacia occurred in the past after a series of very wet years ( Kriticos et al., 2003b ). More severe cyclones will both disperse weed seeds through wind and floods, and open up gaps for weed invasion in areas of pristine native vegetation, especially in the wet tropics.
- Adapting mechanisms to climatic change
It is clear that both crops and weeds will respond to climate change, but the overall winners of their competition in the field will be the colonizing species, because of their superior adaptations and wide ecological amplitudes (i.e., the limits of environmental conditions within which an organism can live and function). Although it is not possible to be specific, under climate change, weed management will become more important in the future at every scale, from farmlands to regional landscapes. As colonizing species become abundant, and possibly more aggressive in many regions, humans will have to adapt to manage weed populations more effectively, to maintain productive landscapes, and achieve food security.
Control of weeds, pests, and diseases are all likely to be more difficult and more expensive under climate change, and there will have to be more emphasis on regional cooperation for preventing the spread of certain weeds, pests, and diseases (as in the case of control of diseases, such as HIV). Given that some well-known invasive species are likely increase their biogeographical ranges, and other, relatively mild species may become aggressive invaders, all countries need to be able to conduct risk assessments at the appropriate level, for national planning to reduce the new threats posed by weeds. Global and regional co-operation is essential to establish new networks and the capacity to implement early detection and rapid response systems. Increased gathering of information, through local and regional surveys of distribution and abundance of potential invaders, sharing of such information and increased border protection of countries through quarantine, are likely to be of greater importance in the future. More effective integration of on-ground control methods (manual, mechanical, chemical, and biological control) with broader pest control at the farm level will be part of the solutions. What this means is that natural resource managers need to cooperate more with each other, and weed managers and researchers need to be even more effective than before. A new paradigm in weed management might include the view: ‘Do what you have been always doing… better…’ because the stakes are much higher now.
Projected changes in climate and crop yields in the latter part of the 21st century suggest that there will be yield increases in mid and high latitudes (Canada, Japan, European Union, and New Zealand). These regions are recognized as having sufficient technology-based adaptive capacity to face the changing global climate. In contrast, yield decreases are predicted for tropical and sub-tropical regions of lower latitudes, mainly developing countries, including the Indian sub-continent, Middle East and Southeast Asia, with important regional differences ( Parry, 1998 ). In the latter regions, presently characterized by persistent poverty and food insecurity, temperature maxima are already near the optimum under the current climatic conditions. Modelling indicates that warming may lead to decreased yield and production with an increase in risk of hunger ( IPCC, 1996 ).
The agricultural systems in many developing countries are more vulnerable to climate change, because they are dependent on declining natural resource bases, are labor intensive and less capital and technology dependent. The increasing population pressure on natural resources in developing countries is well known; it has already led to pronounced degradation of land and water resources and increased the risk of hunger. Under this scenario, in Africa, predictions are that by 2080, cereal production will decrease by 10% and the consequent risk of hunger will increase by 20%, although such effects can be partly offset by various farmer adaptations, technological changes, and CO 2 fertilization effects ( Parry, 1990 ; Rosenzweig and Hillel, 1998 ).
Nevertheless, it is also predicted that the aggregate agricultural production in developing countries may not change much, as climate change occurs. Despite this prediction, there are specific regions within some countries that would be disproportionately affected by climate change, leading to increased poverty. Most experts agree that the future of global agriculture will be shaped by the: (a) dynamics of change and developments in science and technology; (b) sharing of knowledge and transfer of technology to developing countries; (c) expected production gains in developed countries (mainly Europe); and (d) impacts of trade liberalization.
Technically, adapting to climate change will require significant transformation of agriculture production across the globe, by tapping three main sources for growth: (a) expanding the land area, (b) increasing the land cropping intensity (mostly through irrigation), and (c) boosting yields. The view that we may be approaching the ceiling for all three sources is not supported at the global level, although severe problems exist in specific countries and even whole regions ( Parry, 1990 ). There will be major changes of land use, probably involving changes in farming locations. For instance, in tropical and sub-tropical countries, flood-prone areas will be less attractive to cropping, because of increased rainfall and flooding frequency. On the other hand, areas previously not farmed, due to varying degrees of aridity, salinity or low productive potential, may become important, also due to modified rainfall patterns.
