This paper assesses the potential effects of climate change on avocado production, and suggests strategies for adaptation.
New Zealand and Australia Avocado Grower’s Conference ’05. 20-22 September 2005. Tauranga, New Zealand.
The avocado industry is affected in a range of ways by climate since it affects growth, disease risk, fruit quality and industry location (see Table 1). Amongst many other
considerations, management and infrastructure decisions attempt to account for these climate effects and risks. Such decisions will usually use the historical climate as a guide to future conditions.
There is increasing evidence that human activities are already changing the global climate, and that more change seems likely. Consequently, historical conditions may become increasingly less pertinent as a guide to industry activities or industry adjustment. This paper assesses the evidence for climate change, drawing particularly on the IPCC Third Assessment Report (IPCC 2001a, b, 2002), and explores the potential impacts and implications of such changes for the avocado industry in Australia.
It is certain that the atmospheric concentration of various gases and particulates has changed over the past century, and there is much evidence that they are now higher than at any time in the past 420,000 years (Petit et al., 2000).
The concentration of these atmospheric constituents has consequences for the absorption of solar radiation by the atmosphere, and thus global and regional climates. In the case of the main ‘greenhouse gases’, notably carbon dioxide (CO2) (Fig 1), methane (CH4) and nitrous oxide (N2O), the sign, magnitude and longevity of this effect is well-established; and the increased concentrations of these gases results in a net warming of the globe (IPCC, 2000, 2001a). In the case of the minor or more transient gases (such as the tropospheric ozone-precursors), the effect is known to be warming, but the degree, duration and distribution of the warming around the planet is less certain.
For particulates, the direct effect can either be warming (for dark, highly absorptive particles such as soot) or cooling (for reflective particles such as sulphate), and their impact depends partly on their location in the atmosphere (IPCC, 2001a). Particulates can cause or prevent the formation of clouds, which in turn either cool or warm the earth, depending on their type and location. Thus, the net effect of particulates remains uncertain in both sign and magnitude.
Nevertheless, independent evidence from observations of the climate of the past century and a half, strongly implies that the total global radiant energy (gases and particulates) is having a warming effect on the world.
Table 1. How climate affects critical stages in avocado production.
|Climatic factor||Impact on avocado growth and development|
|Storm damage (incl. cyclones)||Fruit loss, tree and infrastructure damage|
|Frost when flower buds, flowers or freshly set fruit are present||Frost affects flower buds, flowers and small fruit, thus reducing crop yield|
|Cold weather during flowering, fruitset and during the first few weeks after fruitset||Night temperatures of 10 to 15oC or lower, for extended periods during flowering and for a few weeks after flowering can reduce pollination, fruit set and fruit retention|
|Very low humidity and high winds at flowering||Desiccation of flowers and fruitset failure|
|Heat stress and high solar radiation||Sunburn damage to fruit and exposed branches Research in some horticultural crops suggests enhanced radiation increases fruit and tree susceptibility to pathogens|
|High summer temperatures||Smaller sized ‘Hass’ fruit|
|Higher maximum and minimum temperatures||Earlier maturity under higher temperature conditions and vice versa|
|Diurnal temperature variation||The greater the diurnal temperature range the greater the chances for male and female flower parts being open at the same time, and therefore achieving pollination and fruit set|
|Periods of warm weather and high rainfall leading to soil saturation||Increased incidence and severity of Phytophthora cinnamomi|
|Wet conditions for 48 hours or longer||Fruit can become infected by anthracnose and other fruit rot diseases (e.g. stem end rot) if fruit are wet for 48 hours or more. Stress during fruit development appears to make them more susceptible to these diseases|
|Warm season, especially when combined with high humidity||Increased incidence of insect pests, eg. fruit spotting bug and Monolepta sp.|
|Hot sites and hot windy conditions||Higher evaporation rates and thus greater need for irrigation and mulch|
|Combination of suitable soils and climate||Industry location|
It is very likely that the global mean atmospheric temperature near the earth’s surface has risen by 0.73oC since 1850 when measurements began, and is now higher than at any time during at least the past thousand years. About three-quarters of the change observed since 1850 is attributed to human actions (IPCC, 2001a).
