The role of climatic variability on Eucalyptus regeneration in southeastern Australia

Abstract In the forested landscapes of southeastern Australia, bushfires and timber harvesting are the primary catalysts for regeneration in Eucalyptus regnans, E. delegatensis, and high elevation mixed species (HEMS) forests. Quantifying the role of climate, topography and edaphic conditions on plant regeneration is important for understanding current and future risks of climate change. In this study, we investigated the post-disturbance regeneration dynamics in these forests. We sought to determine the direct impact of climate variability on regeneration and the interactive effects of climate, topography, and edaphic factors on the regeneration success of Eucalyptus. Data was collated from regeneration surveys conducted in 881 coupes, totaling ~55,000 ha of forest harvested and regenerated between 2004 and 2012. The time-period analyzed included the Millennium Drought, an intense and prolonged drought that lasted from 1996 to 2010. To test the influence of climatic, topographic, and edaphic variables on the occurrence and abundance of Eucalyptus regeneration, we used machine learning models. We found that regeneration success was closely tied to the timing of the Millennium Drought. Between 2004 and 2005 Eucalyptus seedlings occurred in less than 50% of survey plots across all three forest types. Declines in regeneration during the period of drought were greater in E. regnans and E. delegatensis than HEMS forests, suggesting that regeneration in the HEMS forests is more resistant to drought. We found that seasonal precipitation and temperature had the greatest influence on regeneration success of Eucalyptus. Responses varied by species, however autumn precipitation had the strongest influence on abundance of regeneration of E. regnans and E. delegatensis, while summer temperature was more important for HEMS species. Our findings highlight the importance of seasonal and annual climate variability on Eucalyptus regeneration and portend potential declines in regeneration success in a warmer and drier future for E. regnans and E. delegatensis.