WORLD SURVEY of CITRUS IRRIGATION Million Tons Were
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WORLD SURVEY OF CITRUS IRRIGATION Walter Reuther 1 World citrus production has risen during the past The above background information indicates that century from less than 1 "Jillion to more than 44 million -citrus,as the most important tree fruit crop, amply justifies metric tons in 1973. Around 6 million tons of fresh fruit international training programssuch asthis short courseon of this total found its way into international trade, about irrigation. Your Florida hosts are to be commendedon 14 million tons were processed,and the rest were consumed their vision and desireto servecitrus growerseverywhere in fresh within the country of origin. The 1973 world arrangingthese sessions-especiallyconsidering that citrus production of all deciduoustree fruits combined (apples, in Florida waslargely unirrigated until recently. I am proud pears, peaches,plums, apricots, sweet and sour cherries) to be invited to participate. totalled about 42 million tons, to put this into perspective I will discuss briefly here the origin and natural with other fruits. The next most important table fruit habitat of citrus, summarizethe hiStoryof citrus irrigation, with respectto total worldwide tonnage is bananas,with survey the current worldwide citrus irrigation picture, and about 35 million metric tons in 1973, of which about 6.7 discuss the possible implications of some of the newer million tons were exported (1). Thus, citrus has now irrigation methodsfor the future. surpassedall other fruits to become not only the most important fresh table fruit on a worldwide basis,but also ORIGIN AND NATURAL HABITAT is secondonly to bananasin volume of internationaltrade. Citrus is produced in a large number of countries Most of the cultivated species within the genus (FA0 lists abOut 90) in a belt extending approximately Citrus appearto be indigenousto the more humid tropical between400 N and 400 S latitude around the world, with and near-tropicalregions of China,southeast Asia, including the heaviest concentration in the subtropical regions the easternareas of India and Bangladesh,and the islandsof between 200 and 400 N latitude (Fig. 1). Large-scale Philippines and Indonesia. Primitive man undoubtedly intensiveculture of citrus is heavily concentratedin frost- played an important role in selectivelydisseminati!'g and prone, irrigated subtropical areas, despite its origins in planting seed of the most attractive edible speciesand frost-free tropical regions with favorable annual rainfall varietiesin this generalarea before the dawn of history (8). distribution (Table 1). The reasonsfor this are complex, The humid tropical origin of citrus is supported by its and involve human as well as climatic, soil, and varietal generalmesophytic nature. It hasa moderatelyshallow and factors (5). I predict that tropical regionswill gradually spreadingrooting habit not well adaptedto withstanding begin to contribute more significantly to world citrus long droughts. Its flat, broad evergreenleaves and thin, production in the future, mostly in areas where succulent bark of its twigs do not have specialized supplementalirrigation will be required (6). xerophytic features for greatly limiting transpiration or The popularity of citrus as a fruit crop seemsto withstanding temperatureextremes. The absenceof a cool continue to soar, despitethe more rapid rise in production temperature-induceddormancy requirementfor flowering costs than market prices in recent years in most of the and its susceptibility to damageby freezing also indicate major producing countries. Projections of the Food and its tropical origin. Agriculture Organizationof the United Nationssuggest that There are no citrus speciesindigenous to the New total world production may rise to 56 million tons by World, and hence it is of interest to note the ecological 1980 (10). This would be an incredible increaseof a third conditions which favor natural spreadof introducedspecies in lessthan a decadel without cultural attention by man. Feral grovesof sour orangesand sweet orangesand to a lesserextent rough lemons and West Indian limes gradually developed in certain specializedhabitats as a'result of seedspread by 'Professor of Horticulture, Depertment of Plant Sciences, Uni- man or animals from cultivated sourcesafter the intro- venity of ~ifomi., Riwnide. duction of citrus to Florida in the sixteenth c»ntury. 