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What do you do when life gives you lemons? If you work for SAEON, you crunch the numbers and plan ahead, that’s what...
‘When life gives you lemons, make lemonade’ is a proverbial phrase coined over a hundred years ago to encourage optimism and a can-do attitude in the face of adversity or misfortune.
But is this positive outlook applicable to the fate of lemons, and other citrus, in a 21st century world facing the adversity of climate change?
Citrus (in the broadest sense referring to all members of the genus Citrus) likely originated in tropical Southeast Asia and since first domesticated in as early as 1500 B.C., has been cultivated extensively around the world. Currently citrus and hybrids thereof are grown commercially in 142 countries, representing the highest value fruit crop in international trade.
Areas of cultivation within the tropics and subtropics (Figure 1) dominated production during 2009-2010, but with yields decreasing with increasing latitude and the cooler climatic conditions associated with the transition to the temperate zone. Seven countries account for approximately three-quarters of global citrus fruit production, with Iran (2%), South Africa (2%), Pakistan (2%) and Japan (1%) contributing the most to the remaining quarter (Figure 2).
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Citrus production in a changing climate
Understanding climate change is complicated, as minimum temperatures are increasing at a faster rate than maximum temperatures, and higher latitudes are warming faster than the tropics. Understanding the impacts of climate change, on agriculture for example, is doubly difficult and requires the interpretation of a great deal of crop data (for example yields, mortality, timing of life cycles) and climate data (primarily temperature and precipitation) from as many different localities, and over as many years as possible.
In a warming environment, minimum temperatures in the subtropics will fall below 0oC less frequently than they do at present. Theoretically, this will benefit crops of tropical origin with limited frost tolerance, such as citrus, that are currently being cultivated at the poleward extremes of their cold tolerance ranges. Increasing maximum temperatures in the subtropics will result in climatic conditions more closely approximating those of the tropics and this, too, will benefit tropical crops.
However, this same warming may be responsible for cueing life cycle events such as the release from dormancy and the initiation of reproduction, and for hastening plant life cycles; the consequences of which are perceived as the early arrival of spring events such as leafing and flowering, or early fruit ripening. Plants growing at these latitudes are therefore facing a unique challenge: their environments are warming and their yearly life-cycle and life-cycle stages may be advancing, but not necessarily all at the same rate.
Consider a citrus tree that has been triggered into early flowering or leafing by warmer than normal minimum winter temperatures (which does not mean that the occasional night with temperatures below 0oC does not still occur). In this scenario, the periods of new growth and the incidence of frost overlap - with disastrous consequences for the completion of the plant life-cycle and the all-important production of fruit.
Understanding the behaviour of plants - especially crop plants located at their latitudinal climate extremes - to warming climates is critical for the crop management decisions and forward planning that form part of climate change adaptation.
Flowering dates and the incidence of frost
Scientists from SAEON’s Ndlovu Node, the School of Geography, Archaeology & Environmental Studies at the University of the Witwatersrand, Johannesburg (South Africa), and the Department of Geography at Golestan University, Gorgan (Iran) recently gained some insight into these behaviours, and investigated the flowering dates of citrus and the incidence of frost over a 51-year period. The collaborating countries are both minor, yet important, contributors to global citrus production that cultivate these crops under less-than-optimal subtropical and near-temperate conditions. Limited crop data from South Africa restricted the analysis to Iran only.
Peak flowering dates (85% of trees in an orchard) for orange, tangerine, sweet lemon, sour lemon and sour orange were extracted from daily flowering records spanning the period 1960-2010 for two government orchards in the Iranian cities of Shiraz and Kerman. Last frost dates (the last day in the spring season with minimum temperature below or equal to 0oC) and the number and magnitude of frost events occurring after peak flowering were calculated from daily temperature records provided by the Iranian Meteorological Organization. Maximum and minimum temperature records for the same period were checked to confirm overall climate warming for the region.