In temperate countries, global warming will reduce climatic constraints on agriculture, which is likely to expand and extend into uplands. In Europe, a 10℃ warming may raise climatic limits to cultivation by approximately 150m ( IPCC, 1996 , 2001 ). Changes in the types of crops grown are also likely in regions where there are substantial increases in the temperature of the growing seasons, and in areas where agricultural productivity is currently limited by temperature. In many situations, tropical and sub-tropical crops with higher thermal requirements would become more attractive. In all areas of the world, there will be a need to have stress tolerant and hardy crop cultivars, including more drought-tolerant cultivars, to face the uncertainties of climate change. As rainfall patterns change and areas become prone to drought, irrigation will be crucial to maintain world food supplies and its role is expected to increase under climate change. One in five developing countries will face water shortages and water availability is already critical in west Asia and north Africa and will be so also in south Asia in 2030 ( IPCC, 1996 , 2001 ). Greater efficiency in water use needs to be achieved, and a new irrigation infrastructure will have to be installed, to substitute for moisture losses due to increased transpiration.
Maintaining soil fertility will be challenging, because in some areas, increased rainfall will cause increased leaching, while in other areas, warming may increase productive potential, so that yields can be maintained without additional fertilizer. Adopting farming methods that reduce the costs of production and minimize environmental damage, while maintaining or even increasing production will be crucial. In this regard, no-till or conservation agriculture, which can raise crop yields by 20-50%, will have a major role under climate change. Experts agree that 80% of increased crop production in developing countries still has to come from intensification of agriculture, which involves: (a) increased cultivable land; (b) higher yield crops; (c) increased crop diversification and multiple cropping; and (d) shorter fallow periods. However, regions other than tropical Latin America and Sub-Saharan Africa face a shortage of suitable land, and in these regions intensification through improved management and technologies will be the main source of production growth. The development and dissemination of new science and technology-based solutions will be much sought after for more holistic and integrated pest and weed management. Taking ‘no regrets’ actions, i.e., undertaking those strategies that make sense for reasons other than climate change, are seen as important. Two such approaches are breeding more allelopathic crops and modification of crops by introducing genes that will confer more competitiveness, allied with yield components, and increased resistance to pests.
In the past, environmental policies for agriculture have traditionally focused largely on practices of soil conservation, reducing land and water quality and reducing the impacts of excessive use of herbicides and pesticides in farming landscapes. More recently, agriculture has turned attention to conserving biological diversity on rural landscapes.
Given that agriculture is a major contributor of the greenhouse gases methane and nitrous oxide, it seems prudent to expand these policies to limit the emissions of CO 2 , CH 4 , and N 2 O from agricultural practices. It is also necessary to encourage agriculture to more aggressively adopt and expand on agroforestry opportunities for carbon sequestration benefits. On a farm level, this will require revitalizing wellestablished conservation farming practices, including avenue cropping, minimum tillage, allelopathic crop residues, and similar ecological approaches to holistic management of populations of weeds, pests, and pathogens.
Humans must act to reduce the primary root cause: the high rate of CO 2 emissions, by a variety of approaches, such as decreased burning of fossil fuels, eradicating large-scale deforestation, and reclamation of large wilderness areas for agricultural or other human uses. Among the most feasible actions to mitigate the CO 2 buildup involve some combination of conserving energy, substituting alternative energy sources (e.g., solar, wind and hydropower) for fossil fuels, and reducing the deforestation occurring in the tropics.
The trend of increasing concentrations of greenhouse gases and an enhanced greenhouse effect is likely to continue in the coming decades, presenting serious threats to both agricultural systems and natural ecosystems. Climate change is therefore the biggest challenge faced by humanity. The response of crops, weeds, or natural vegetation communities is inexorably linked to the climate modifications that humans have exacerbated. This review has provided an overview of some key issues and the complex and multiple-driver nature of global change.
Overall, climate change can be expected to favor invasive plants over established, and slow growing native vegetation, especially if accompanied by an increase in extreme conditions, such as droughts alternating with very wet years. Pioneering species with various physiological adaptations and wide ecological amplitudes are better equipped to adapt to new climatic conditions. Weeds generally have excellent propagule dispersal mechanisms, often by human activities or by birds, and are likely to spread rapidly into new areas, quickly exploiting changing climatic conditions that favor their establishment.
