Canopy nectar production and the impact of logging and climate in Grey Ironbark Eucalyptus paniculata (Smith) forests

Abstract

Nectar in tall forest canopies is a significant, but poorly quantified, resource for Australian fauna, as well as the European Honeybee Apis mellifera. We investigated the impact of logging on nectar production in the canopy of Grey Ironbark Eucalyptus paniculata (Smith) forests in southern Australia. Using cherry-pickers and cranes we measured nectar production in large and small trees in replicate sites in each of recently logged, young regrowth and old regrowth forest over three consecutive years (2004?2006). We focused on over-night nectar production, although nectar was produced during both the day and night. Logging history and tree size, when considered individually, had no significant effect on nectar production per flower, although the two factors showed a significant interaction. However, these differences were relatively minor in comparison to the negative relationship with drought. Little nectar was produced per flower under any logging history in drought. During good conditions nectar production varied depending on logging history. When scaled up to the forest stand, logging history had a marked effect on nectar production with old regrowth forest producing seven times as much sugar per ha as recently logged forest. Young regrowth forest 15?20 years old produced nectar quantities intermediate between that of recently logged forest and regrowth forest. At the compartment scale, current practices require the retention of old forest and the typical extent of this retention reduced the difference between old regrowth forest and recently logged forest to a factor of two times. Nectar production per flower was low and a limited resource in autumn 2004 and late-winter 2005, but was copious and in surplus in early summer 2006. Nectar standing crops at the flower scale appeared to be determined by an interaction between environmental conditions (drought) that negatively influenced nectar production and the feeding activity of flower visitors at the time, which itself is affected by prevailing temperatures and nectar attributes, such as sugar concentration and regional nectar availability. We suggest that management actions should focus on minimising nectar depletion in poor flowering years when the nectar resource is limiting.