Ericaceae in Malesia: vicariance biogeography, terrane tectonics and ecology

Abstract

The Ericaceae are cosmopolitan but the main clades have well-marked centres of diversity and endemism in different parts of the world. Erica and its relatives, the heaths, are mainly in South Africa, while their sister group, Rhododendron and relatives, has centres of diversity in N Burma/SW China and New Guinea, giving an Indian Ocean affinity. The Vaccinioideae are largely Pacific-based, and epacrids are mainly in Australasia. The different centres, and trans-Indian, trans-Pacific and trans-Atlantic Ocean disjunctions all indicate origin by vicariance. The different main massings are reflected in the different distributions of the subfamilies within Malesia. With respect to plant architecture, in Rhododendron inflorescence bracts and leaves are very different. Erica and relatives with the ‘ericoid’ habit have similar leaves and bracts, and the individual plants may be homologous with inflorescences of Rhododendron. Furthermore, in the ericoids the ‘inflorescence-plant’ has also been largely sterilised, leaving shoots with mainly just bracts, and flowers restricted to distal parts of the shoot. The epacrids are also ‘inflorescence-plants’ with foliage comprised of ‘bracts’, but their sister group, the Vaccinioideae, have dimorphic foliage (leaves and bracts). In Malesian Ericaceae, the four large genera and the family as a whole have most species in the 1500–2000 m altitudinal belt, lower than is often thought and within the range of sweet potato cultivation. The same belt is also most diverse for ferns, birds-of-paradise and bowerbirds. Distribution maps of Malesian Ericaceae are presented, with related species indicated. Concentric patterns of distribution are frequent and could be attributable to evolution around shrinking seas. In New Guinea the main axial range is a composite structure, with the southern part formed by the old Australian craton and the northern and eastern parts formed from 32 tectonostratigraphic terranes or microplates. These formed independently and then docked with each other and the craton. The tectonic boundary between the craton and the accreted terranes is also an important biogeographic boundary. Many taxa have distributions linking different accreted terranes but are not on the craton. Movement and integration of island arcs and more substantial terranes has led to complex patterns of disjunction throughout Malesia, but there is an underlying parallel-arc structure, seen clearly in New Guinea. The tectonic provinces in Borneo are also reflected in the two main biogeographic tracks: Kuching-–Kinabalu and Kutei Mts./Mt. Kemul–Kinabalu, the Lupar River boundary and the Meratus Mts. centre. The islands of the Riouw Pocket (Corner 1978a) are notably surrounded by many groups which are absent there, including mangrove and Ericaceae species. However, other mangrove and coastal taxa occur along a CW Sumatra – Riouw Pocket – SW Borneo track. In Malesia, Rhododendron and Vaccinium mangrove and coastal species are mainly restricted to the west, in Sumatra–Borneo and often the Riouw Pocket; and are represented in New Guinea by related montane taxa. Van Steenis (1963, 1984) drew attention to mangroves uplifted by tectonics and the Malesian Ericaceae in general could be largely derived from mangrove forms. Rhododendron species occur as epiphytes in mangrove, and many Ericaceae occur in calcareous and saline sites. The very rapid rates of tectonic uplift occurring in New Guinea would raise a mangrove community or a coastal ‘padang’ to the upper montane zone in just one million years.