More effective management solutions will therefore be required to reduce the threat posed by aggressive colonizers, which can make production of food and management of land and water resources much more difficult. Global change is a somewhat deceptively simple expression for what is actually an exceedingly complex array of dynamic processes and specific interactions and manifestations in different regions ( Rosenzweig and Hillel, 1998 ). Climate change, sea level increases, higher CO 2 concentrations, UV radiation, and tropospheric ozone are but a few of the potentially fateful factors involved. In dealing with an issue as complex as climate change, there are many uncertainties, including the disordered behavior of the physical climate and our inadequate understanding of that system, especially in regard to the interactions of oceans, clouds, and ice. Still other uncertainties are the fast pace and unknown directions of future social, political, and technological changes. Such uncertainties and unpredictable developments will affect how the Earth’s ecosystems and our agricultural landscapes respond to climate change, and ultimately, how humans will respond.
However, climate is not the only factor that will be changing as the 21st century unfolds. Population growth and varying economic and technological changes are likely to affect the environment no less than will climate change per se . Furthermore, the socio-economic and technological conditions will seriously interact with agriculture as well, and our ability to sustain effective production, whilst ensuring sustainable land use. To define how and what we may realistically achieve is a problem in itself, but taking no action is not an option.
Conclusion
Although the uncertainties of future climate changes are large, we already know that changes will alter the balance between weed species, rice production systems, and ecosystems. Irrigated systems are likely to suffer mainly from the indirect effects of climate change. In these systems, herbicides are the dominant weed control method and they are likely to become less effective due to CO 2 increases and more frequently occurring weather extremes. Moreover, water-saving production methods in response to water-scarcity will be implemented in these systems and cause severe increases in weed competition. We hypothesize that in irrigated, temperate rice systems, temperature and rainfall variability increases will have less impact than CO 2 increases. Higher CO 2 concentrations will probably make rice and C 3 weed species (particularly rhizomatous perennials such as Oryza longistaminata, Leersia hexandra, Bolboschoenus maritimus, Sacciolepis africana , and Cyperus halpan ) more competitive against C 4 weeds, while mechanical control will become more difficult due to the stimulating effect on belowground growth.
Rainfed production systems will be impacted by the direct effects of climate change as these systems harbor most of the C 4 and all the parasitic weed species. They are most vulnerable to rainfall irregularities and soil degradation. Here we suggest that the area infested with parasitic weeds Striga asiatica , S. hermonthica, S. aspera and Rhamphicarpa fistulosa could increase, particularly in places where soil degradation and erratic rainfall become prevalent. Furthermore, because of their likely higher drought and heat tolerance, C 4 species like the perennial grasses Imperata cylindrica, Paspalum scrobiculatum , and Cynodon dactylon , the annual grasses Rottboellia cochinchinensis, Digitaria horizontalis, Eleusine indica, Dactyloctenium aegyptium, Pennisetum purpureum , and Echinochloa colona , and the sedges Fimbristylis littoralis (annual), Cyperus rotundus and C. esculentus (perennial) are likely to become more competitive in rainfed rice. Droughtand heat-tolerant rice cultivars will gain popularity. Should these tolerance traits not be combined with a certain degree of weed competitiveness or resistance against parasitic weeds, adaptive and competitive (C 4 and hemiparasitic) weeds might prevail.
While atmospheric CO 2 levels are certain and temperatures are highly likely to increase, the spatial distribution of future rainfall remains much more uncertain ( Giannini et al., 2008 ). This uncertainty about such a vital vegetative growth factor, combined with a lack of understanding about the interaction between different environmental factors that are likely to change, means that any predictions of future distribution of plant species must be evaluated carefully. The net effect of climate change on weeds will depend on the composition of local weed populations and the CO 2 × temperature × water availability interaction effects. These effects should be investigated for different species, ecosystems, and agroecological zones, in the context of subsistence agriculture and emerging social issues and resource scarcity. Weed management strategies should be diversified to lessen dependency on herbicides and mechanical control, and targeted to likely future problem species such as hemiparasitic and perennial rhizomatous weeds. Moreover, future climate change adaptation strategies for rice-based production systems, such as new cropping system designs or improved stress-tolerant cultivars, should simultaneously address possible implications for weed competition.