Australian annual mean temperatures have increased by 0.82oC since 1910, with rapid increases, particularly since 1950 (Smith, 2004), with night-time temperatures increasing faster (0.11oC/decade) than daytime temperatures (0.06oC/decade). Night- time (minimum) temperatures have particularly risen sharply in the northeast of Australia. There are also trends from 1957 to 2003 of increasing frequency in hot days (35oC or more) of 0.08 days per year and a decreasing trend in cold nights (5oC or less) of 0.16 nights per year (Hennessy et al., 2004a).
Since 1900, annual Australian-average rainfall shows a moderate increase (7.9mm/decade), but it is dominated by high year-to-year variability (Smith, 2004).
While north-eastern Australia has become wetter since 1950, much of eastern and southern Australia has become drier. This is due to a weakening or southward shift of the frontal systems that bring most rain to these regions (Marshall, 2003) and generally wetter conditions during the 1950’s.
Rainfall intensity in eastern Australia has increased from 1910 to 1998, but has decreased in the far southwest of Australia (Haylock and Nicholls, 2000) over this same time period. Over New South Wales, extreme daily rainfall intensity and frequency has decreased from 1950 to 2003 (Hennessy et al., 2004b).
The frequency of tropical cyclones in the Australian region has decreased since 1967 (Hennessy, 2004c), along with an increase in cyclone intensity, possibly as a result of a shift in areas of formation. Explosively developing cyclones, including East Coast Lows off the New South Wales coast, have increased between 1979 and 1999 (Lim and Simmonds, 2002).
There have also been climate changes in regions where the avocado industry is strongly represented. We have selected eight sites (Fig 2) to assess such changes: Mareeba, Bundaberg, Nambour, Gatton, Toowoomba, Coffs Harbour, Mildura and Manjimup, these sites representing important avocado production regions in Australia.
Temperature affects avocados in many ways, including influencing timing and reliability of flowering, fruit growth, ripening and fruit quality. There are strong trends of increased mean annual temperature across the eight sites (Table 2), ranging from 0.49oC per century (Mildura) to 3.26oC per century (Nambour) with an average of 1.8oC. About 60 to 80% of the warming arises from change in night-time temperatures (except for Mildura where all the warming is from daytime temperatures) and 55 to 70% of the warming is from temperature increases in the May-October period (again with the exception of Mildura which is 35%).
There were weak trends of declining rainfall in most sites and most seasons, however, these were mostly not statistically significant, with the exception of Bundaberg where rainfall during November to May showed significant declines (Fig 3). These apparent declines may in part have been the result of a period of above average rainfall experienced in the late 1950’s.
Fruit set in avocado is sensitive to late frosts during spring. In frost-affected avocado- growing regions in Australia, there have been significant decreases over the past five decades in the number of frosts (Table 3), the date of last frost, the length of the frost period and the date of the first frost (i.e. the first frost is slightly later -see Fig. 4). These changes are largely explained by the general increases in minimum temperatures experienced at these sites, although at some sites, increases in atmospheric humidity (see dewpoint temperatures in Table 2) and variations in rainfall also play a role. If minimum temperatures increase as projected, and providing rainfall doesn’t decrease too markedly, then the historical trends towards lowered risk of frost are likely to continue.
Anthracnose post-harvest fruit rot and insect pest attacks are favoured by warm and humid conditions. As noted above, avocado production regions are becoming warmer but there is also a general increase in atmospheric water vapour content (humidity) as measured by dewpoint temperature (Table 2).
Periods of heat stress (for example -days with temperatures above 35oC) can adversely affect fruit set and fruit size. Fruit can also be burnt by high radiation loads. Generally across the eight sites, there are trends towards increasing numbers of heat stress days and higher radiation loads (Table 4).
Fruit set is also enhanced by high diurnal temperature ranges (the difference between daytime maxima and night-time minima); this too has been decreasing in most of the eight sites over the past 48 years (Table 4). A low diurnal temperature range decreases the chances of male and female flower parts being open at the same time to achieve pollination.
Irrigation demand is strongly affected by evaporation rates. High quality, long-term and consistent measurements of evaporation are rare. We have used the Penman-Monteith equation (FAO) to estimate potential evaporation based on more commonly measured atmospheric variables. However, wind-run is not varied and where this has changed, we will not have represented this in our estimates which show increases in potential evaporation across all sites (Table 4). This is likely to have resulted in progressively higher irrigation requirements over time, assuming that all other factors (e.g. technology, pricing, availability) were not changed.
