Note: I am grateful to Chris Sayer, Californian avocado grower, for documenting climate effects on avocado cropping via Twitter.
We are on track to retain more and more energy in our climate system
Increasing carbon dioxide emissions from burning fossil fuels will increase the energy retained in our climate system by at least an additional 4.5 Watts per square meter by 2100. This suggests a rise in global average temperature of around 2.5 to 2.7o C.
As ocean surface temperatures rise they release more water vapor. As air temperatures rise water vapor levels rise (every 1 degree centigrade allows air to hold about 7% more moisture). This results in heavier downpours. In large land masses such as South America and USA, coastal areas and high mountain chains are hit hardest. Smaller land masses surrounded by oceans are also affected.
In large continental masses the difference between high temperatures inland and lower temperatures on the coast can cause strong, drying winds; and as inland temperatures rise with climate change, so do wind speeds.
Rising temperatures perturb high atmospheric circulation patterns, causing large pulses of cold air to extend out from Arctic and Antarctic poles. These can bring unusually intense cold periods and even affect subtropical jet stream patterns, causing warmer and drier conditions.
These are examples of some of the more obvious effects. What other knock-on effects that may occur in the interconnected complexity of the weather system are harder to predict.
What does it mean for avocado growers?
In some areas, higher temperatures may affect the ability of avocado flowers to set. In other areas cultivars that do poorly due to a higher heat requirement for flower set may begin to bear well. If marine coastal fogs decrease in spring, avocado pollination may be enhanced. Initial trends suggest reduction in summer, but it is highly complex and uncertain. Winners and losers. Growers in the more challenging dry Mediterranean climate areas will likely to struggle with sunburn damage to foliage and trunks.
As mentioned, strong drying winds are likely to increase in strength. Autumn winds can break branches carrying heavy fruit loads. Stronger winter winds can blow off both leaves and immature fruit.
Increases in climate variablity may mean successive challenges within a year – frosts, damaging storms, waterlogged soils, then drought and dessicating winds.
Increased evapotranspiration may ultimately lead to salination, especially where irrigation water already contains undesirable salt levels. Extended periods of drought may reduce the level of natural aquifers.
Managing the challenges
Most weather-related problems associated with avocado culture are already being managed.
Wind damage can be mitigated with shelterbelts, staking, hedgerowing.
Frost damage can be mitigated with sprinklers.
Drought can be managed with irrigation and mulching.
Salinity can be managed with salt-tolerant rootstocks and irrigation leaching. New technology can help guide appropriate decisions.
Sunburn can be dealt with via talc/kaolin clay sprays.
Rainstorms can be managed with improved subsoil drainage, planting on mounds, ditching, and soil structural improvements via deep rooting cover crops.
At this point, it seems likely that future shifts in duration and intensity of adverse climatic events can continue to be dealt with by existing and new management practices. High temperatures in Spring that exceed a critical threshold for fruit set, and summer temperatures sufficient to damage fruit seem to be the only exception.
Is there really any need to genetically ‘improve’ avocado as a crop to meet higher wind energy, higher global average temperatures, and more intense rainfall effects?
Breeding avocado is more expensive than breeding other crops
Avocado is very difficult to ‘breed’ in the classical sense of controlled pollinations. Worse, time from seed to fruit ranges from about 5 years to 15 years. It is an expensive and difficult crop to breed, even with the aid of new genomic techniques. New characters could be added to meet climate change (wood tensile strength was mentioned, but characters affecting tree size, trunk caliper, and so on could be considered). Each new character added requires generations of breeding cycles, and sometimes very very large populations to find rare genetic recombinations.
Solving problems with management techniques is much quicker than waiting for plant breeding results.
And it seems to me the only problem that can’t be solved by management is heat stress at flowering and summer fruit maturity.
