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Pollination ecology of Eucalyptus globulus subsp. globulus and Eucalyptus nitens (Myrtaceae)

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posted on 2023-05-26, 02:46 authored by Andrew HingstonAndrew Hingston
Tasmanian native blue gum Eucalyptus globulus subsp. globulus and its closely related southeastern Australian mainland congener E. nitens are the major trees grown in eucalypt plantations in temperate regions of the world. Plantation stock are mostly grown from seeds, that are increasingly being collected from seed orchards of trees selected for characters desired by the forest industry. Seed production and fitness of the resultant trees are dependent largely upon pollen transfer between flowers on different trees, because of the partial self-incompatibility in these two species. The unsuitability of the pollen to transfer by wind necessitates the harnessing of animals to transfer pollen as they forage at flowers. This research aimed to determine which animals were effective pollinators of these tree species in Tasmania. These two species have contrasting floral forms, associated with enormous differences in nectar production, that resulted in their flowers being used by different animals as food sources. The small flowers of E. nitens produced only 0.3 - 0.6 mg of nectar sugar per day and, accordingly, were visited exclusively by small, mostly native, insects. Introduced honey bees (Apis mellifera) and bumble bees (Bombus terrestris), being larger, more energy demanding insects, were rarely seen visiting flowers of E. nitens, and birds were never seen attempting to feed from these flowers. In contrast, the large flowers of E. globulus produced 37- 56 mg of nectar sugar per day, rendering them attractive to energy demanding birds and exotic bees, as well as the less energy demanding smaller insects. Single visits to flowers of E. globulus by swift parrots (Latham us discolor) resulted in statistically significant increases in seed production above the levels occurring in unvisited flowers. Although other bird species were not sufficiently assessed by this method to determine whether they are also effective pollinators, analyses of their foraging behaviour and pollen loads suggest they are. In contrast, experiments indicated that insects were poor pollinators of E. globulus. Single visits to flowers by insects, including honey bees and bumble bees, did not result in statistically significant increases in seed production above the levels occurring in unvisited flowers. Even prolonged exposure to insects throughout the life of a flower failed to result in the production of as many seeds as that following a single swift parrot visit, despite insects often consuming all of the daily nectar production. Hence, seed production and the fitness of plantation trees should be enhanced by management practices that benefit populations of native flowervisiting insects in seed orchards of E. nitens and birds in orchards of E. globulus.

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