Acknowledgements
This work was carried out with the support of “Cooperative Research Program for Agriculture Science & Technology Development (Project title: Development of prediction system for management of rice weeds under climate change, Project No. PJ01052603)” Rural Development Administration, Republic of Korea.
References
Alberto A.M.P. , Ziska L.H. , Cervancia C.R. , Manalo P.A. 1996 The influence of increasing carbon dioxide and temperature on competitive interactions between a C3 crop rice (Oryza sativa) and a C4 weed (Echinochloa glabrescens) Aust. J. Plant Physiol. 23 (6) 795 - 802    DOI : 10.1071/PP9960795
Asch F. , Dingkuhn M. , Sow A. , Audebert A 2005 Droughtinduced changes in rooting patterns and assimilate partitioning between root and shoot in upland rice Field Crop Res 93 (2) 223 - 236    DOI : 10.1016/j.fcr.2004.10.002
Auld B.A. 2004 The persistence of weeds and their social impact Int. J. Social Eco. 31 (9) 879 - 886    DOI : 10.1108/03068290410550665
Bailey S.W. 2004 Climate change and decreasing herbicide persistence Pest Manag. Sci. 60 (2) 158 - 162    DOI : 10.1002/ps.785
Baker H.G. 1965 The Genetics Colonizing Species Academic Press New York, USA Characteristics and modes of origin of weeds 147 - 172
Barrett C.B. , Moser C.M. , McHugh O.V. , Barison J. 2004 Better technology better plots or better farmers? Identifying changes in productivity and risk among Malagasy rice farmers Am. J. Agr. Econ. 86 (4) 869 - 888    DOI : 10.1111/j.0002-9092.2004.00640.x
Bazzaz F.A. , Carlson R.W. 1984 The response of plants to elevated CO2 1 Competition among an assemblage of annuals at 2 levels of soil-moisture Oecologia. 62 (2) 196 - 198    DOI : 10.1007/BF00379013
Bjorkman O. 1976 Adaptive and genetic aspects of C4 photosynthesis.; Metabolism and Plant Productivity University Park Press Baltimore, MD, USA 287 - 309
Boese S.R. , Wolfe D.W. , Melkonian J.J 1997 Elevated CO2 mitigates chilling-induced water stress and photosynthetic reduction during chilling Plant Cell Environ 20 (5) 625 - 632    DOI : 10.1111/j.1365-3040.1997.00082.x
Bouma J. , Varallyay G. , Batjes N.H. 1998 Principal land use changes anticipated in Europe Agr. Ecosyst. Environ. 67 (2) 103 - 119    DOI : 10.1016/S0167-8809(97)00109-6
Bunce J.A 2000 Acclimation of photosynthesis to temperature in eight cool and warm climate herbaceous C3 species: Temperature dependence of parameters of a biochemical photosynthesis model Photosynthetic Res 63 (1) 59 - 67    DOI : 10.1023/A:1006325724086
Cater D.R. , Peterson K.M 1983 Effects of a C2-enriched atmosphere on the growth and competitive interaction of a C3 and a C4 grass Oecologia 58 (2) 188 - 193    DOI : 10.1007/BF00399215
Chloupek O. , Hrstkova P. , Schweigert P 2004 Yield and its stability, crop diversity, adaptability and response to climate change, weather and fertilisation over 75 years in the Czech Republic in comparison to some European countries Field Crop Res 85 (2) 167 - 190    DOI : 10.1016/S0378-4290(03)00162-X
Cochrane V. , Press M.C. 1997 Geographical distribution and aspects of the ecology of the hemiparasitic angiosperm Striga asiatica (L) Kuntze: A herbarium study J. Trop. Ecol. 13 (3) 371 - 380    DOI : 10.1017/S0266467400010579
2008 Briefing Notes. Invasive Pl. Climate Change http://www.weedscrc.org.au/documents
Darwin R. , Kennedy D. 2000 Economic effects of C2 fertilization of crops: transforming changes in yield into changes in supply Environ. Model Assess. 5 (3) 157 - 168    DOI : 10.1023/A:1019013712133
