Explosive flowering, nectar production, breeding systems, and pollinators of New Zealand mistletoes (Loranthaceae)

Abstract

The six New Zealand loranthaceous mistletoes fall into two groups based on pollination biology. Four (Alepis flavida, Peraxilla colensoi, P. tetrapetala, and Trilepidea adamsii) are bird pollinated with hermaphrodite flowers while the other two (Ileostylus micranthus and Tupeia antarctica) are dioecious or sub-dioecious and insect pollinated. We provide data on the pollination biology of the five extant species (Trilepidea is extinct). The two Peraxilla species and Trilepidea have recently been shown to have explosive flowers. Here we show that Alepis has weakly facultatively explosive flowers. The world distribution of explosive mistletoe flowers suggests that the syndrome has arisen a number of times independently within the family, and is found in about half the putatively ancestral genera. The principal avian visitors to the bird-pollinated species were tui and bellbirds; introduced species are numerically unimportant as pollinators. The nectar production schedules of Alepis and Peraxilla encourage single-visit pollination as little nectar is produced after the flowers open. However, when bird densities are high, buds of Peraxilla may be forced open prematurely, encouraging multiple visits. Alepis is highly self-compatible and its flowers achieve good seed set in the field even when all pollinators are excluded by mesh bags, partly because pollen contacts the already-receptive stigma before the bud opens. Unvisited Peraxilla buds do not open, but their morphology allows a low level (11–22%) of self-pollination in the bud. The sex ratio of Tupeia at Wainui (Banks Peninsula) was 3:1 female:male, while Ileostylus at Wakefield (Nelson) was subdioecious with a 2.5:1:1 female:male:hermaphrodite ratio. Hermaphrodite Ileostylus plants set seed even when pollinators are excluded, so self-pollination is possible in this species. Results suggest different species vary in their susceptibility to pollen limitation affecting reproduction. The insect-pollinated species have unspecialised pollination syndromes and are probably adequately pollinated by native and/or introduced insects; Ileostylus is also self-compatible. The bird-pollinated species are more susceptible with specialised pollination systems depending principally on endemic birds, but the effect is reduced in Alepis as its flowers can open themselves and it achieves good seed set even when pollinators are excluded. Peraxilla spp. seem most at risk with obligately-explosive flowers and only low seed set without pollinators. Historical declines in the North Island have been greatest in the species with the most exacting pollination requirements. Long-term conservation of these mistletoes will require conservation of tui and/or bellbirds. © 1997 Taylor & Francis Group, LLC.