A classification of tropical tree crops - particularly those grown for their fruit - is presented based on:
The classification reveals a pattern in the confusing multitude of tree crops; its usefulness is enhanced because of similarities in ecology and fruitfulness of the species within a class.
The contrast between the fruitful, continuously-growing, single-stemmed species and the rather unfruitful, rhythmically-growing branched species is striking. The ecology and husbandry of the two sub-classes of single-stemmed species and their effect on fruitfulness can be described in simple terms, since fruiting is closely linked with growth. The branched species form a large and diverse class. The relationships between branching and rhythmic growth and between branching and floral differentiation are briefly considered. It is concluded that fruitfulness in single-stemmed plants improves with the growth rate, but in branched trees, a balance has to be struck between growth and fruiting: moreover in the latter, specific stimuli are usually needed to trigger flower initiation and/or bloom. This leads to a different set of research priorities for the two classes of crops.
In virtually all branched tree crops grown for their fruit, vegetative growth and sexual differentiation are separated to some extent, either spatially or temporally. Spatial separation finds expression in cauliflory and shoot dimorphism; some characteristics of the species in these two categories are given. Temporal separation is effected through asynchronous or synchronous growth rhythms; sexual differentiation takes place during a quiescent phase in shoot growth. As the growth rhythm changes with tree age and degree of synchrony depends on the climate, further classification of this large category is difficult, the more so since far too little is known about the growth rhythm of most species. Concerted efforts to describe the phenology of these species are strongly recommended.
The study of tropical tree crops is fragmented. The major disciplines: forestry, plantation crops, fruit crops stand on their own and do not adequately cover such fields as agroforestry; they often leave a large category of 'miscellaneous tree crops' unattended. Moreover, nearly everywhere, research is organised on a commodity basis; insights gained in one crop are seldom applied to other crops in order to generalise the findings. Consequently, the student of tropical tree crops enters a jungle in which the multitude of trees obscures the view of the forest.
This paper is meant to open up this jungle by grouping species in a classification based on:
-growth habit: branching, position of inflorescences;
Since the habit and growth rhythm of a species reflect its adaptation to the environment, the species in one class have ecological features in common.
The key was initially intended to relate fruit crops to plantation crops in order to let the study of fruit crops benefit from the sustained research efforts in plantation crops. Hence the key is applied primarily to species grown for their fruit, and some generalisations are made with respect to fruitfulness. Because of this emphasis on fruit, crops such as banana, pineapple and passionfruit are included, even though they are not tree crops.
The proposed classification is inspired by the more general classification of trees according to architectural models by Halle, Oldeman and Tomlinson (1978). These models are also based on tree habit and growth rhythm, but the emphasis is on vegetative growth.
1. Single-stemmed species
1.1 Continuous growth, concurrent floral development (oilpalm. coconut. papaya)
1.2 Continuous growth culminating in flowering
a. largely supported by current photosynthesis (pineapple. sisal. banana)
b. largely supported by accumulated reserves (sago. sugar-palm. bamboo)
2. Branched species
2.1 Floral development concurs with extension growth (mainly small trees. shrubs and vines: capulin. rose mallow. passionfruit)
2.2 Extension growth and floral development separated
2.21 according to loci:
a. cauliflory (cacao. jackfruit. durian)
b. shoot dimorphism (coffee. apple)
2.2.2 in time:
a. growth rhythm asynchronous (nutmeg, sapodilla)
b. growth rhythm synchronous (clove, mango, rambutan, some deciduous species: rubber, kapok, mombin)
A simple, characteristic, unbranched versus branched habit distinguished the two main classes. A stem is said to be single if it can be considered as a separate entity in the completion of its vegetative growth and fruiting. Therefore 'single-stemmed' includes branching at the base as in banana or branching towards the end of the crop cycle as in pineapple. The unbranched habit is usually associated with continuous growth; this applies to all crops given as examples. Branching, on the other hand, is more often linked with rhythmic growth: the shoot grows in flushes alternated by quiescent periods.