Fuhrer J. 2003 Agroecosystem responses to combinations of elevated C2 ozone and global climate change Agr. Ecosyst. Environ. 97 (1) 1 - 20    DOI : 10.1016/S0167-8809(03)00125-7
Giannini A. , Biasutti M. , Held I.M. , Sobel A.H 2008 A global perspective on African climate Climatic Change 90 (4) 359 - 383    DOI : 10.1007/s10584-008-9396-y
Haden V.R. , Duxbury J.M. , DiTommaso A. , Losey J.E. 2007 Weed community dynamics in the system of rice intensification (SRI) and the efficacy of mechanical cultivation and competitive rice cultivars for weed control in Indonesia J. Sustain. Agr. 30 (4) 5 - 26
Ingram J.S.I. , Gregory P.J. , Izac A.M. 2008 The role of agronomic research in climate change and food security policy Agr. Ecosyst. Environ. 126 (1-2) 4 - 12    DOI : 10.1016/j.agee.2008.01.009
Houghton J. , Meira Filho L. G. , Callander B. A. , Harris N. , Kattenberg A. , Maskell K. , (Eds.) 1996 Intergovernmental Panel on Climate Change Cambridge University Press Cambridge, U.K. (Eds.) Climate Change 1995: The Science of Climate Change
Houghton J. T. , Ding Y. , Griggs D. J. , Noguer M. , van der Linden P. J. , Dai X. , Maskell K. , Johnson C. A. 2001 Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change Cambridge University Press Cambridge, UK 881 -
Kanampiu F.K. , Kabambe V. , Massawe C. , Jasi L. , Friesen D. , et al. 2003 Multi-site, multiseason field tests demonstrate that herbicide seed-coating herbicide-resistance maize controls Striga spp. and increases yields in several African countries Crop Prot et al. 22 (5) 697 - 706    DOI : 10.1016/S0261-2194(03)00007-3
Kriticos D.J. , Alexander N.S. , Kolomeitz S.M. 2006 Predicting the potential geographic distribution of weeds in 2080 Proc. 15th Aust. Weeds Conf. Weed Management Society of South Australia Adelaide 27 - 34
Kriticos D.J , Sutherst R.W. , Brown J.R. , Adkins S.W. , Maywald G.F. 2003 Climate change and biotic invasions: A case history of a tropical woody vine Biol. Invasions. 5 (3) 147 - 65    DOI : 10.1023/A:1026193424587
Kriticos D.J. , Sutherst R.W. , Brown J.R. , Adkins S.W. , aywald G.F. 2003 Climate change and the potential distribution of an invasive alien plant: Acacia nilotica ssp. Indica in Australia J. Appl. Ecol. 40 (1) 111 - 24    DOI : 10.1046/j.1365-2664.2003.00777.x
Krupnik T.J. , Rodenburg J. , Shennan C. , Mbaye D. , Haden V.R. 2011 Trade-offs between rice yield, weed competition, and water productivity in the Senegal River valley.; Innovation and Partnerships to Realize Africa’s Rice Potential Africa Rice Center Cotonou, LB Proceedings of the Second Africa Rice Congress Bamako 22-26 March 2011 231 - 239
Latif M.A. , Islam M.R. , Ali M.Y. , Saeque M.A. 2005 Validation of the system of rice intensification (SRI) in Bangladesh Field Crop Res 93 (2) 281 - 292    DOI : 10.1016/j.fcr.2004.10.005
Mohamed K.I. , Bolin J.F. , Musselman L.J. , Townsend P.A. 2007 Integrating New Technologies for Striga Control - Towards ending the witch-hunt World Scientific Singapore Genetic diversity of Striga and implications for control and modelling future distributions 71 - 84
Mohamed K.I. , Papes M. , Williams R. , Benz B.W. , Peterson T.A 2006 Global invasive potential of 10 parasitic witchweeds and related Orobanchaceae Ambio 35 (6) 281 - 288    DOI : 10.1579/05-R-051R.1
Naylor R.E.L. , Lutman P.J. 2002 Weed Management Handbook 9ed Edition, British Crop Protection Council Blackwell Science Oxford, UK 1 - 15
Oechel W.C. , Strain B.R. 1985 Direct Effects of Increasing Carbon Dioxide on Vegetation University Press of the Pacific Honolulu, HI Native species responses to increased atmospheric carbon dioxide concentration
