TREE DIVERSITY ON SMALL PLOTS IN MADAGASCAR : A PRELIMINARY REVIEW

Jean-Prosper Abraham Rakotonirina Benja2, Mady Randrianasolo 3, Jorg U. Ganzhorn 4, Vololoniaina JEANNODA 2 & Egbert Giles LEIGH, Jr 5

Introduction

Madagascar is an island larger than France (590 000 km2) stretching from 11° 57' to 25° 32' south latitude (Koechlin, 1972). While still part of East Gondwanaland (which was composed primarily of India, Australia and Antarc¬ tica), Madagascar split off from Africa between 160 and 130 million years ago (Storey, 1995), before toads (Bufonidae) reached the shores of the Indian Ocean 130 million years ago (Blommers-Schlosser & Blanc, 1993). By 1 10 million years ago, Australia and Antarctica were quite separate from Madagascar and India (Lawver et al., 1 992). India finally split off from Madagascar and began its voyage to Asia about 85 million years ago (Storey et ai, 1995). Madagascar has a continental diversity of habitats : «spiny forest » resembling the vegetation of southern Arizona’s Sonoran desert in the southwest, rain forest on the eastern slopes, which extends to a strip of the northwest coast, deciduous dry forest along much of the west coast, and the endless rolling grasslands and savannas in the center of the island (Humbert & Cours-Darne, 1965 ; Koechlin, 1972 ; Koechlin et al., 1974). As some genera have done in Hawaii (Carlquist, 1980), genera such as Diospyros and Terminalia have radiated into all the major types of natural forest and bush in Madagascar, suggesting that Madagascar was a quasi -independent experiment in plant evolution. This view is supported by the high proportion of endemics — 94 % among Madagascar’s trees and shrubs (Koechlin, 1972, p. 156).

Is Madagascar small enough to limit the diversification and evolution of its plants ? On small, isolated islands such as Mauritius or Réunion, which, to be sure, are much younger (Réunion was formed by volcanic activity only three million

Rev. Ecol. (Terre Vie), vol. 51, 1996.

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