Projections of future change
A selection of climate models, driven by a range of scenarios of human development, technology and environmental governance, project the global mean temperature to rise a further 2 to 5.8oC during the 21st Century (IPCC, 2000). This is a large range, with about half of the variation in projected temperatures being due to uncertainties in the climate models, and the other half due to uncertainties regarding greenhouse gas emissions which are closely tied to social, economic and technological aspects of our future. The projected warming is not evenly distributed around the globe: continental areas warm more than the ocean and coastal areas, and the poles warm faster than equatorial areas. When translated to Australia, there are anticipated to be substantial increases in temperature over and above those already experienced.
For the eight avocado growing sites assessed, expected changes by 2030 are about 0.3 to 1.7oC, with much greater changes by 2070 (Table 5). The changes up to 2030 are consistent with the existing trends in mean temperature with extrapolations of current trends in all cases intercepting with the range of temperatures expected in 2030 (Fig. 5).
A mean warming of 0.4 to 2.0oC is anticipated over most of Australia by the year 2030, relative to 1990, and 1 to 6oC by 2070 (CSIRO, 2001) -see Table 5. Mean temperature change is likely to be greatest inland and least on the coast. Most warming is expected to occur in spring and summer, and least in winter. There is no strong indication whether the diurnal temperature range is likely to change. In contrast, the current trend towards lower frost risk is likely to continue in all frost-affected sites. However, whilst there is an expectation of a 10 to 50% increase in days over 35oC by 2030, the occurrence of hot spells is more likely to increase in frequency in inland growing areas (e.g. Mildura) and to a lesser extent on the coast.
A tendency for less rainfall is expected in the south-west of WA (-20 to +5% by 2030, – 60 to +10% by 2070). In much of eastern Australia, projected ranges are uncertain (e.g. -10 to +10% by 2030 and -35 to +35% by 2070). Recent analyses indicate that Queensland coastal rainfall may on balance decline but that this may vary with season. When broken down by seasons, spring rainfall tends towards decreases ranging from zero to -20% by 2030 and zero to -60% by 2070. Autumn shows a tendency for decreases with changes from +7% to -13% by 2030 and +20% to -40% by 2070. Summer (+7% by 2030 and +20% by 2070) and winter (+13% by 2030 and +40% by 2070) showing no particular directional changes (Cai et al. 2003).
There is uncertainty as to the likely change in the frequency and strength of El Niño
(ENSO) events. Even in the absence of increases in El Nino events, projected changes in atmospheric moisture balance (rainfall minus potential evaporation) will lead to drier conditions over Eastern and Southern Australia and Eastern New Zealand, with a greater likelihood of droughts. These more frequent droughts are likely to be accompanied by higher temperatures, which will compound the problem. This, combined with expectations of increased evaporation, suggest increased irrigation demands.
Rainfall intensity is expected to generally increase with warmer temperatures, as the air can hold more moisture (about 6 to 8% per oC) enabling more intense precipitation. If rainfall intensity does increase, this may increase soil erosion risk and also may increase the frequency of waterlogging: conditions where Phytophthora cinnamomi is problematic. Scenarios of rainfall intensity (e.g. Hennessy, 2004c) indicate considerable geographical diversity in possible responses, with a tendency for a decrease in rainfall extremes along the east coast in autumn and winter, although most models project an increase in the intensity of extreme rainfall in spring and summer on the north-east coast. The projections for the Sunraysia/Riverland region were similar to the east coast. In contrast with the consistency between historical temperature trends and projected temperature changes, historical trends in rainfall intensity at the eight sites in avocado- growing regions show generally consistent decreases.
Australian tropical cyclone frequency and regions of occurrence show little change under enhanced greenhouse conditions, but there is a 56% increase in the number of storms with wind speeds exceeding 30 ms-1 (108 km per hr) and an increase in the number of storms south of latitude 30oS (Walsh et al., 2004). However, the confidence in these projections is only moderate due to issues with representation of ENSO in the models.
Climate change may bring slightly higher extreme wind speeds to Mildura, especially in spring and summer, but lower wind speeds on the north coast of NSW and the Queensland coast, with exceptions under cyclonic conditions.
