Plant-geography of east Malesia

Abstract

A floristic analysis is given of the flora of the Lesser Sunda Islands. It is concluded that their flora must formerly have been richer, especially in rain-forest plants Irom both east and west Malesia. Impoverishment of the flora started at the onset of the Glacial Age, when these islands acted as an important bridge from continental south east Asia through the Philippines, Celebes, to north Australia, facilitating exchange of drought-indifferent or seasonal drought-requiring plants between Asia and Australia. Before the Pliocene the Lesser Sunda Islands contributed little to effective contact and exchange. Two main contacts between the Malesian and Australian floras must have occurred, the first in the Upper Cretaceous to Paleocene, or even earlier, and the second, and final, contact before the end of the Miocene. These contacts were interrupted by a long period, throughout which the Australian flora was isolated. During the first contact Fagoideae and several other taxa of the Laurasian flora attained a bipolar-Antarctic range. Other Laurasian elements, which became thoroughly integrated with the primitive Australian-Gondwanean flora and subsequently evolved with it, probably exist but are difficult to trace. In the Miocene, a further, more abundant influx of the Asian/Malesian element into the Australian flora took place. This influx is more easily traced in the present Australian flora, especially its tropical moiety, a number of endemic species evolving from such immigrant taxa. The New Guinean tract had a rather poor archipelagic Australian-Gondwanean flora from the Oligocene until the Miocene, when this a.chipelago gradually became continuous land. With the developing orogeny over its whole length, it became a colossal expanse, with congenial soils and climate. The diversity of niches thus created allowed an explosive development of the Malesian element. Of the ancient Australian-Gondwanean element already present, part possibly became extinct, while part became integrated. The reciprocal emigration of the Australian element was always impeded because of its adaptation to nutrient-deficient soil types. Such soils are very scarce in New Guinea, Malesia and south-east Asia generally. As a result of this study, the relation between scleromorphy and nutrient-deficient soil types in Australia and South Africa has been reappraised. Scleromorphy can be traced in the tropical rainforest and is exemplarily expressed in the heath forest of west Borneo, and adjacent Banka and Billiton. It is deduced that scleromorphy is bound to soil, not to climate. The poor flora of the primitiae of the New Guinean flora is the result of the dearth of nutrient-“delicient soils and the later competition from the Malesian rain-forest. It also accounts for the proportionally good representation of the Australian element in the mountain flora of New Guinea and other Malesian islands. These botanical interpretations and deductions are in accord with recent tentative geophysical maps and datings. Several hitherto obscure facts are logically explained by this hypothesis: the overwhelming Malesian element of the present New Guinean flora; its explosive development in that island since the Miocene; the presence of the important demarcation between the Asian/Malesian and Australian element in the Torres Strait; the very low percentage of the Australian element in the Malesian flora, and the presence of various Laurasian representatives of different antiquity in the Australian flora.