Eucalypt nectar studies conclude
Accessing flowers 30 metres off the
ground required the use of a cherry-
picker and crane.
Photos by Brad Law
Research into the nectar production of two species of eucalypts in State forests on the New South Wales south coast is sure to be of interest to both beekeepers and forest managers. Dr Brad Law reports.
In Australia, nectar is an abundant food for our wealth of native nectarivores and, more recently, European honeybees (Apis mellifera). For the first time, the effect of timber harvesting on canopy nectar production in tall forest trees has been investigated by NSW Department of Primary Industries researchers, with funding from the Honeybee Program of the Rural Industries Research and Development Corporation (RIRDC) and Forests NSW.
State forests provide the major honey resource for the beekeeping industry in NSW. While Forests NSW has a number of management practices already in place to retain nectar-producing trees during forestry operations, there is no information on how much nectar is produced by retained trees or how much nectar regrowing young trees produce after logging. Indeed, beekeepers have expressed concern about the effects of logging on nectar production, especially the perception that young trees do not produce as much nectar as mature trees.
The two eucalypt species chosen for research, spotted gum (Corymbia maculata) and grey ironbark (Eucalyptus paniculata), are of prime importance to nectarivorous wildlife, the timber industry and beekeepers.
Not surprisingly, the most exciting part of our research was accessing flowers in forest canopies over 30 metres high using cranes and cherry-pickers. With fantastic views of the south coast in the background, we measured nectar in flowers bagged overnight to determine how much nectar is produced when pollinators are excluded. We also measured un-bagged flowers to determine how much nectar is available to pollinators. Both large and small trees were measured in forest with different logging histories: recently logged, regrowth and mature.
After measuring thousands of flowers, we can conclude that nectar production in spotted gum on a per flower basis was not affected by logging history nor tree size.
When individual flowers are scaled up to the forest stand, mature forest with large trees and many more flowers produced almost ten times as much sugar per hectare as recently logged forest, with regrowth being intermediate.
However, at the compartment scale, the difference between mature forest and recently logged forest was reduced to a factor of two times when the extent of areas excluded under current logging practices was considered. Most importantly, nectar was not a limiting resource in 2005 as extensive flowering was recorded across the south coast.
We also surveyed local beekeepers with questionnaires and found that honey yields in 2005 were extremely high. A typical 1000 hectares of spotted gum forest flowering from April – August yielded five tonnes of honey. Consistent with a nectar surplus in 2005, we found that honey productivity was comparable across the three different logging histories.
But not every year is as good as 2005, with flowers measured in 2003 providing a strong contrast. Few trees were in flower and nectarivores, especially birds and honeybees, left virtually no nectar behind by mid-morning. Beekeepers reported that hive bees were not producing honey under these conditions.
Results for grey ironbark showed similarities to spotted gum with regard to the impact of logging, but the species differed markedly in other aspects of nectar production.
The results of this study will help promote sustainability by raising the awareness of forest managers about the importance of the nectar resource for both native fauna and honeybees, and beekeepers about current forest management practices.