The answers to the heat stress problem
Heat stress at flowering may prevent fruit set. Summer fruit can be severely damaged in inland heatwaves. Regular and prolonged whole-tree heat stress is likely to damage important physiological processes. It is ‘game over’ at that point. Growers must choose:
- relocate to a suitable climate
- grow a different crop
- fund a cross-species genetic modification or gene editing program that allows flower set under high heat conditions AND confers physiological resistance to whole-tree heat stress
The only growers that are likely to relocate (within a country, or to a different country entirely) are corporate growers. Relocating is expensive. Only corporate growers have the capital (or can attract it). Inevitably there is a delay of 4 or so years before the first useful crop volumes flow from new plantings. Corporates can wait years for a return on investment.
Smaller family-based farmers have less financial flexibility. In some hotter inland areas away from ameliorating coastal ocean effects, avocado orcharding will have to be abandoned.
Most medium to smaller growing enterprises are far more likely to change crop, give up growing (especially if their land is being encroached by suburbia), or grow a different (probably short-cycle) crop.
In short, moving countries is an effective heat stress mitigating measure, and requires no further plant breeding effort.
Avocado nodal groups already carry climate resilience
Avocados of the tropical group (so-called ‘West Indian) grow and fruit well in spite of tropical temperatures. ‘Guatemalan’ avocado nodal group generally fruit well in the warm temperate+ to subtropical zone of the South American Andes. Many of the more northerly ‘Mexican’ node tolerate cooler conditions, including some frost resistance.
The driver of this wide band of climate tolerance seems to me to be a once contiguous wild avocado population experiencing wide temperature differences with both latitude and altitude. (More complex local climate effects within parts of the broad range may also have played a part.)
Localised population genetic diversity has probably been fragmented and thus ‘pruned’ by human deforestation from shifting ‘slash and burn’ agriculture, and more recently by forest clearing for coffee growing. The case of the cultivar Nabal illustrates the point:
“The parent tree was accidently destroyed in June 1917, by a laborer who was planting coffee. It stood among coffee bushes in the Finca ‘Santa Lucia,’ 7a Calle Poniente, near the Alameda de Santa Lucia, Antigua, Guatemala. “Wilson Popenoe, 1917
If Popenoe had not collected budwood and shipped to the USA over a hundred years ago, the Nabal cultivar would not exist today. And neither would cv. Reed, as Reed is a seedling of Nabal.
‘Guatemala’ is a relatively small geographic area, and possibly has greater depletion of diversity than its much larger neighbour. It seems to me, the greatest amount of avocado genetic diversity is likely to be in local trees in isolated villages, farmlets, and wild places throughout Mexico. And as some parts of Mexico experience dry heat, perhaps some genetic adaptation has occurred.
Genes for heat tolerance can’t be seen
Heat tolerance is probably mediated by a number of physiological processes regulated by a number of interacting genes. ‘Classical’ avocado breeding is primarily guided by easily measurable physical attributes – fruit size, flesh quality, fruit storage quality, crop load, tree size and so on. Avocados historically have been selected in climates well suited to avocado culture and production. The ability of the plant to tolerate heat isn’t revealed until plantings are tried in marginal climatic zones.
Problems in breeding for heat tolerance
If tolerance to heat has evolved in the hottest part of avocado’s natural range, then that is where germplasm should be sought. Not in commercial orchards, but farmlets, villages, towns, wild places.
Historically, the primary selection filter was fruit quality, coupled with reliable productivity. Tolerance to heat and cold was a bonus. But as the ‘pedigree’ of avocado in commercial use is narrow (excluding tropical avocados) there may not be any marked heat tolerance within this rather small subset of avocado diversity. (This is not to say that there is no useful selectable variability in existing germplasm collections. Avocado cultivars are very heterogenous, and their flowering system tends to maintain that heterogeneity.)