Climate change could feasibly affect the avocado industry in many ways. Potential effects of the climate changes outlined above are briefly documented, based on current understanding of avocado physiology.
Table 6 Climate change impacts on avocado growth and development.
Potential impacts on avocado growth and development
Less diurnal temperature range –
Reduced chances of overlap between open stages of male and female flower parts. Therefore less potential for pollination and fruitset, especially in single variety plantings.
Warmer nights –
Less chance for fruitset failure in areas that currently experience crop failures due to cold nights during flowering.
We assessed the potential change in frequency of heat stress conditions using scenarios of future temperature change in conjunction with flowering periods, drawn
from Newett et al (2003), for each of the eight sites. Generally, the frequency of heat stress days is likely to increase only marginally if temperature increases by 2030 are at the lower end of the projection range, but increase by two to five-fold if temperature increases are at the upper end of the range. The increase in heat stress frequency with temperature is essentially exponential (Fig 6). If temperature increases proceed as indicated in the climate change scenarios, heat stress days would be commonplace during flowering at all of the eight sites, by 2030. Where there is an increase in the number of days with temperatures above 35oC during flowering, fruit development and on-tree fruit ‘storage’ are affected, and reduced fruit set and damage to fruit will also occur.
Temperature is the main factor responsible for the change from the vegetative to the reproductive phase of avocados; subtropical avocado cultivars can only produce flower buds if kept under a cool temperature regime. Research showed that ‘Hass’ did not flower at 30/25, 25/20 or 24/19°C day/night temperatures but flowered when kept for 3-4 months at 15/10, 18/15, 20/15 and 23/18°C. Under the last two regimes, flowering was delayed and its rate was much lower. For ‘Hass’, the 23/18°C regime is probably close to the critical point for flowering (Gazit and Degani, 2002).
Carbon dioxide (CO2) concentration
The effects of short-term atmospheric CO2 enrichment (150 to 2000 μmol CO2 mol-1) on photosynthesis of ‘Hass’ avocado are reported in Schaffer and Whiley (2002). It was found that net CO2 assimilation measured in leaves increased as the atmospheric CO2 concentration increased. Witjaksono et al (1999) report that better shoot growth and greater biomass accumulation occurred in a CO2-enriched environment than under ambient CO2 conditions.
Dry matter partitioning in ‘Hass’ avocado trees grown at both 350 and 600 μmol CO2 mol-1 is reported in Schaffer and Whiley (2002). There was a greater allocation of dry matter to the trunks of avocado trees growing at 600 μmol CO2 mol-1 compared with trees at 350 μmol CO2 mol-1 (Fig 7). Janse van Vuuren et al (1997) showed that the bulk of tree carbohydrate reserves in avocado are found in the roots and wood, and that low starch reserves at the beginning of a new reproductive season facilitate an off-year for yield. They state that for the producer it is important to promote the build up of reserves.
Research (Schaffer and Whiley, 2002) showed that by 45 days after flowering, trees grown at 600 μmol CO2 mol-1 held more fruit than those at 350 μmol CO2 mol-1. Since there is a direct relationship between fruit retained at 40-50 days after flowering and final yield, it is likely that increased atmospheric CO2 concentrations will benefit productivity of avocado.
Fig. 7 Partitioning of dry matter in ‘Hass’ avocado trees grown for 6 months in atmospheres of either 350 or 600 μmol CO2 mol-1. Columns represent means (n=6) ±SE. (Schaffer and Whiley, 2002). ( Figure not provided)
There appear to be many potentially significant impacts of climate changes on the avocado industry, some of which may be positive, some negative. There is a need to identify management strategies to either offset negative impacts or to take advantage of positive responses. Previous assessments of such adaptations have been made for other industries (e.g. Howden et al. 2003). One of the general conclusions from these analyses is that the best defence against future climate change is to continue to develop the capacity and knowledge to manage current climate variability more effectively.
Most of the anticipated climate changes point towards the need for a very high standard of orchard management in order to respond to the challenges that expected changes pose. Some of the expected changes may even see a need to consider a shift in orchard location (e.g. Schulze and Kunz, 1995). There is also a need to adapt marketing plans to accommodate anticipated changes in harvest times. Therefore the following potential management implications for growers and the industry may need to be considered.