28 29 Fig. 1. Distributionof intensivecommercial citrus-producing -- of the world. These "wild" groves were Invariably located on compara- of temperature, wind velocity and fluctuations in water tively rich, low but well-drained moist lands found in the table. vicinity of rivers or lakes known locally as "hammocks". Of course, neither the native habitats nor adopted Such habitats in their native state supported good stands of ones are infallible indications of optimal conditions for tall hardwood trees (oak, magnolia, hickory, etc.) and an commercial exploitation of crop plants, but merely indicate admixture of sabal palms and pines in many situations. the basic requirements for survival. But it is clear that citrus The topsoil was porous, acid, high in organic matter and cannot be placed in the class of such drought-resistant fertility, and often overlaid a calcareous subsoil. Citrus tree crops as the olive, almond, or carob. Millions of trees tended to form a dense undergroWth under the hectares of these crops have been grown for centuries shading canopy provided by the foliage of the taller native hardwoods and palms in such natural groves, varying from Table 1. Estimated distribution of world citrus production, 1973. 1 to 100 or more hectares (9). The main attributes of this hammock environment favoring feral citrus growth were a World Pf"oduction Subtropical TropicalZ reasonably uniform year-round supply of moisture in the Group (tons x 10001 regions regions first meter of topsoil, but without prolonged periods of water-logging, and a protective canopy of forest tr'ees. %of total Such a protective leaf canopy not only reduces the trans- 29,338 91 9 pirational demand for soil moisture, but also affords signif- Oranges Mandarlm 6,128 97 3 icant protection against moderate frosts. My observations Lemons and limes 4,008 75 25 in Florida, Central and South America, the Caribbean Gr~fruit 3,727 95 5 Islands and Southeast Asia indicate that citrus species Unclassified 762 25 75 10 must have substantially the environment of a tropical Total 43,953 90 rain forest to successfully invade native plant communities. The key factors for survival of seedlings appear to be a Source: FAD Production Yearbook (1). fairly well-drained and fertile topsoil with a fairly uniform year-round moisture supply, and freedom from extremes ZBe~n 20 N and 20 S latitude. 30 without irrigation in subtropical regions of the Mediter- citrus was among the earliest tree crops planted and ranean Basin with between 350 and 700 mm (14 and 28 irrigated in this region. inches) of annual rainfall, but having a 4- to 6-month Citrus spreadfrom southeastAsia acrossAsia Minor, summer period without effective rainfall. Some unirri- then~ to the African continent, and from Egypt alongthe gated olive, carob, and almond orchards are being pulled irrigated valleys whose rivers flowed into the out and replacedwith citrus ,in Spain and elsewherein the Mediterranean.Most of these have mild winter climates Mediterraneanbut only in areaswhere irrigation projects with low frost hazardssuitable for citrus. provide new sourcesof water. Citrus often cannot even The first mention of citrus in Europeanliterature was survive,much lessproduce a commercialcrop, in a typical about 310 B.C., by Theophrastus.He saw and accurately ami-arid wet-winter, dry-summer Mediterranean-type described citron trees and fruit when he accompanied climate. However,some citrus is grown without irrigation Alexanderthe Great to Persiaand Media.The odds arethat in the Mediterranean,for example, in Epirus province of these were irrigated trees growing in the gardensof the northwestern Greece and in adjacent Albania near the wealthy. Ionian Sea. Annual rainfall in this area is in the rangeof Only very sketchy historical information is available 1,000 to 1,100 mm (39 to 43 inches),with only about 2 or on North African and Europeancitrus culture beforeabout 3 months of drought in summer, and hence does not the twelfth century, although it is clear that the citron, qualify as a typical Mediterraneanclimate. lemon, sour orange,and pummelo were spreadthroughout Some citrus is grown for the local market without Asia Minor and the MediterraneanBasin before that time. irrigation in hot, humid tropical areaswith a monsoon- Spanish documents indicate citrus fruits from windmill- type rainfall distribution, despite 4 or 5 months of severe irrigated orchards were being exported to Europe from drought in most years.Some areas of the lower Magdalena Majorca in the twelfth century. Arabic writers of the River Valley of Colombia are typical. Seasonalrainfall is thirteenth century describethe culture, including irrigation, highly variable, ranging between 1,000 and 3,000 mm of citrus. In fact, present day irrigation methods used in (39 and 118 iAches)per year, with 4 to 5 months in the Spain and Portugalare essentiallythose introduced by the December-April period devoid of effective rainfall in Moors from North Africa beginningin the eighth century. 2 yearsout of 3. Citrus is grown without irrigation on some Present-daywater rights and water laws throughout the exceptional d~p, well-drainedretentive soils but