Using these data, the scientists aimed to determine the nature and rate of shifts in flowering dates and last frost dates in an effort to understand the impact of this aspect of climate warming on the Iranian citrus industry.
Orchards in both cities experienced significant increases in minimum (0.5-0.7oC/decade) and maximum temperatures (0.3oC/decade) over the 51-year study period, confirming overall climate warming in Iran in recent decades. As is typical from other climate change research, minimum temperatures are increasing faster than maximum temperatures.
Further, orchards in both cities face considerable frost risk, with mean monthly minimum temperatures in winter falling below or remaining only fractionally above the 0oC frost threshold. Peak flowering dates were observed to advance for all five citrus types in both cities, with oranges showing the smallest change, flowering - on average - six days earlier (0.11d/yr) in 2010 than they did five decades ago. Conversely, sweet lemon currently flowers 33 days earlier (0.65d/yr) than in 1960.
It was not surprising that the researchers revealed strong correlations between peak flowering dates and increasing maximum and minimum temperatures. These orchards also experienced advances in their last frost dates in recent decades - marginal for Kerman at 0.04d/yr and more substantial for Shiraz (0.47d/yr).
The Iranian citrus industry is therefore facing the scenario outlined previously. Minimum and maximum temperatures are increasing, and spring flowering and the date of the last frost event are advancing - but at demonstrably different rates. These different rates are the consequence of flowering date being the product of both minimum and maximum temperatures, while frost events are driven by minimum temperature alone.
The risk posed by frost
In short, spring flowering is advancing quicker than the last frost event of winter is retreating, resulting in an extended period during which the new growth is susceptible to damage from frost. This is particularly apparent from the orchards in Kerman where the number of days between peak flowering and the last frost date ranged from 0-3 days prior to 1988, but from 0-15 days for the period 1988-2010.
The number of frost events occurring after flowering also appears to have increased, reaching a maximum of four in 2005. Furthermore, an increasing number of years since 1988 (41%) have experienced frost events after peak flowering has occurred. Evidence also supports an increase in the number of citrus types that are now faced with frost risk (Figure 3).
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Is it all doom and gloom?
So, is the long-term prognosis all doom and gloom for the Iranian citrus industry? It would certainly appear that any positive benefits of a warming climate, particularly for a tropical crop being grown in extra-tropical climes, are being nullified by a paradoxical increase in frost risk.
Flowering ahead of the last frost date places the flowers and immature fruit at considerable frost risk - a risk that has increased over the past 50 years and which ultimately impacts negatively on fruit yield and profit. However, increasing minimum winter temperatures appear to be lessening the severity of these late winter frost events. Sub-zero temperatures following peak flowering before 1988 ranged from -2oC to -5oC, while from 1988-2010 temperatures rarely dropped below -2oC (Figure 3).
Fortunately, frost severity and the associated risk will continue to ameliorate as increasing trends see temperature minimums remaining above 0oC within a projected 30 years. This permits some level of persistence and adaptation within the industry.
Understanding and managing for increasing frost risk, at least in the coming few decades, therefore must make up part of the uncertainty being grappled with by Iranian citrus famers. Contributing to this uncertainty is a suite of climate-specific plant behaviours which were not the focus of this study. Increasing temperatures may result in longer, desirable growing seasons, the economic value of which will not be realised if the chilling requirement during the dormant period is not met. Likewise, warmer temperatures may favourably boost yields, but only if water remains non-limiting.
Long-term observation is crucial
Within this uncertainty is general agreement on two issues: This study reinforces the absolute value of long-term data - in this case pertaining to the behaviour of citrus in Iran, and how past and current trends can be interpreted to inform the adaptation strategies essential for sustaining agricultural capacity and ensuring sustained economic and food security under climate change.
Despite this study from Iran, the scientists involved emphasise that the research necessary for underpinning adaptation strategies remains sparse for much of the Middle East, as well as central Asia, South America and Africa - including South Africa, and are limited largely by a lack of suitable and corresponding crop and climate records.
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