Table 7. Potential implications for avocado orchard management as a result of anticipated climate change.
|Issue||Potential management implications|
|Lower diurnal temperature range||Introduction of pollinator varieties in blocks currently planted to single varieties. More extensive use of growth regulators to achieve better fruitset.|
|Warmer night temperatures||Areas previously considered too cool may now have potential as avocado production sites. Areas such as the Toowoomba Range previously experiencing frequent fruitset failures may become more viable.|
|Higher summer temperatures||Increased irrigation requirements, increased water storage capacity, more accurate moisture monitoring systems and more efficient irrigation systems. Re-locate to cooler micro-climates or more southerly locations. Use of overhead evaporative cooling irrigation systems (Blight et al., 2000). Greater need to reduce incidence of stress on ‘Hass’ by more careful management, use of mulch and use of growth regulants such as uniconazole (Sunny®) to improve fruit size (Wolstenholme, 2001).|
|Significantly warmer autumn and winter temperatures||In the long term it may be necessary to cease orchard operations in areas where it becomes too warm in autumn and winter to produce a good flowering. ‘Shepard’ may be successfully grown in more southerly districts.|
|Earlier maturity times and reduced on-tree fruit ‘storage’||Plan for earlier harvest times and address associated marketing issues. Assess market access and timing.|
|Phytophthora cinnamomi activity||Even greater attention required for the control of this disease:-Better drainage, eg. higher row mounding. Closer monitoring and attention to root phosphonate levels. Greater use of mulch. More effort directed to attaining recommended soil pH of 5.5 and high soil calcium levels. More precise soil moisture and nutrient management. Use of more tolerant rootstocks. Stricter orchard phytosanitary procedures.|
|Insect activity||Closer monitoring and more responsive management of insect pests and predators. Better control mechanisms|
|Increasing number of heat stress days||Greater need for effective management of P. cinnamomi to improve leaf cover. Efficient and effective irrigation scheduling and application system. Use of overhead evaporative cooling irrigation systems. Application of sunburn protection for fruit eg. Surround®, bentonite. Selectively harvest exposed fruit prior to expected high temperature periods. Select sites with greater water holding capacity in the root zone, eg. deeper loams. Greater use of mulch.|
|Increase in number of cyclones and storm events||Consider re-location of orchards to areas less prone to cyclones and other extreme weather events such as hail. Re-visit the use of windbreaks to reduce damage to trees and fruit. Canopy management systems implemented, not only to improve productivity and fruit quality but also to keep tree size smaller to reduce structural damage in storms.|
|Increase in frequency of droughts||Selection of orchard sites less prone to drought. Installation of adequate water harvesting and storage structures to capitalise on high rainfall events when they do occur. Greater need for the selection, installation and maintenance of effective moisture monitoring and irrigation scheduling systems. Installation and maintenance of more efficient and effective irrigation systems capable of adequately watering the whole orchard using less water and with a quicker turn-around. Effective use of under-tree mulching to reduce temperature in root zone, help maintain soil organic matter levels in the face of increased soil temperatures and reduce unnecessary evaporation. Allow for possible increases in irrigation demand and also consider risks to supply reliability.|
In order for adaptation to climate change to be successful there will be a need to incorporate both pre-emptive and reactive adaptive strategies. These will need to occur in conjunction with already changing social, economic and institutional pressures. With this in mind, adaptation measures aimed at reducing the negative impacts of climate change will have to reflect and enhance current ‘best-practices’ designed to cope with adverse conditions such as drought. Whilst a range of technological and managerial options may exist as indicated in Table 7, the adoption of these new practices will require:
Adaptation strategies that incorporate the above considerations are more likely to be of value, as they will be more readily incorporated into existing on-farm management strategies.
We are in the early stages of assessing the impacts and consequences of climate change in horticulture in Australia. Growers are already managing avocado production within a very variable climate. The best defense in managing the impacts of climate change in any system is to improve on the management of current climate variability. The following are challenges for growers, industry and scientists to address as climates continue to change
Better understand and take advantage of CO2 fertilization, and its effects on yield
We wish to thank Tony Whiley, Steve Crimp and Peter Hofman for their guidance and suggestions during the development of this paper.
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