I don’t suppose anyone has ever attempted to characterize what ‘heat tolerance’ in avocado is composed of (in terms of which genes affecting which physiological processes, and in what manner). It is certainly possible to do this, and a similar process has been carried out by Prof. Neena Mitter at the University of Queensland to identify genes involved in Phytopthora resilience. Prof. Mitter’s work identified 17 genes involved in a variety of physiological processes apparently responsible for the Phytopthora resilience phenomenon. ‘Heat tolerance’ may not involve as many genes, but is likely to be multigenic.
If genes are identified, then the process of constructing and patenting gene-markers can begin. An expensive process, and probably an expensive product.
Plant exploration is also expensive, and in the case of invisible characters, difficult. I also doubt anyone has gone in search of ‘heat tolerant’ avocado plants in Mexico.
If an apparently heat tolerant seedling can be found, the fruit will likely be inferior – small, big seed, probably thin skinned. Transferring the wild genes from putative ‘heat tolerant’ individuals into commercial cultivars would be a long job.
Why breed for heat tolerance?
Why bother? Who would do it? Who would benefit?
Outside the tropical avocados, at least, this question implies that breeding for heat tolerance, whether by conventional or genetic manipulation techniques, would only happen if Hass variety was threatened on a relatively wide scale.
In such an ’emergency’, I would hope ‘science diplomacy’ would steer the task, with a cooperative effort for the common good, and the tab picked up by everyone.
Temperatures that would threaten Hass avocado
Temperatures above 35 °C (95 °F) have been quoted as adversely affecting fruit set. I am not sure if this is due to warm dry air (perhaps plus wind) desiccating the flowers in the crucial late spring period, or if this is a temperature level that triggers abiotic stress gene up-regulation. This might result in flower or fruitlet drop, or maybe early-stage pollen or ovule damage or death.
Will temperatures rise above 35 °C in the critical spring period?
If so, will such effects be localised to more inland and lower latitude areas of the Mediterranean-climate avocado producers (USA, Israel, Spain, North Africa)?
Will the speed of climate warming outpace the speed of breeding more heat tolerant avocado cultivars?
We don’t have answers. Yet.
Increasing general resilience
Avocado is not an important food in the well-fed West. Objectively, at only 2% protein and about 20% oil, it hardly stands out nutritionally. In contrast, for example, low THC hemp seed has much more protein (around 25%) and contains about 30% oil (which contains exceptionally high levels of the 2 essential fatty acids linoleic acid and alpha-linolenic acid), and unlike avocado, hemp is relatively drought tolerant. But avocado – with corn and beans – is an important food for poor Mexican folk. The trees barely need cultivation, are long-lived and fairly reliable producers.
But avocado will always have a place in Western domestic economies, even in times of economic constraint.
Therefore, the avocado ‘resilience system’ should be strengthened as a matter of general food security. Security is the essence of a social good, and its delivery is the responsibility of government.
In the case of securing maximal resilience in the avocado system all governments should cooperate in funding avocado germplasm conservation everywhere (resilience depends on multiple redundancies), but most particularly in the natural range.
The accent should shift from selecting existing cultivars with ‘good’ fruit or fruiting to collecting a range of diversity, including diverse tree forms. Mutants, particularly dwarfing mutations, should also be curated widely.
Universities and other research organisations should continue to cooperate to better understand and explore the gene-environment nature of avocado.
Breeders, both private and public, should generally share information to aid insights into avocado breeding. Non-commercial cultivars should also be shared (and phytosanitary requirements rigorously observed).
If avocado fruit are a public good, then the consuming public should contribute a small amount of each avocado purchase price to an international, consensus-driven Trust tasked with maintaining avocado diversity in perpetuity for the common good.
Ironically, a somewhat similar arrangement has been put in place for conserving germplasm of coffee, a tree whose industrial culture is responsible for destroying a part of avocado diversity forever.
Perhaps coffee drinkers should pay ‘reparations’ for the damage they have done. The money could then go to a community fund to enduringly avocado germplasm for the common good.
David Putland, 2011. Climate change and climate policy implications for the Australian avocado industry