The Australian cotton industry is looking to the future through explicit research focusing on what impact climate change and extreme weather events will have on cotton production, and seeking to understand how we can adapt to environmental changes.
Worldwide cotton production has broadly adapted to growing in temperate, subtropical and tropical environments, but growth and production systems in Australia may be challenged by future climate change. Changes in climate factors such as warmer air temperatures and extreme fluctuations in precipitation as a result of rising carbon dioxide (CO2) concentration may significantly impact plant growth and crop productivity.
Prior research utilising controlled environment glasshouses and field studies in the US have provided an excellent foundation of understanding potential impacts. However, there has been no specific research into climate change impacts for modern Australian cotton systems, and little research attempted to assess the combined interactive effects (temperature x CO2 x water) of climate change on cotton productivity; especially in the field.
Over the past several years, a range of research initiatives led by CSIRO and Western Sydney University, and supported by CRDC, have been underway to better understand responses of the Australian cotton system to this changing environment. Research has included simulation modelling, glasshouse and field-based studies, which has revealed some key insights. On-going research requires a multi-faceted approach that incorporates model simulations, glasshouse and field studies to better our understanding and knowledge of cotton system and plant-soil responses to projected environmental conditions for Australian cotton regions.
Climate change will have both positive and negative effects on cotton production. Increased CO2 may increase yield in well-watered crops and higher temperatures will extend the length of the growing season. However, warmer temperatures also accelerate the rate of crop development and could potentially shorten the time to maturity, which may then impact crop management decisions.
Higher temperatures also have the potential to cause significant fruit loss, reduce water use efficiencies, lower yields and alter fibre quality.
Environmental conditions that encourage excessive shading by the leaves may lead to fruit loss throughout the season. Consequently, fruit loss may exacerbate excessive vegetative growth and further loss of fruit, due to a lower fruit load to restrict vegetative growth.
A prediction of more frequent extreme weather events such as droughts, heatwaves and flooding pose significant risks to improvements in cotton productivity. Inter-annual yield variability is likely to be greater, with benefits from increases in yield potential during favourable seasons, but also large reductions in yield during the seasons affected by extreme weather events.
Research into integrated effects of climate change (temperature, humidity, CO2 and water stress) on cotton growth, yield and quality will be important. This includes the development of cultivars tolerant to abiotic stresses (especially for more frequent hot, water-challenged (deficit and waterlogged) conditions), and better understanding of whole system management strategies to maximise production and minimise losses of cotton grown in variable environments. In a systems context, climate change is a multi-factor complex issue that is likely to require more than one solution to address the multi-dimensional, integrated impacts on Australian cotton production systems.
This overview highlights some of the recent research that outlines legacy effects, climate change across Australian cotton regions, and the impacts of extreme weather events on cotton systems.
The positive effect of elevated CO2 on cotton growth and yield is generally consistent across studies, however, single-season experiments do not account for ‘legacy effect’ on subsequent crops.
Cotton plants under elevated CO2 produce N-poor litter, which can reduce soil N availability for subsequent crops through reduced decomposition rate. As a third of cotton’s N uptake comes from mineralised N, reduction in decomposition can strongly limit the yield response to elevated CO2 in subsequent seasons.
In glasshouse experiments, elevated CO2 strongly reduced yield in the second year, particularly at ambient temperature. Conversely, warmer air temperature had a consistent effect and seemed to negate the negative effect of elevated CO2 on yield.
Assessing the strength of this legacy effect in the field will be critical in developing fertiliser recommendations to mitigate the potential negative impact of elevated CO2 on cotton yield in the future.
Climate change across cotton regions
There have been substantial increases in atmospheric CO2 concentration since the beginning of the industrial age. Atmospheric CO2 during the past 800,000 years ranged between 170 and 300 µmol mol-1 in response to natural transitions between glacial and inter-glacial periods.
However, atmospheric CO2 has been rapidly increasing over the past 200 years due to world-wide industrial activity from a pre-industrial concentration of about 280 µmol mol-1 to 406 µmol mol-1 in 2017 (Tans and Keeling, 2018), with projections for more rapid increases in the future. It is projected that atmospheric CO2 concentration will rise to 450 µmol mol-1 by 2030.
As a consequence of rising greenhouse gases in the atmosphere, including CO2, global air temperatures have also been increasing throughout many regions. Global average air temperature has warmed by more than 1°C since records began in 1850, and each of the last four decades have been warmer than the previous decade (CSIRO and Bureau of Meteorology, 2018). Australia’s climate has increased 1°C since 1901 with an increase in the frequency of extreme heat events (CSIRO and Bureau of Meteorology, 2018).
In a recent study, eight locations across Australia’s cotton-growing regions have been assessed to explore temperature trends from: (a) 1957 to 2017 (60 years); (b) 1957 to 1996 (39 years); and (c) 1997 to 2017 (20 years). All eight locations exhibited a trend for an increase in the accumulation of the number of day degrees (a measure of heat accumulation throughout a growing season, from September to April) during the period 1957 to 2017 (Figure 1). Furthermore, from 1957 to 1996, there was an increase in the number of day degrees at Emerald, and during the period 1997 to 2017 there was an increase in the number of day degrees at Griffith and Moree.
Although the slopes of each regression were mostly positive, suggesting a possible increasing trend in day degree accumulation, the variation in the number of day degrees between years was large over a relatively short timeframe. However, the significant increasing trend in the number of day degrees from 1957 to 2017 for all eight locations indicates an increase in the number of hot days and warmer night-time air temperatures.
