Ericaceae in Malesia: Vicariance Biogeography, Terrane Tectonics and Ecology
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311 Ericaceae in Malesia: vicariance biogeography, terrane tectonics and ecology Michael Heads Abstract Heads, Michael (Science Faculty, University of Goroka, PO Box 1078, Goroka, Papua New Guinea. Current address: Biology Department, University of the South Pacific, P.O. Box 1168, Suva, Fiji. Email: [email protected]) 2003. Ericaceae in Malesia: vicariance biogeography, terrane tectonics and ecology. Telopea 10(1): 311–449. 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. 312 Telopea 10(1): 2003 CONTENTS Introduction ...................................................................................................................... 313 Affinities of Ericaceae with other families .................................................................. 313 Ericaceae subfamilies and their biogeography ............................................................ 314 Conclusion on biogeography of the subfamilies ........................................................ 319 Global biogeography ...................................................................................................... 319 Shoot architecture and habit .......................................................................................... 324 Ericoid shrubs as largely sterilised inflorescences ...................................................... 325 Hypocotyl and lignotuber .............................................................................................. 330 Ericaceae ecology ............................................................................................................ 333 Ericaceae as weeds of active margins .......................................................................... 337 Altitude .............................................................................................................................. 345 Floral biology .................................................................................................................... 349 Ericaceae in Malesia – systematic treatment ................................................................ 349 Biogeography of the Malesian mountain flora and the myth of long distance dispersal .................................................................................................... 389 Biogeography of regions in Malesia .............................................................................. 394 Chronology ...................................................................................................................... 425 Mangrove ancestry of Malesian Ericaceae .................................................................. 426 Terrane tectonics and biogeography ............................................................................ 429 Centres of origin, chronology, and adaptation ............................................................ 431 Rhododendron: adaptive radiation or non-adaptive amalgamation? ........................ 432 Conclusions: biogeography and evolution in Ericaceae ............................................ 434 Acknowledgments .......................................................................................................... 435 References .......................................................................................................................... 436 ‘Fortune reigns in gifts of the world, not in the lineaments of nature’. – Shakespeare, As You Like It. Heads, Ericaceae in Malesia 313 Introduction Rhododendron L. (c. 900 species) and Erica L. (c. 860 species) are the largest genera in Ericaceae. Both are ecologically important and widely cultivated in Europe, but they are not highly speciose there. Erica has a clear centre of diversity in Cape Province, South Africa, and Rhododendron has one in N Burma/SW China and a second in New Guinea. Ericaceae (including Epacridaceae) are hardy plants, better able than most families except grasses to withstand the environmental extremes of mountain tops, arctic conditions, volcanoes, swamps, rocks, tree trunks and branches, and acid, oligotrophic soil which is often sandy or peaty. However, apart from a few saprophytic members they nearly all require high light levels, and many need a good supply of moisture. This paper deals with the Ericaceae of the lands between Asia and Australia (the East Indies) that botanists refer to as Malesia. The affinities of Ericaceae with other families From the 1960s to the 1980s the Russian–American (Takhtajan–Cronquist) classification of the dicotyledons (e.g. Cronquist 1981) was widely taught and very influential. However, the three largest groupings — Dilleniidae, Rosidae and Asteridae — were often not adopted by tropical botanists. Philipson (1974, 1975, 1977), for example, placed Ericales (Dilleniidae), and Cornales, Araliales, and Pittosporaceae (Rosidae) together with the Asteridae on the basis of ovule morphology. (Philipson revived and extended Warming’s early observations which had fallen into disrepute through van Tieghem’s uncritical and complete reliance on them). Philipson (1977) also removed Ebenales, Primulales and Theales from Dilleniidae to a position near this group, and a broad Ericales, including these orders, is also supported by palynology. This vast group of all the Asteridae plus many ‘dilleniid’ and ‘rosid’ families comprises about a third of the dicots (Olmstead et al. 2000). Although Philipson’s work drastically undermined the Cronquist–Takhtajan groups, it was soon taken up by Dahlgren (1980). (Anderberg’s 1992 claim that the new grouping of Ericales s.l. with Asteridae was ‘definitely an innovative approach’ of Dahlgren was perhaps a little over-enthusiastic). Recent results from molecular studies are much more closely aligned with the Philipson–Dahlgren system than with the Russian–American classification: for example Olmstead et al. (2000) indicated a broad Ericales (including Actinidiaceae, Primulales, Ebenales, Theales and Lecythidaceae etc.) as the sister group of the Euasteridae. It is interesting that both the Philipson–Dahlgren ideas and the DNA cladograms