Predominant colonization of Malesian mountains by Australian tree lineages

Abstract

Aim Massive biota mixing due to plate‐tectonic movement has shaped the biogeography of Malesia and during the colonization process, Asian plant lineages have presumably been more successful than their Australian counterparts. We aim to gain a deeper understanding of this colonization asymmetry and its underlying mechanisms by analysing how species richness and abundance of Asian versus Australian tree lineages in three Malesian subregions change along environmental gradients. We hypothesize that differing environmental histories of Asia and Australia, and their relation to habitats in Malesia, have been important factors driving assembly patterns of the Malesian flora. Location Malesia, particularly Sundaland, the Philippines and Wallacea. Taxon Seed plants (trees). Methods We compiled plot‐level data of environmental variables and tree abundances from three Malesian subregions. For each species, we inferred its geographical ancestry (Asian or Australian) based on published phylogenetic studies and the fossil record. We used proportions of Australian versus Asian species and individuals per plot to test how they are related to environmental parameters and geographical position using logistic regression models. Results Proportionally more Australian (and fewer Asian) tree species and individuals occurred (a) at higher elevations, (b) on sites over ultramafic parent material and (c) closer to their source region Australia with a significant increase of Australian elements east of Wallace's line. The trend was stronger for individuals than for species. Main conclusions Long‐term environmental similarities between source and sink habitats have shaped the assembly of the Malesian flora: Tree lineages from tropical Southeast Asia predominantly colonized the Malesian lowlands and rich soils, whereas trees from montane refuges in Australia were more successful in the newly emerging Malesian mountains and on poorer soils. The biogeographical patterns caused by the Malesian Floristic Interchange point to the importance of phylogenetic biome conservatism in biotic interchanges and resemble those resulting from the Great American Biotic Interchange in the Neotropics.

Could the environment limit dispersal between Sunda and Sahul?

“…Limitation of species that lack coping strategies subsequently dictates the composition of ultramafic plant communities (Pillon et al, 2010). In southeast Asia, this results in an increase in Sahul lineages in ultramafic communities (Brambach et al, 2020). Lineages most successful in the non-ultramafic soils widespread in Sunda and Sahul should be limited by the ultramafic soils of Wallacea.…”

First fossil fruits of Elaeocarpus (Elaeocarpaceae) in East Asia: Implications for phytogeography and paleoecology

“…In Australia, Elaeocarpus is essentially restricted to mesic forest communities (Rozefelds & Christophel, 1996a). Brambach et al (2020) recognized that Gondwanic lineages like Elaeocarpus, along with podocarp conifers, dominate some high altitude forests in the Malesian region, but also occur at lower altitudes. In China, Elaeocarpus occurs in warm and wet rain forests and other evergreen forest communities ranging from lowland to montane areas (Tang & Phengklai, 2007).…”

“…As in the Malesian region (Brambach et al, 2020), Elaeocarpus in South China often occurs in elevated and montane areas. The extant Chinese species that were compared with the fossils typically occur in evergreen montane forest.…”

Environmental stress influences Malesian Lamiaceae distributions

“…Likewise, altitude poses difficulties through reductions in temperature and soil fertility (Asner et al, 2014 ; Grubb, 1977 ). In Malesia, we can compare the effects of these stressors and dispersal distances between islands upon species distributions (Brambach et al, 2020 ; Joyce et al, 2020b ; Kooyman et al, 2019 ; Trethowan, 2021 ).…”