Current climate projections indicate Australia will exhibit more heatwaves (air temperatures greater than 35°C). Recent examples were during the 2016-17 cotton season, where high temperature records were broken across the country. Moree, in the Gwydir Valley in North West NSW, recorded 54 consecutive days exceeding 35°C. The previous record was 11 days above 35°C. Mungindi, north of Moree, measured 49 consecutive nights of 20°C or above. The previous record was 27 nights.
Climate projections also indicate that there will be changes in rainfall distribution, including an increase in the intensity of drought and flooding. Drought conditions directly affect dryland crops during the season and reduce water availability for irrigated cotton systems. On the other hand, Australia’s cotton is often grown on heavier soil textures (clay soils), so crops may experience yield losses due to waterlogging during heavy rainfall events.
Crop simulation studies
Crop simulation studies assessed the potential impacts on lint yield, water use, and water use efficiency across nine Australian cotton locations covering diverse irrigated and dryland scenarios at Emerald, Dalby, St George, Goondiwindi, Moree, Bourke, Narrabri, Warren, and Hillston. The results of these simulations are summarised in Table 1.
Leaf, Plant and Crop Level Effects
The integrated effects of warmer air temperatures and elevated atmospheric CO2 concentration on cotton growth, physiology and soil microbiology have been studied in a number of glasshouse and field studies in recent years. In both field and glasshouse studies, elevated atmospheric CO2 increased vegetative biomass and photosynthetic rates of cotton compared with plants grown at current CO2 levels. In glasshouse studies, elevated CO2 improved leaf and plant-level water use efficiency of cotton, which was associated with improved photosynthesis and biomass production, rather than decreases in water use. However, these studies also showed that improved water use efficiencies were negated by warmer air temperatures, as more water was required to grow the plants.
The field studies showed similar outcomes, but other crop level issues emerged. Increased vegetative biomass and reduced water use efficiency became evident as water consumption also increased. Crops had excessive vegetative growth with large leaf areas significantly increasing transpiration. Further reductions of water use efficiency were then associated with high temperatures, as well as the excessive shading caused by the leaves, leading to the shedding of fruit throughout the season. In turn, this continued to exacerbate the vegetative growth and the loss of fruit because there was little fruit load to restrict vegetative growth.
Studies have been conducted to explore differences between cotton cultivars in projected climatic scenarios. As future environments are anticipated to produce larger cotton plants with potentially greater requirements for water, plants with smaller, more compact vegetative growth habits and higher photosynthetic rates (eg Sicot 71BRF) may have an advantage over cultivars with substantial plant biomass and leaf area (eg DP16). Therefore, there may be variation in plant performance that could be utilised in the selection of breeding lines for future environments; however, implementation in capturing the needs for climate change in breeding programs remains a challenge.
Studies have indicated that projected climate change may impact nutrient availability and soil microbial communities. This is important to consider given the key role soil microbes play in nutrient cycling and availability, and the importance of nitrogen use efficiency in cotton systems.
Most climate change effects on soil communities are linked to changes in plant responses, thereby generating plant-soil feedbacks.
Low soil N may reflect greater plant N uptake and thus may not necessarily mean low nitrification rate. Additionally, limitations in plant growth may result in greater soil N levels than when plants are actively growing and utilising N from the soil.
Recent studies determined that climate responses of soil physicochemical (physical and chemical) properties and nitrification rate were also related to crop growth stage; only responding when the crop reached the early flowering stage. The studies also found that the changes were related to the abundance of microbial nitrifiers in the soil. Specifically, warmer air temperatures did not significantly change potential nitrification rates, and these alterations were dependent on the growth stage of the crop.
Changes in the rate of nitrification processes, and functional microbial communities that affect nitrification, could potentially lead to alterations in soil nitrogen availability, which may subsequently affect cotton crop productivity and nitrogen use efficiency.
Impacts of extreme weather events
Climatic projections include far greater variable weather conditions in the future, which is likely to have a more severe impact on cotton productivity. Projected climatic conditions are likely to increase inter-annual yield variability because of the high yield potential in seasons where there are no extreme climatic events, but large reductions in yield during seasons that are affected by extreme climatic events.
Simulation models demonstrated an increase in the number of days above 35°C across all locations in the study (ranging from Emerald to Hillston). There was also a reduction or no change in the number of cold shocks (≤ 11°C) throughout the majority of the growing season, with the exception of increased cold shocks in some NSW growing areas in January and February.
Furthermore, there were reported increases of two to 16 per cent and four to 17 per cent in mean rainfall and rainfall variability, respectively, within cotton growing season for the period centred on 2030, which will have significant consequences on farming system. For example, nitrate is highly mobile in soil, and thus susceptible to leaching in flooded conditions.
Glasshouse studies have shown that flooding caused a rapid loss of N from the soil, contributing to a reduction in growth and yield of cotton, particularly at warmer temperatures.
This article appears in the Summer 2019-20 edition of CRDC's Spotlight magazine. It's is a condensed version of a more comprehensive review of the recent research into effects of climate change and extreme weather events on Australian cotton systems undertaken by Katie Broughton and Michael Bange (CSIRO), David Tissue, Linh Nguyen and Brajesh Singh (Western Sydney University), Yui Osanai (University of New England), Qunying Luo (University of Technology Sydney) and Paxton Payton (USDA), whose efforts are gratefully acknowledged. The full version is available on the CottonInfo website.