Vol. 5 No. 2
How will weed management change under climate change? Some perspectives.
Author(s): N. CHANDRASENA
Abstract: Human activities, including expanded fossil fuel use and deforestation, have caused atmospheric CO2 to increase significantly from a preindustrial concentration of about 280 μL L-1 to a current estimate of about 370 μL L-1. Even if CO2 emissions are immediately scaled back, levels are expected to double sometime during this century. An increase in CO2 and other greenhouse gases is likely to cause an increase in global surface temperature. Rainfall patters are likely to change across many areas of the globe and extreme events, like drought and cyclones, are predicted to be more prevalent and intense. The resultant major climate changes will affect the growth of plants, through modification of their photosynthetic performance and other physiological changes. As CO2 rises, C3 plants are likely to benefit more, and respond with increased net photosynthesis, growth, and yield, compared to C4 plants. Therefore, higher atmospheric CO2 is predicted to stimulate the yields of most of the world’s major crops, which are C4 plants. An important question being asked is: Given that many of the most troublesome agricultural weeds are C4 plants, will the competitive ability of these weeds be reduced relative to C3 crops as climate change occurs? As ‘colonising plants’, weeds have many biological traits, including wide ecological amplitudes, which give them advantages over other plants to exploit more successfully disturbed habitat and changed environmental conditions. Also, there are a large number of C3 weeds in the world, which may become more aggressive in many situations, under elevated CO2 and warmer conditions. Under such changed climatic conditions, the likely scenario is that both C3 and C4 weeds will become more competitive, with potentially negative consequences for the environment, as well as agricultural productivity across different regions of the globe, negating some of the otherwise beneficial effects of CO2 ‘fertilization’ of the C3 world crops. It is also probable that many colonising plants will extend their bio-geographical ranges as global environmental changes occur, and weed management in the field will become more costly and difficult. Humans have no option, but to adapt to effects of elevated CO2 and warming of the planet, which they exacerbated. However, climate change is not the only factor that will be changing as the 21st century unfolds. Population growth and varying economic and technological changes will increasingly affect the environment no less than will climate change. Developed countries, due to technological advancements, will adapt more effectively to respond to climate change, including the likely increased impacts of weeds. On the other hand, burdened by population pressure and declining natural resource bases, many developing countries will not be so well placed to face climate change and its flow-on effects, such as water and food scarcity. With regarding to managing weeds, our adaptive responses need to be based on better knowledge of how plant communities will respond to climate change. Rather than ad hoc responses, scientists will have to re-evaluate their approaches and more rigorously apply scientific and ecological knowledge to effectively manage ecosystems, one of which is the agricultural field. Tools available for ecological weed management include breeding allelopathic crops cultivars and drought and stress-resistant varieties; minimum-tillage or conservation farming; agro-forestry and the use of allelopathic crop residues. Sustainable weed management under climate change will have to be more holistic and better integrated with pest management, where possible. Re-vitalizing the above-mentioned ecological approaches is a must. A crucial element in this response strategy will have to be adequate public education about the threats posed by the changing climate. Early detection, preventative weed management, border protection and risk management approaches, will have critical roles in the containment of invasive species In many ways, weed scientists and weed managers have to ‘do what they have been doing better’, under future climate change scenarios. Humans must also take drastic action to reduce the primary root cause of climate change - the high rate of CO2 emissions, by a variety of approaches. This would involve burning less fossil fuel, stopping large-scale deforestation occurring in the tropics, preventing reclamation of large wilderness areas for agricultural use and protecting conservation areas from invasive species. Other actions to mitigate the inevitable CO2 build up involve some combination of conserving energy, and the increased use of alternative energy sources (e.g. solar, wind and hydropower) as substitutes for fossil fuels.
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