Two sub-classes of single-stemmed species are distinguished. The further division of the second sub-class is useful but arbitrary, as indicated in the key by the qualification 'largely supported by'. The large and diverse class of branched species is also divided into two sub-classes, based on the time and place of floral development in relation to shoot growth.
In sub-class 2.1 sexual differentiation and vegetative growth occur side by side, because the inflorescence is borne on the shoot, which grows continuously.
The important branched trees grown for their fruit nearly all belong to the second sub-class (2.2), in which extension growth and floral differentiation are separated to some extent, either spatially or temporally. Spatial separation (division 2.2.1) finds expression in cauliflory and shoot dimorphism. Temporal separation (division 2.2.2) is effected through asynchronous or synchronous growth rhythms; sexual differentiation takes place on the shoot during a quiescent phase following the termination of the flush. The growth rhythm changes with tree age and climate; because of its variability, the distinction between asynchronous and synchronous rhythms is not sharp.
3. THE SINGLE-STEMMED SPECIES
The class of single-stemmed species grown for their fruit is comparatively small. With the exception of papaya, all species are Monocotyledons. The growth habit is simple: a single meristem produces all aerial parts in orderly succession.
The number of leaves soon becomes stable, since new leaves replace falling leaves (coconut), or because the meristem turns floral (banana). The demands of a virtually constant leaf area can be met by a steady growth rate of the roots; thus the habit of growth is consistent with a constant top: root ratio. This in turn explains the continuous growth of these successful mono-axial species.
Growth has precedence over flowering and fruiting, but fruitfulness improves progressively with higher growth rate, as shown in Table 1 for oilpalm: the harvest index is much higher at high growth rates. As a result the crops respond very favourably to husbandry and reach high and predictable yields. In fact, the top producers among fruit crops (papaya, banana, pineapple) and oil crops (oilpalm, coconut) are single-stemmed species. Small wonder that these crops are widely grown within their ecological limits. They are of great importance to rich and poor, also because the fruit ripens throughout the year.
Since under good management both biological yield and the yield of fruit are already high, no spectacular yield increment is to be expected. However, in view of the large acreages, small gains still assume great importance. Research workers should concentrate on refinements in growing techniques, based, for instance, on water or nutrient response curves, to maintain high growth rates. There appears to be little scope for crop manipulation apart from breeding.
In the first sub-class, an inflorescence emerges in the axil of each leaf after the short juvenile period. A period of adverse growing conditions (e.g. moisture stress in oilpalm, Table 1) sets back flowering and fruiting much more than growth. Hence these crops only thrive in equable tropical climates. Water shortage, for instance, has to be prevented through a good rainfall distribution (oilpalm), access to groundwater (coconut) or irrigation (papaya). If closely spaced, the plants elongate, leaf axils become acute and again fruiting suffers much more than growth. In illustration 1, data from Sly and Chapas (1963) demonstrates the increment in height and the sharp decline in yield for oilpalm at close spacings.
Hence oilpalm, coconut and papaya are designed for undisturbed growth in both time and space. For maximum yield, the grower has to sustain a high growth rate throughout the plant's life.
Pineapple and banana are the only important fruit crops in the second sub-class. They flower terminally after the completion of extension growth. This separation of growth and fruiting in time makes the plants more flexible. Sub-optimal growing conditions merely postpone the attainment of adequate size for flowering. Consequently these crops are not restricted to non-seasonal climates. They produce good crops even in the sub-tropics, although the duration of the crop cycle increases as growth rate diminishes.
For banana this is shown by the effect of altitude on crop duration and yield in Table 2. The near equality of yield per day indicates that in this example, the slow-down in floral development is made good by a larger share of photosynthates for the fruit. Also the terminal inflorescence tolerates interplant competition. The fruit becomes smaller at close spacing, but over a range of density this is compensated by the higher plant number per hectare.
To be continued in July, 1986 newsletter.
Editor's note: the Tables were not included in the current newsletter.
DATE: May 1986
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