Community-level plant–pollinator interactions in a Palaeotropical montane evergreen oak forest ecosystem

The montane terrain of northern Laos is covered by species-rich subtropical evergreen oak forests, home to endemic tree genera such as <i>Mytilaria</i> (Hamamelidaceae), and characterised by the coexistence of several honeybee and bumblebee species. We explored community-level plant–pollinator interactions of this unique little-known ecosystem. Extensive direct observations on flowering phenology and flower-visitor assemblages of 288 plant species of 82 families were conducted in a montane forest ecosystem in Houaphanh and Xiangkhouang Provinces, Laos, from 2005 until 2016. Based mainly on the extensive flower-visit data, we assessed the pollination system of each plant species. Five sympatric honeybee species (<i>Apis dorsata</i>, <i>A. laboriosa</i>, <i>A. cerana</i>, <i>A. florea</i> and <i>A. andreniformis</i>) were common on various types of flowers, and floral preferences differed among species. Long-tongued bees belonging to Bombini and Anthophorini (Apidae) were species-rich and frequent visitors on various deep flowers, especially Acanthaceae, Balsaminaceae, Lamiaceae, Rubiaceae and Zingiberaceae. Character displacement by tongue length was observed among the bee species, and many relaxed species-specific and species-semispecific interactions were observed between the bees and the deep-flowers. Four plant species, in the genera <i>Mytilaria</i>, <i>Chloranthus, Dioscorea</i> and <i>Cryptocarya</i>, were visited exclusively by thrips. Two plant species, in the genera <i>Lysimachia</i> and <i>Thladiantha</i>, had oil-secreting flowers, which were specifically visited by the oil-collecting bees <i>Macropis</i> and <i>Ctenoplectra</i>, respectively. The dominant pollination system assessed was general insect pollination (31%), followed by long-tongued bee, small bee, honeybee, dipteran, lepidopteran, beetle, wasp, carpenter bee, thrips (e.g. the endemic genus <i>Mytilaria</i>), bird and hemipteran pollination. Our results suggest that the plant–pollinator interactions in the Palaeotropic montane ecosystem are characterised by significant contribution of the five honeybee species and species-rich, morphologically diverse long-tongued bees, both of which have contributed to shaping the remarkable diversity of angiosperms with deep flowers.