Olesen J.E. , Bindi M. 2002 Consequences of climate change for European agricultural productivity, land use, and policy Eur. J. Agron. 16 239 - 262    DOI : 10.1016/S1161-0301(02)00004-7
Parmesan C 1996 Climate and species’ range Nature 382 765 - 766    DOI : 10.1038/382765a0
Parry M.L. 1990 Climate Change and World Agriculture Earthscan London
Parry M.L. 1998 The Bawden Memorial Lectures 1973-1998 Silver Jubilee Edition British Crop Protection Council Surrey, UK The impact of climate change on European agriculture 325 - 338
Parry M.L. , Rosenzweig C. , Iglesias A. , Livermore M. , Fischer G. 2004 Effects of climate change on global food production under SRES emissions and socio-economic scenarios Global Environ. Chang 14 (1) 53 - 67    DOI : 10.1016/j.gloenvcha.2003.10.008
Patterson D.T. , Musser R.L. , Flint E.P. , Eplee R.E. 1982 Temperature responses and potential for spread of witchweed (Striga lutea) in the United States Weed Sci 30 (1) 87 - 93
Patterson D.T. , Westbrook J.K. , Joyce R.J.V. , Lingren P.D. , Rogasik J. 1999 Weeds, insects, and diseases Climatic Change 43 (4) 711 - 727    DOI : 10.1023/A:1005549400875
Patterson D.T. 1985 Weed Physiology CRC Press Boca Raton, Florida. USA Comparative eco-physiology of weeds and crops 101 - 129
Patterson D.T 1995 Weeds in a changing climate Weed Sci 43 (1) 685 - 701
Phoenix G.K. , Press M.C 2005 Effects of climate change on parasitic plants: The root hemiparasitic Orobanchaceae Folia Geobot 40 (2) 205 - 216    DOI : 10.1007/BF02803235
Reilly J. , Schimmelpfennig D 1999 Agricultural impact assessment, vulnerability, and the scope for adaptation Climatic Change 43 (4) 745 - 788    DOI : 10.1023/A:1005553518621
Rosenzweig C.R. , Hillel D. 1998 Climate Change and Global Harvest Oxford University Press Oxford
Stern N. 2006 Stern Review: The Economics of Climate Change Treasury Office of the Government of the United Kingdom and Northern Ireland, UK London
Seckler D. , Barker R. , Amarasinghe U. 1999 Water scarcity in the twenty-first century Water Resour. Dev. 15 (1) 29 - 42    DOI : 10.1080/07900629948916
Wand S.J.E. , Midgley G.F. , Jones M.H. , Curtis P.S. 1999 Responses of wild CO4 and CO3 grass (Poaceae) species to elevated atmospheric CO2 concentration: A meta-analytic test of current theories and perceptions Global Change Biol 5 (6) 723 - 741    DOI : 10.1046/j.1365-2486.1999.00265.x
Watling J.R. , Press M.C 2000 Infection with the parasitic angiosperm Striga hermonthica influences the response of the CO3 cereal Oryza sativa to elevated CO2 Global Change Biol 6 (8) 919 - 930    DOI : 10.1046/j.1365-2486.2000.00366.x
Ziska L.H. , Teasdale J.R. , Bunce J.A 1999 Future atmospheric carbon dioxide may increase tolerance to glyphosate Weed Sci 47 (5) 608 - 615
Ziska L.H. 2003 Evaluation of the growth response of six invasive species to past, present and future carbon dioxide concentrations J. Exp. Bot. 54 (381) 395 - 404    DOI : 10.1093/jxb/erg027
Ziska L.H. 2008 Rising atmospheric carbon dioxide and plant biology: The overlooked paradigm. DNA. Cell Biol. 27 (4) 165 - 172    DOI : 10.1089/dna.2007.0726
Ziska L.H 2000 The impact of elevated CO2 on yield loss from a CO3 and CO4 weed in fieldgrown soybean Global Change Biol 6 (8) 899 - 905    DOI : 10.1046/j.1365-2486.2000.00364.x
Ziska L.H. , Bunce J.A. 1997 Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected CO4 crops and weeds Photosynth. Res. 54 (3) 199 - 208    DOI : 10.1023/A:1005947802161