Sympatric and allopatric formes of the genus Ravenala in forests and open habitats in eastern Madagascar

Four variants, or forms, in the currently monotypic Ravenala,madagascariensis Sonn. (the "traveler's tree") have been observed on the eastern slopes of Madagascar, from sea level up to an altitude of 1000 m in submontane forests: (1) the first form, locally called "malama", is the most strikingly divergent variant, especially with respect to its juvenile phase. It grows in the understory of undisturbed submontane rainforest. The secondarily spiral arrangement of the leaves, the long decurrent blade, the absence of a leaf stalk and the arrangement of the leaf blades to form a torus, are very different from the usual growth habit of Ravenala. The adult stage of this "malama" is characterized by the classical fan-shaped leaf arrangement. (2) the second forest form, locally called hiranirana is, according to our surveys, more abundant in forest gaps and more or less disturbed primary forest than in truly undisturbed forest. The juvenile phase of this second form is more similar to the usual fan-shaped ravenala, with a well developped leaf stalk and a relatively wide blade, but a slight and regularly alternate shift of the leaf blades, is characteristic of the "hiranirana". It persists in the adult phase after the trunk has formed. (3) the third form, locally called "bemavo", is the commonest form growing on the deforested slopes between 300 and 600 m elevation. This form constitutes the socalled "Ravenala forests". It is characterised by a well arranged fan-shape of the leaves at all stages of development, at least in these open areas, and the yellow color of its leaf sheaths. This is the form whose leaves are most used by local people to make roofs, and whose trunks are most used to make floors and walls. (4) the fourth form, locally called "horonorona", grows mainly at elevations lower than 300 m. It is common in the swampy areas along the coast, where it grows with Typhonodorum and species of Pandanus. This form usually presents a reduced leaf fan and an obliquely oriented trunk base. This last character is related to the multistemmed habit of this form. Indeed, "horonorona" differs strikingly from the other three recognized forms in its cespitose habit. This low elevation form is the one most commonly cultivated throughout the world despite the inconvenience of its suckering habit and low cold tolerance. From an evolutionary point of view, we suggest that the suckering habit and the perfect fan-shaped arrangement of the leaves, with the consecutive protecting self-shading of the blades, is the primitive condition. Thus the origin of Ravenala probably first evolved in more or less inundated lowland forests (which are today mostly destroyed), so the original Ravenala may have resembled the fourth form "horonorona" which now lives in deforested lowland sites, on either poorly or well drained soils. Afterwards, Ravenala presumably invaded steep slopes covered with originally quite open forest. Finally, forms similar to the extant "malama" (1): which is well adapted to the shade of the closed forest understory, invaded the more closed forests on less steep slopes, The establisment and persistance of Ravenala in open deforested areas may be a relatively recent event. The contractile roots, observed in all the forms, may be crucial for adaptation to the different habitats. Indeed, these contractile roots allow the stem apex to sink into the soil. This pattern of growth and establishment is responsible for the augmentation of the stem diameter through the initiation of numerous roots along the buried stem base. This growth habit stabilizes the plant on unstable inundated slois and steep slopes. II aslo protects the young stem apex against fire in the regularly burnt "savoka" and other savannah-like slopes of eastern Madasgascar. The "Malama" is most obviously divergent from the others. Thus future studies will probably lead us to begin by separating this form taxonomically. No matter what way these forms are classified, their different eco-morphological characters explain how Ravenala can grow in such strikingly different environments.